Organic monolayers and fluoropolymer brushes : functionalization, stability and tribology
Bhairamadgi, N.S. - \ 2014
Wageningen University. Promotor(en): Han Zuilhof; Cees van Rijn. - Wageningen : Wageningen University - ISBN 9789461739636 - 178
unimoleculaire films - organische verbindingen - organische fluorverbindingen - colloïden - polymeren - adhesie - frictie - oppervlakteverschijnselen - unimolecular films - organic compounds - organofluorine compounds - colloids - polymers - adhesion - friction - surface phenomena
This thesis deals with the adhesion and friction properties of densely grafted and covalently bound fluoropolymer brushes on silicon surfaces with varying thickness and fluorine content. A novel surface-functionalizing method is described using the thiol-yne click (TYC) reaction. The TYC reaction is highly useful for the attachment of functional (bio-)molecules and immobilization of radical initiators onto a surface with high density. Next, the hydrolytic and thermal stability of 24 different types of monolayers on Si(111), Si(100), SiC, SiN, SiO2, CrN, ITO, PAO, Au and stainless steel surfaces was evaluated. Subsequently, based on this outcome, highly stable fluorinated polymers are described as obtained using surface-initiated atom transfer radical polymerization (SI-ATRP) reactions. The effects of thickness and fluorine content on tribological properties of these layers were studied. The adhesion and friction properties were investigated using colloidal probe atomic force microscopy under dry and ambient conditions. The solvent-free lubricating properties of obtained fluoropolymer brushes have been characterized in detail, and demonstrate their potential for e.g., MEMS/NEMS devices.
Colloids at liquid interfaces: dynamics and organization
Ershov, D.S. - \ 2014
Wageningen University. Promotor(en): Jasper van der Gucht, co-promotor(en): Martien Cohen Stuart. - Wageningen : Wageningen University - ISBN 9789461738943 - 127
colloïden - oppervlaktechemie - grensvlak - oppervlakteverschijnselen - capillairen - vloeistoffen (liquids) - colloids - surface chemistry - interface - surface phenomena - capillaries - liquids
This thesis deals with spherical microparticles trapped at liquid interfaces. It focuses on two aspects of their behavior: firstly, the effect of the curvature of a liquid interface on interparticle interactions and their organization; secondly, the mobility of particles at visco-elastic interfaces.
In Chapter 2of this thesis we showed that it is possible to induce capillary interactions between spherical microparticles with homogeneous surface chemistry by tailoring the curvature of the liquid interface. If the interfacial curvature is anisotropic, the constraint of constant contact angle along the contact line can only be satisfied if the interface is deformed locally. These deformations create excess surface area, which changes when two particles approach each other. This leads to a change in the surface free energy, which manifests itself as a capillary interaction between the particles.
To study the effect of curvature on the interactions between particles, we created oil-water interfaces of different shape (ellipsoid, dumbbell, torus and squares) and added spherical negatively charged particles that adsorbed at these interfaces. On all these interfaces, we observed quadrupolar capillary interactions that organized the particles into square lattices. The order of this organization increased with increasing curvature anisotropy, indicating that capillary interactions are stronger as well. By contrast, on flat interfaces or on spherical droplets with homogeneous curvature, no attractive interaction was observed and only at very high surface coverage did the particles order in a hexagonal lattice, as a result of repulsive interactions.
In Chapter 3we studied the interface deformations around particles at curved interfaces and the resulting capillary interactions theoretically. We used the finite element method to solve the Young-Laplace equation for the shape of the interface around a particle and calculated the interaction potential between the particles numerically.
The main finding of these calculations is that for an anisotropically curved interface, with two different local principal curvatures, the particle deforms the interface in two ways simultaneously: concave deformation along one principal direction and convex – along the other, thus creating a deformation field with quadrupolar symmetry. Two particles with such deformations interact favorably only if the overlapping deformations are similar (concave-concave, convex-convex), which occurs when they approach each other along one of the two principal directions. Since the two local principal directions are always perpendicular, particles interacting along them will tend to arrange into a square pattern.
As a consequence of the quadrupolar deformation field, two particles approaching each other along a line forming 45 degrees with the principal axes will repel each other (which is confirmed by our observations), because in this case the deformation fields overlap with four different “petals” (2 pairs of concave-convex), and the excessive surface area doesn’t reduce upon approaching, but increases. A system of two particles oriented at an angle with respect to the principal axis is therefore subjected to a torque rotating the axis of the system so that it gets aligned with one of the two principal directions. The torque magnitude reaches its maximum when the system’s axis is at an angle of 45 degrees with respect to the principal direction and decreases to 0 when the axis is aligned with one of the principal directions.
The family of interaction potentials we obtained allows for calculating the minimum deviatoric curvature needed to initialize capillary interactions strong enough to compete with thermal energy, so that a stable organization can be expected. The calculated value was very close to the deviatoric curvature where ordering was observed experimentally in Chapter 2.
In Chapter 4we studied the mobility of 3 mm polystyrene particles in a monolayer of 1.5 mm core-shell microparticles deposited at flat air-water interfaces; all the particles present in the system were stabilized by negative charges.
In this exploratory chapter we made an attempt to characterize the mechanical properties of such monolayers by analyzing the mobility of the larger tracer particles in the monolayer. With increasing particle density of the monolayer, we observed that the mean-square displacement of the tracer particles was reduced, which can be interpreted as an increase of the viscosity of the monolayer. At very high densities the motion of the particles became subdiffusive and confined, pointing at elasticity of the monolayer. We also studied correlated movements between neighboring particles in an attempt to do two-point interfacial microrheology. A comparison between the one-point and two-point methods shows clear indications of heterogeneous dynamics of the tracer particles. Our results therefore call for a further development of two-point microrheology at interfaces.
In Chapter 5we used tracer particles to study the properties of thin cross-linked actin networks deposited at the surface of oil droplets. These networks are a model system for the intracellular actin cortex. We used the generalized Stokes-Einstein relation to extract the complex frequency-dependent shear modulus of such networks from the movement of the added tracer particles. We studied the effects of the length of actin filaments and the cross-linker concentration on the mechanical properties of these layers.
The advantage of this system is that actin networks are freely accessible from the water phase, and therefore can be subjected to in-situ addition of cross-linkers, enzymes or other chemicals of interest. Using this, we managed to show strong stiffening after addition of myosin motor proteins and ATP, which we ascribed to contraction of the actin-myosin network.
Structure of binary mixed polymer Langmuir layers
Bernardini, C. - \ 2012
Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers. - S.l. : s.n. - ISBN 9789461732149 - 200
polymeren - colloïden - colloïdale eigenschappen - oppervlakteverschijnselen - polymers - colloids - colloidal properties - surface phenomena
The possibility of preparing 2D stable emulsions through mixing of homopolymers in a Langmuir monolayer is the core topic of this thesis. While colloid science has achieved well established results in the study of bulk dispersed systems, accounts on properties of mixed monomolecular films are fewer, and seldom systematic. The aim of this investigation is to contribute to a deeper understanding of the subject, in order to explore opportunities to apply the acquired knowledge to the fabrication of technologically relevant materials. In particular, this study focused on a possibly applicable, innovative strategy for the manipulation of the morphology and the patterning of mixed Langmuir monolayers: the possibility to stabilize and control a dispersion of homopolymers through the addition of a lineactant (the equivalent of a surfactant in three dimensional systems), able to adsorb preferentially at the interfacial contact line of polymer domains, thereby lowering the interfacial energy (line tension) in the system and favoring an effective dispersion of one component into the other.
The state of the art of the preparation and investigation of 2D colloids is the subject of Chapter 2, which is a comprehensive review on several systems able to yield phase–separated Langmuir monolayers, and includes a general definition of the concept of a 2D colloid, the most relevant instrumental techniques and experimental tools available, a summary of several systems suitable for preparing 2D colloid dispersions, an introduction to the concept of lineactant, and several examples, both experimental and theoretical, in which compounds acting as lineactants have been investigated. This review clearly shows that the polymer–based mixtures are a poorly explored subject, when compared to amphiphiles of natural origin, and so the rest of the thesis has been devoted to the investigation of polymer–based Langmuir monolayers.
This investigation has been carried out with two parallel approaches: classical experiments at the Langmuir trough and morphological characterization of the Langmuir monolayers with the Brewster Angle Microscope have been performed, along with Self–Consistent Field modeling of the same systems. The setup of the SCF model and comparison of SCF calculation with experimental data from the reference experiments are dealt with in Chapter 3. Surface pressure isotherms at the air/water interface were reproduced for four different polymers, poly–l–lactic acid (PLLA), poly (dimethylsiloxane) (PDMS), poly (methyl methacrylate) (PMMA), and poly (isobutylene) (PiB). The polymers are all insoluble in water, but display a different degree of amphiphilicity; therefore the four isotherms differed strongly. The polymers were described through a SCF model on a united atom level, taking the side groups on the monomer level into account. In line with experiments, the model shown that PiB spread in a monolayer which smoothly thickened at a very low surface pressure and area/monomer value. The monolayer made of PMMA had an autophobic behavior: a PMMA liquid did not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only formed after the collapse of the film at a relatively high pressure. The isotherm of PDMS had regions with extreme compressibility which were linked to a layering transition. Finally, PLLA wetted the water surface and spread homogeneously at larger areas per monomer. The classical SCF approach features only short–range, nearest–neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, a power–law van der Waals contribution was taken into account in this model. Two–gradient SCF computations were performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact was obtained, together with the associated line tension value. The SF–SCF molecularly detailed modeling of PLLA, PDMS, PMMA, and PiB monolayers, spread at the air/water surface, has proven to be consistent with experimental data: the incorporation in the model of a detailed molecular description of the monomeric features of the four compounds examined has been crucial to reproducing the features of the adsorption and pressure/area isotherms.
In Chapter 4, the same approach was applied to the description of polymer mixtures spread at the air/water interface. The aim of this chapter was to analyze topics such as 2D phase separation and partitioning in mixed polymeric Langmuir monolayers. Two of the four polymers studied in Chapter 3 were selected in order to obtain a mixed Langmuir monolayer. A system consisting of water–insoluble, spreadable, fluid–like polymers was prepared. The polymers were polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), combined, in some cases, with a minority of PDMS–b–PMMA copolymer. Both Langmuir trough pressure/area isotherm measurements and Brewster angle microscopy (BAM) observations were performed, and complemented with molecularly detailed self–consistent field (SCF) calculations. It was shown that PDMS undergoes a layering transition that is difficult to detect by BAM. Addition of PMMA enhanced contrast in BAM, showing a two–phase system: if this consisted of separate two–dimensional (2D) PMMA and PDMS phases, a PDMS–PMMA diblock should accumulate at the phase boundary. However, the diblock copolymer of PDMS–PMMA failed to show the expected “lineactant” behavior, i.e., failed to accumulate at the phase boundary. The calculations pointed to a non-trivial arrangement of the polymer chains at the interface: in mixtures of the two homopolymers, in a rather wide composition ratio, a vertical (with respect to the air/water interfacial plane) configuration was found, with PMMA sitting preferably at the PDMS/water interface of the thicker PDMS film, during the PDMS layering phase transition. This also explained why the diblock copolymer was not a lineactant. Both PMMA and PDMS–b–PMMA were depleted from the thin–thick PDMS film interface, and the line tension between the phases consequently increased in the binary mixtures, as well as in the ternary ones. The results shown in this chapter proved that gaining an accurate control over thin film structures at the microscopic level is a far from trivial task, and the acquisition of fundamental knowledge is necessary in order to interpret experimental data in an appropriate way.
As a consequence, in Chapter 5 an investigation based solely on SCF modeling was carried out, in order to analyze which polymer blends could have the possibility to undergo lateral phase separation in two dimensions. Specifically, the model system investigated consisted of water–supported Langmuir monolayers, obtained from binary polyalkyl methacrylate mixtures (PXMA, where X stands for any of the type of ester side groups used: M, methyl–; E, ethyl–; B, butyl–; H, hexyl–; O, octyl–; L, lauryl–methacrylate). In particular, the conditions which determined demixing and phase separation in the two–dimensional system were addressed, showing that a sufficient chain length mismatch in the ester side group moieties is able to drive the polymer demixing. When the difference in length of the alkyl chain of the ester moieties on the two types of polymers was progressively reduced, from 11 carbon atoms (PMMA/PLMA) to 4 carbons only (POMA/PLMA), the demixing tendency was also reduced; it vanished, indeed, for POMA/PLMA. In the latter case the polymer/subphase interactions affected more the distribution of the polymer coils in the blend monolayer: mixing of the two polymers was observed, but also a partial layering along the vertical direction.
Lineactancy was also considered, by selecting the mixture in which phase separation was best achieved: a third component, namely a symmetrical diblock copolymer of the type PLMA–b–PMMA, was added to a PMMA/PLMA blended monolayer. Adsorption of the diblock copolymer was observed exclusively at the contact line between the two homopolymer domains, together with a concomitant lowering of the line tension. The line tension varied with chemical potential of the diblock copolymer according to the Gibbs’ law, which demonstrated that PLMA–b–PMMA indeed acted as a lineactant (the two–dimensional analog of a surfactant) in the model system made of a binary demixed PMMA/PLMA Langmuir monolayer.
In conclusion, the requirements needed to achieve polymer blend demixing in a Langmuir monolayer are the following: spreadable, insoluble polymers, with the same amphiphilicity degree, combined to a certain chemical mismatch of the side moieties are necessary in order to cause lateral demixing at the air/water interface. The polyalkyl methacrylate example investigated in the chapter represented a suitable model system, since the methacrylate backbone guarantees that the different polymers have the same affinity towards the water subphase, while the different ester moieties drive the occurrence of lateral demixing. The dependency of the lateral demixing on the difference in length between the two ester side groups chosen was demonstrated. A rather complex interplay of forces regulates the distribution of the polymer coils in the monolayer: subtle alterations of this complex balance might favor the dewetting of the mixture in a single domain, together with the layering of the blended polymers along the direction normal to the air/water interface, as well as accumulation of one polymer at the domain edge, instead of the occurrence of the lateral phase separation. Furthermore, the possibility to control emulsification of two–dimensional demixed polymer blends was proven. This was achieved by use of a diblock copolymer, which acted as a lineactant by adsorbing at the contact line of the polymer domains. The calculations demonstrated the possibility to extend the lineactant concept, first elaborated in the context of lipid membrane investigations, to the field of study of polymer thin films.
Van eiwitten in platland tot bionanotechnologie in Wageningen
Norde, W. - \ 2010
Wageningen : Wageningen Universiteit - ISBN 9789085855767 - 28
eiwitten - oppervlakten - grensvlak - nanotechnologie - bionanotechnologie - oppervlaktechemie - oppervlakteverschijnselen - proteins - surfaces - interface - nanotechnology - bionanotechnology - surface chemistry - surface phenomena
Physics of associative polymers : bridging time and length scales
Sprakel, J.H.B. - \ 2009
Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers, co-promotor(en): Jasper van der Gucht. - S.l. : s.n. - ISBN 9789085853657 - 237
polymeren - colloïden - micellen - reologische eigenschappen - oppervlakteverschijnselen - polymers - colloids - micelles - rheological properties - surface phenomena - cum laude
cum laude graduation (with distinction)
Modification of silicon nitride and silicon carbide surfaces for food and biosensor applications
Rosso, M. - \ 2009
Wageningen University. Promotor(en): Han Zuilhof; Remko Boom, co-promotor(en): Karin Schroen. - [S.l. : S.n. - ISBN 9789085853794 - 221
organische verbindingen - unimoleculaire films - microfiltratie - nanotechnologie - oppervlaktechemie - oppervlakteverschijnselen - organic compounds - unimolecular films - microfiltration - nanotechnology - surface chemistry - surface phenomena
Silicon-rich silicon nitride (SixN4, x > 3) is a robust insulating material widely used for the coating of microdevices: its high chemical and mechanical inertness make it a material of choice for the reinforcement of fragile microstructures (e.g. suspended microcantilevers, micro-fabricated membranes-“microsieves”) or for the coating of the exposed surfaces of sensors (field-effect transistors, waveguide optical detectors). To a more limited extent, silicon carbide (SiC) can find similar applications, and this material also starts to be more and more applied in coating and sensor technologies.
In all these applications, control over the surface properties of inorganic materials is crucial, for example to avoid blockage of membranes during filtration, or to provide sensor surfaces with specific (bio-)recognition properties. In this thesis, a variety of methods is developed to obtain and study robust functional coatings on SixN4 and SiC. These enable a whole new range of applications involving biocompatible and bio-specific surfaces, while retaining the bulk mechanical, structural, electrical or optical properties of the inorganic substrates.
Chapter 2 and 3 of the thesis give an overview of the great potential of covalent organic monolayers: Chapter 2 presents the formation of alkylthiol, alkylsilane and alkene monolayers, as well as a number of applications in biocompatible surfaces, micro- and nanopatterning of surfaces and sensing. The emphasis of this review chapter is put on the possible combinations of the bulk properties of inorganic materials (electrical, optical, structural) and the surface properties of organic monolayers (wettability, biospecificity, biorepellence). Chapter 3 is focused on biorepellent surfaces in the field of filtration with microfabricated membranes. Indeed, silicon nitride microsieves, despite their high permeability and structural homogeneity, are prone to pore blocking, when submitted to biological solutions. The chapter gives a review of the available surface modification techniques involving organic coatings that can minimize or even prevent this surface contamination. These coatings involve highly hydrophilic oligomers and polymers, which have been widely explored for organic surfaces. Covalent organic monolayers formed onto inorganic surfaces can extend the applications of these biorepellent coatings to microdevices like SixN4 microsieves (as also discussed in Chapters 7 and 8)
Chapter 4 and 5 present the thermal functionalization with highly stable alkene-based organic monolayers of the surfaces of silicon-rich silicon nitride (Chapter 4) and silicon carbide (Chapter 5). This work was motivated by the substantial knowledge of similar monolayer formation on silicon surfaces1,2 and the initial success of simple functionalizations on silicon nitride.3 The strong covalent attachment of the coating molecules with the substrates makes the obtained hybrid structures much more resistant to chemical degradation than other types of monolayers on these substrates. The reaction proceeds via attachment of the terminal double bond of alkenes with the surface groups (Si-H in the case of silicon nitride surfaces or –OH for silicon carbide surfaces). Besides methyl-terminated surfaces, functional coatings can be obtained by the use of bi-functionalized alkenes (Figure 1), also allowing further surface reactions and the attachment of bio-recognition elements, through covalent attachment of diverse chemical (carboxylic acid, amine) or biological (oligo-peptides, protein) moieties.
Figure 1. Modification of SiC and Si¬xN4 surfaces with alkyl monolayers
Chapter 6 describes a modification of this method, where UV irradiation is used instead of heat to initiate the modification of both silicon nitride and silicon carbide. For both materials, this method allows the grafting of heat-sensitive compounds, needs less starting material (using only a liquid film) and provides monolayers with higher quality (as e.g. indicated by grafting density and stability) and higher reproducibility. Here again the attachment of diverse functionalities is possible, via formation of activated esters. After hydrolysis and activation of such grafted ester, amines can be attached in high yield (> 80 %), as demonstrated using X-ray photoelectron spectroscopy (XPS). Besides the homogeneous modification of plain surfaces, this method also opens the way to surface patterning of silicon nitride and silicon carbide and the modification of mechanically sensitive microfabricated devices.
In Chapters 4 to 6, the chemical functionalizations are studied using X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and static water contact angles. Si-C bonds are formed preferentially upon reaction of SixN4 surfaces with alkenes, similarly to what is reported for pure silicon surfaces, albeit that no measurement could totally exclude the presence of C-N bonds. The wet etching of SiC yields hydroxyl-terminated surfaces, and an IRRAS study reveals the attachment of alkenes via a Markovnikov-type addition (O-C bond formed on the second carbon of the double bond). The stability of these monolayers is reported in acidic and basic conditions, and it was shown that UV initiation yields even more stable monolayers, probably due to some cross-linking of the alkyl chains.
Chapter 7 explores the biorepellence of UV-initiated monolayers on silicon nitride surfaces Oligomers of ethylene glycols (3 or 6 units: methoxy-tri(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)3(CH2)9CH=CH2; EO3, and methoxy-hexa(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)6(CH2)9CH=CH2; EO6) are attached on the silicon nitride surfaces. The adsorption of two proteins, bovine serum albumin (BSA) and fibrinogen is used to test the biorepellence of the monolayers, in comparison with bare oxidized silicon nitride. Both proteins adsorb readily onto bare SixN4 surfaces, with adsorbed amounts of 1.25 and 2.7 mg.m-2 for BSA and fibrinogen, respectively, of which more than 80 % is irreversibly bound. In contrast to this, when oligomers are attached to the surface, this adsorption decreases to under the detection limit of the method used for this experiment (optical reflectometry). The ex situ study of surfaces with AFM and water contact angles also indicates that some of the monolayers completely prevent the adsorption of proteins.
Figure 2. Biorepellent behavior of oligoethylene oxide coated SixN4 surfaces
Chapter 8 describes the applications of the biorepellent coatings used in Chapter 7 (EO6) to silicon nitride microsieves, in order to improve the filtration of biological solutions and liquid food products. The EO6 coatings are successfully formed on microfabricated membranes with pore diameters of 0.45 micrometer, using the UV-initiated monolayer formation described in Chapter 6. This work shows that these coatings could be applied without loss of permeability due to wettability or pore blocking. Moreover, AFM showed that these coatings significantly decrease the adsorption of proteins on the surface between the pores.
Chapter 9 describes an alternative functionalization technique for inorganic surfaces, namely the use of plasma oxidation of alkyl monolayers to reproducibly form aldehydes (among other oxidized species) onto surfaces. The method described here for silicon and silicon nitride surfaces, is developed for the functionalization of sensitive devices and substrates. The formation of methyl-terminated alkyl monolayers from linear terminal alkenes is one of the easiest to perform, since linear monofunctional alkenes are readily available, their purification is easy (distillation) and their grafting conditions are very flexible (liquid state, heat-resistant, UV-resistant > 250 nm). Once these stable monolayers are formed, a short plasma treatment (0.5 to 2 s) is able to form oxidized functionalities within the top few angstroms of the surface, while the underlying alkyl chains retain their initial packing and insulation properties of the inorganic substrate. The grafting of gold nanoparticles shows that micron-sized patterns can be formed using a soft contact mask to protect a limited area of the monolayer. Alternatively, the aldehydes can be used to attach biotin and avidin onto SixN4 surfaces. The selective adsorption of biotinylated BSA onto the avidin-modified surfaces shows that the plasma treatment of methyl-terminated monolayers is a fast and efficient method to produce surfaces displaying high specific biochemical interactions.
In the chapter 10, some of the most striking effects that are described in the previous chapters are put into a wider perspective. Especially the formation and stability of monolayers is discussed, also in relation to biofunctionalization, biorepellence, and opportunities for surface engineering are proposed.
Ketens en grenzen
Fleer, G.J. - \ 2007
Wageningen : Wageningen Universiteit - 34
polymeren - colloïdale eigenschappen - oppervlaktechemie - oppervlakteverschijnselen - polymers - colloidal properties - surface chemistry - surface phenomena
Protein/polysaccharide complexes at air/water interfaces
Ganzevles, R.A. - \ 2007
Wageningen University. Promotor(en): Martien Cohen Stuart; Fons Voragen, co-promotor(en): Harmen de Jongh; Ton van Vliet. - [S.l.] : S.n. - ISBN 9789085046141 - 151
polysacchariden - eiwitten - adsorptie - oppervlaktechemie - oppervlakteverschijnselen - polysaccharides - proteins - adsorption - surface chemistry - surface phenomena
KEYWORDS:protein, polysaccharide,b‑lactoglobulin, pectin, electrostatic interaction, complex coacervation, adsorption, air/water interface, oil/water interface, surface pressure, surface rheology, spectroscopy
Proteins are often used to create and stabilise foams and emulsions and therefore their adsorption behaviour to air/water and oil/water interfaces is extensively studied. Interaction of protein and polysaccharides in bulk solution can lead to the formation of soluble or insoluble complexes. The aim of this thesis was to understand the influence of (attractive and non-covalent) protein/polysaccharide interaction on adsorption behaviour at air/water interfaces (and oil/water interfaces) in terms of adsorption kinetics, and rheological and spectroscopic characterisation of the adsorbed layers. The approach was to first identify the relevant parameters (like charge density, charge distribution or molecular weight of the ingredients) in the mixed protein/polysaccharide adsorption process. Subsequently, for each parameter a range of ingredients was selected/prepared allowing variation of only this single parameter. After investigation of the phase behaviour in bulk solution of the different protein/polysaccharide mixtures to be used, the role of each parameter in mixed protein/polysaccharide adsorption was studied. The parameters most thoroughly assessed were: protein/polysaccharide mixing ratio, polysaccharide charge density and molecular weight and the sequence of adsorption. The majority of the measurements were performed withb‑lactoglobulin (in combination with various polysaccharides e.g. pectin or carboxylated pullulan) at air/water interfaces, at standard conditions of pH 4.5 and low ionic strength (< 10 mM). In addition, experiments were performed at higher ionic strengths, different pH's, with different proteins or at an oil/water interface, to extend the insight in mixed protein/polysaccharide adsorption. This results obtained lead to a generic mechanistic model of mixed protein/polysaccharide adsorption.In conclusion, protein/polysaccharide interaction can be exploited to control protein adsorption at air/water interfaces. Any parameter affecting protein/polysaccharide interaction (e.g. ingredient parameters like polysaccharide molecular weight, charge density and distribution or system parameters like charge ratio, pH and ionic strength) may be varied to obtain the desired adsorption kinetics, surface rheological behaviour, or net charge of the surface layer. The choice of simultaneous protein/polysaccharide adsorption (in the form of complexes) versus sequential adsorption (first the protein, than the polysaccharide) provides an extra control parameter regarding the functionality of mixed adsorbed layers.
Faradaic and adsorption-mediated depolarization of electric double layers in colloids
Duval, J.F.L. - \ 2003
Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Herman van Leeuwen. - Wageningen : s.n. - ISBN 9789058089403 - 238
colloïden - elektrokinetische potentiaal - oppervlakteverschijnselen - oppervlaktechemie - colloids - electrokinetic potential - surface phenomena - surface chemistry - cum laude
cum laude graduation (with distinction)
|ENVISAT - land surface processes
Su, Z. ; Roerink, G.J. - \ 2001
Delft : Netherlands Remote Sensing Board (BCRS), Programme Bureau, Rijkswaterstaat Survey Department - ISBN 9789054113560 - 114
remote sensing - land - atmosfeer - oppervlakte-interacties - interacties - warmtebalans - modellen - oppervlakteverschijnselen - aardoppervlak - klimaat - meteorologie - verdamping - Azië - remote sensing - land - atmosphere - surface interactions - interactions - heat balance - models - surface phenomena - land surface - China
The practical application of scintillometers in determining the surface fluxes of heat, moisture and momentum
Green, A.E. - \ 2001
Wageningen University. Promotor(en): A.A.M. Holtslag; H.A.R. de Bruin. - S.l. : S.n. - ISBN 9789058083364 - 177
meteorologische instrumenten - meteorologische waarnemingen - meting - instrumentatie - instrumenten (meters) - lasers - atmosfeer - warmte - vocht - momentum - oppervlakteverschijnselen - meteorological instruments - meteorological observations - measurement - instrumentation - instruments - lasers - atmosphere - heat - moisture - momentum - surface phenomena
This thesis has collated one review chapter and five experiments concerned with addressing the question, 'how successful is the scintillometer method in determining the surface fluxes of heat, moisture and momentum and under what circumstances does it appear to fail?' Answering this question is important as a workable scintillation method provides the meteorologist with spatial integrated measurements of the surface fluxes at kilometre scales. With such a tool, ground-truth validation of remote sensing systems is possible, water balance studies can be conducted at catchment scales and energy balance experiments extended over slightly non-homogeneous terrain. Using electromagnetic scintillation to infer turbulence quantities is a fairly recent development. Although our interest lies in estimating the surface fluxes Chapter 3 makes it very clear that the foundation of the scintillation method is deeply rooted in at times questionable combination of turbulence and wave propagation theory. The novice must appreciate the important steps and assumptions in the scintillation method. Purchasing a scintillometer off-the-shelf is no guarantee of reliable measurements.
How well this thesis has answered the treatise depends to an extent on the relative performance of the scintillometer method against some benchmark. For these scintillation experiments this benchmark was the eddy covariance method, selected for convenience and familiarity. In many ways this selection is a compromise as different temporal and spatial band-widths are used by each method. Scintillation measures the ensemble average of spatial fluctuations in the refractive-index along the propagation path. The eddy covariance technique makes measurements at one point in space as a function of time. The time average of the eddy covariance method is considered to be an ensemble average. Depending on atmospheric stability the eddy covariance method may require several tens of minutes to integrate the energy from all eddy scales contributing to the surface fluxes. In comparison, the scintillometer can provide statistically stable data within minutes because it only measures in the inertial-convective subrange of frequencies. In light of such differences any comparison between methods should be made with a tongue-in-cheek approach with the state of the atmosphere and surface conditions carefully scrutinised to explain any discrepancies. The strength of using the eddy covariance technique as a comparison is because it identifies any marked deviations from the norm experienced by the scintillation method. Success is in comprehending what caused these deviations not through obtaining the perfect half-hour correlation between methods. The five experiments in this thesis table such deviations and propose explanations for their particular scintillometer type and application. What follows, are the salient facts gleamed from this research.
The experiments using the inner scale meter and the semiconductor laser diode highlighted the pluses and minuses ofdependence for laser scintillometers. This dependence required a correction to the measured signal variance using the spectra of Hill (1978), but it also provided information on the magnitude ofand ultimately a measure of. In the case of the semiconductor laser diode scintillometer,was indirectly determined using a measurement of average windspeed and crop height. This approach proved successful in the calculation of. In contrast, the inner scale meter measuredusing the difference in received signal variances between a gas laser and a large aperture scintillometer having littledependence. This latter approach was particularly sensitive to small signal differences and caused considerable scatter in thecomparisons. Despite a coarse result for,still compared favourably to. In addition to thedependence the laser scintillometers suffered from signal saturation in the presence of strong turbulence. This path limited the laser scintillometers and consequently the inner scale meter to operation over short distances ( L < 100 m). This limitation was less than attractive for the purposes of path averaging fluxes at catchment scale.
The near-infrared large aperture scintillometer was designed to overcome the shortcomings of the laser scintillometers. It is most suited to calculations ofand it successfully did so as a component of the inner scale meter. This scintillometer however performed poorly for the rice paddy experiment. Here<and at times up to 40% of the received signal variance could be attributed to correlated T-Q fluctuations. The rice paddy experiment highlighted the effect of absorption scintillations on the large aperture signal variance. This effect was also apparent in the test of the two-wavelength scintillometer at Ahipara (Chapter 7). Subsequent modification to the scintillometer's electronic filtering alleviated this problem. If large aperture near-infrared scintillometers are still being built based on the original design of Ochs and Cartwright (1980) then theoutput signal may contain the effect of absorption fluctuations in addition to refractive fluctuations.
The optical wavelength scintillometers, whether they are the laser or the large aperture types, struggled to provide a measurement ofbecause they are less sensitive to humidity fluctuations than temperature fluctuations. This was confirmed by observations of the relative contributions made by,, andtoat visible to near-infrared wavelengths. The opposite was shown to be true at microwave wavelengths and so measuringrequires a microwave scintillometer. The two-wavelength combination of microwave and large aperture scintillometers proved successful in calculating bothand, provided the effect of low frequency path-averaged humidity fluctuations was filtered from the scintillometer signals. The microwave scintillometer was shown to be sensitive to inertial-convective fluctuations and capable of calculating.
When mechanical turbulence is minimal and one is interested in unstable atmospheric conditions then the free convection formula developed for the two-wavelength scintillometer provided a reasonable estimate of. However measuring at height and under very unstable conditions means the scintillometer signal variance can be corrupted by the passage of the growing CBL. Because, the scintillometer signal variance can become very small and possibly undetectable from signal noise. Unpublished data from the Ahipara experiment showed the large aperture scintillometer operated at 10 m and can provide reliable estimates ofby also using a free convective scaling formula (De Bruin et al., 1995). This result is in-line with the observations of De Bruin et al. (1995) who also showed coarse measurements ofwere sufficient to ensure reasonable calculations ofand.
Until microwave scintillometers were used in these experiments measurements were confined close to the ground. Microwave technology required installing the scintillometer at some minimum height to avoid surface reflection of the propagated signal. With increased height and distance so grew the requirements to preserve MOST. The valley experiment at Brancott was the first time the effect of advection was observed on the scintillation measurements. The scintillometer footprint was sensitive to the effects of the dry-to-wet transition and the entrainment of the dry and warm air into the newly formed surface boundary layer. Under these conditions the scintillometers could not distinguish the source of the additional signal variance and the scintillometer method failed. It was also highly unlikely under these circumstances the T-Q correlation held at the scintillometer beam height.
In light of these summarised results we present here some recommendations.
under unstable conditions using free convective scaling.
Covalently bound organic monolayers on hydrogen-terminated silicon surfaces = covalent gebonden organische monolagen op waterstof-getermineerde siliciumoppervlakken
Sieval, A.B. - \ 2001
Wageningen University. Promotor(en): E.J.R. Sudhölter; J.T. Zuilhof. - S.l. : S.n. - ISBN 9789058083845 - 192
silicium - unimoleculaire films - interfase - halfgeleiders - oppervlakteverschijnselen - preparaten - silicon - unimolecular films - interphase - semiconductors - surface phenomena - preparations
Monolayers of 1-alkenes and 1-alkynes can be prepared on hydrogen-terminated Si(100) and Si(111) surfaces by a reaction between the organic compound and the Si surface. This reaction, which is schematically depicted below, results in the formation of densely packed, covalently bound, and well-ordered monolayers of the organic compound on the hydrogen-terminated Si surface, that inhibit the oxidation of the underlying Si surface.
In this research, several aspects of this surface modification have been investigated. The scope of the reaction has been explored by using a variety of functionalized and nonfunctionalized alkenes. The results show that many functional groups can be used, provided that: a) the functional group is properly protected, and b) the formation of well-ordered monolayers is not disturbed by too much steric hindrance between these functional groups, once they are in the monolayer. A very interesting property of the resulting functionalized monolayers is that the functional groups can be deprotected and/or further modified, without damaging the monolayer or the underlying Si substrate. This gives access to functionalized monolayers that so far could not be prepared by other methods.
The method for the preparation of the monolayers has been improved by showing that the reaction can also be done using solutions of the 1-alkenes and 1-alkynes in aromatic solvents. The best solvent was found to be mesitylene (1,3,5-trimethylbenzene). In this solvent monolayers are formed that are at least as good as those prepared using neat 1-alkenes/1-alkynes, even at concentrations as low as 0.1 M. This is an important improvement, as it considerably reduces the amount of 1-alkene/alkyne needed in the surface modification.
In the case of the hydrogen-terminated Si(100) surface, there are two hydrogen atoms on each Si surface atom. Upon reaction of this surface with a 1-alkyne, not just one, but two covalent Si-C bonds are formed per organic molecule, as had been demonstrated by a combination of IR spectroscopy, X-ray reflectivity measurements, and quantum chemical calculations. This type of reactivity has so far not been observed for 1-alkynes on other H-terminated (crystalline) Si surfaces.
To get more insight in the structure of the monolayers on a molecular level, they have been investigated by molecular modeling simulations. Large modified Si surfaces, with >30 alkyl chains attached to the Si surface, were investigated, using the approach of two-dimensionally repeating boxes. Calculations without this repeating box approach failed completely, as did calculations using small boxes (<30 alkyl chains). The results show that: a) there is a good correlation between the structure as observed in the simulations and the structure as deduced from a combination of experimental data, and b) that the currently obtained substitution percentage of the Si-H for Si-alkyl groups is close to the maximum substitution percentage that can be reached. This latter conclusion shows that the obtained monolayers are (almost) as densely packed as possible, as desired.
The possibility to use the monolayers for silicon surface passivation has been investigated, determining the effective lifetimes of the minority charge carriers in p-type Si wafers modified with 1-alkenes. The passivating properties of the monolayers are found to be comparable to those of HF and iodine/ethanol solutions, two methods commonly used in semiconductor technology, but the monolayer-modified surfaces are far more stable than these two systems. This shows that these monolayers provide an interesting alternative for Si surface passivation.
|Fundamentals of interfacial and colloid science Vol III: Liquid-fluid interfaces
Lyklema, J. - \ 2000
San Diego : Academic Press - ISBN 9780124605237 - 785
colloïden - colloïdale eigenschappen - grensvlak - oppervlakte-interacties - oppervlaktechemie - oppervlakteverschijnselen - colloids - colloidal properties - interface - surface interactions - surface chemistry - surface phenomena
This volume deals with various aspects of surface tensions and interfacial tensions. Together with the phenomenon of adsorption (enrichment of molecules at interfaces), these tensions constitute the basic characteristics of interfaces. The authors try to keep the treatment systematic and deductive. Recurrent features are that each chapter begins, as much as possible, with the general thermodynamic and/or statistical thermodynamic foundations and the various phenomena are presented in order of increasing complexity. The requirement that the work be both a reference and a textbook is reflected in its being comprehesive as far as the fundamentals are concerned and in its didactic style.
Thin liquid-crystalline polymer films : nucleation, crystallisation, instabilities and growth
Wielen, M.W.J. van der - \ 1999
Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789058081476 - 104
polymeren - afdeklagen - oppervlakteverschijnselen - vloeibare kristallen - polymers - coatings - surface phenomena - liquid crystals
Liquid-crystalline polymers, LCPs, possess an ordered liquid state between their crystalline solid state and their isotropic state. It is not only their ordering that makes this material interesting but actually their hybrid character, i.e., they behave both like liquid crystals and like polymers. In the bulk this class of material has been studied extensively and it is still under further study. However, studies of the surface phenomena and thin films of LCPs are very scarce.
What started as a study with a high practical impact, namely the use of side-chain liquid-crystalline polymers as a primer layer for coatings, evaluated into a thorough investigation on surface phenomena of thin films of ordered fluids.
The thin films were prepared by spin-coating solutions of LCP material, based on alternating copolymers of maleic acid anhydride and a-olefins carrying terminal mesogenic methoxybiphenyl groups, onto silicon wafers. This resulted in smooth isotropic thin (nanometer-scale) films. The main topics investigated on these films are wetting, ordering and dewetting. These phenomena could clearly be made visible by the following three main techniques. By atomic force microscopy (AFM) the surface topography and topology can be imaged and structures of only less than a few nanometers in depth can be made visible. This was done for areas in the range of several mm2 to 0.02 mm2. By optical reflection microscopy this area could be extended to larger areas up to 0.5 mm2, however, without information on the topography. More information on the inner structures in the film was obtained by X-ray reflectometry.
The combination of AFM and X-ray reflectometry turned out to be very powerful in describing the morphology of and structures in thin films. The initial isotropic spin-coated films changed upon annealing above the glass transition temperature, Tg. First randomly oriented micro-crystalline domains are formed which corrugate the polymer-air interface. The size of these corrugations depends on the initial film thickness: it increases with film thickness. At the same time, however, a laterally macroscopic crystal starts to grow from the substrate surface in the direction of the polymer-air interface, at the expense of these domain structures. Finally, a nicely ordered single crystal with parallel ordered bilayers is formed in the film as well as at the polymer-air interface. This ordering was visible in the AFM-images as terrace-like structures with a height corresponding to a bilayer, and in the X-ray spectra as a Bragg peak.
This one-dimensional crystallisation, actually recrystallisation, depends strongly on the temperature due to viscosity effects: the ordering was completed much faster at higher temperatures. An Arrhenius-type plot gives an activation energy of 122 kJ/mol, which we ascribe to the expected reorientations of the mesogenic groups during the recrystallisation process.
When the films are heated for a long period they break up. Holes, which are also visible by optical microscopy, appear which then grow in time. The growth rate of the holes depends on the temperature and film thickness. At the final stage of dewetting only droplets remain on top of a rather stable bilayer. The bilayer is even present above the isotropisation temperature.
This indicates autophobic behaviour which is even more pronounced in the mesophase where a layered film is present, and where dewetting may occur over several ordered layers.
In all cases the dewetting is not linear in time and polymer slippage seems to take place. In the case of polymer slippage, a t2/3 dependence is expected for the growth rate, which is indeed found above the isotropisation temperature. In the mesophase the dewetting differs from the "normal" slippage behaviour and a weaker time dependence is observed. We ascribe this deviation to the mesogen-mesogen interactions which have to be broken-up upon dewetting.
Around the isotropisation temperature there is a strong increase in the (initial) dewetting velocity of over more than 2 orders of magnitude. This acceleration is most probably due to the strongly decreasing viscosity because of the breaking up of the mesogen-mesogen interactions.
Also the effect of chemically and physically modified wafers on the ordering and stability against break-up is investigated. In the case of unmodified silicon wafers, we found that the polymer-substrate interaction induces parallel layering in the film, resulting in stable films. By changing the surface groups by surface modification with several silane compounds with different end groups, the interaction between the polymer and the surface can be tuned. In this way we obtained very hydrophobic surfaces for silanes with methyl or allyl end groups, and more intermediate surface properties for silanes with phenyl, chloride, carbomethoxy or cyano end groups. After surface modification film formation was only possible on the intermediate surfaces.
Above Tg these films show a characteristic behaviour which is strongly temperature dependent. At low temperatures in the mesophase holes nucleate which are encircled by unstable rims. Upon further annealing the rim instability decreases and the dewetting velocity increases. This feature also occurs for another completely different side-chain liquid-crystalline polymer with a methacrylate backbone and cyanobiphenyl groups in the side chains. We ascribe the peculiar dewetting behaviour to the presence of polycrystalline domains in the thin films. Especially their size and orientation and their ability to deform under shear are held responsible for the rim instabilities and, consequently, for the droplets remaining behind in the dry patches.
By adsorbing a monolayer of negatively charged nano-sized silica particles onto a pre-adsorbed positively charged polyelectrolyte a controllable way to roughen a silicon surface is introduced. By sintering at 1000 °C, the particles partly fuse with the substrate thus forming a rough, yet pure silica surface. By changing the radius of the adsorbing silica particles the degree of roughness can be tuned.
When the polymer film thickness exceeds the size of the substrate textures, upon annealing initially smooth films get rough by the formation of ordered domains, i.e., random polycrystalline domains. In time, these ordered domains grow in size and decrease in number. Apparently, the surface structures act as nucleation points for the formation of crystal domains.Big domains grow at the expense of smaller ones (Ostwald ripening).
In the isotropic phase the domains coalesce as an effect of the surface tension. The liquid film then remains stable, which is not the case on smooth substrates. Cooling down from the isotropic phase to the mesophase results in the formation of needle-like structures, so called batonnets, which is a typical texture for smectic A phases as found in bulk experiments.
Copolymer adsorption and the effect on colloidal stability
Bijsterbosch, H.D. - \ 1998
Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789054857907 - 139
adsorptie - membranen - colloïden - oppervlakteverschijnselen - adsorption - membranes - colloids - surface phenomena
The main aim of the work described in this thesis is to study the effect of different types of copolymers on the stability of aqueous oxide dispersions. Such dispersions are a major component in water-borne paints. In order to obtain a better insight in steric stabilisation we first investigated the relation between the adsorbed amount and layer thickness, and paid attention to the effect of the type of copolymer on the adsorbed amount. We also studied the adsorption kinetics as these are relevant for industrial purposes.
An introduction on steric stabilisation is given in Chapter 1. For block copolymers the solvent may be non-selective or selective. In a non-selective solvent both blocks are solvated and the polymer molecules are likely to be in a non-aggregated conformation. However, in a selective solvent the molecules form micelles in which the non-soluble blocks are clustered together, surrounded by a layer of solubilised chains. The adsorption kinetics are expected to be affected by the existence of such micelles. Another important feature for the adsorption of block copolymers is the selectivity of the surface. When only one of the blocks has affinity for the surface this will give rise to selective adsorption. On the other hand, the adsorption of a block copolymer in which both blocks have affinity for the surface is non-selective. The resultant polymer layer will differ for both cases. In thesis we studied selective and non-selective adsorption from a selective and a non-selective solvent. As the architecture of the copolymers is also relevant we paid attention to the adsorption of both block copolymers and graft copolymers.
In Chapter 2 we describe the properties of spread monolayers of polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers at the air-water interface. The surface pressure and the thickness of the layer were measured as a function of the adsorbed amount. The thickness was determined with neutron reflectivity measurements.
Upon compression of the polymer monolayer the surface pressure increases over the entire experimental range of compression. At low coverage the adsorbing PEO block forms a flat "pancake" structure at the surface. When the surface area per molecule is decreased the PEO is pushed out of the surface layer into the solution to form a "cigar" or "brush" structure, which is firmly anchored by the PS block. Some scaling analysis have suggested that this desorption occurs as a first-order surface phase transition. When the polymer layer is compressed further, so that the surface density σincreases, the chains stretch and the thickness H of the layer increases too. Theories predict that H scales as Nσ 1/3, where N is the number of monomers per polymer chain. This is confirmed by our results. However, our experimental data do not show the first-order surface phase transition between pancake and brush. Numerical self-consistent-field calculations also show a gradual transition rather than a first-order phase transition.
In Chapter 3 we present a study on the non-selective adsorption of two series of diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2oxazoline)- poly(ethylene oxide), from aqueous solution on a macroscopically flat silicium oxide surface. The adsorbed amounts in this study, and in that of Chapters 4 and 5, were measured with an optical reflectometer in an impinging jet flow cell. The hydrodynamic layer thickness was determined by dynamic light scattering.
The different blocks in the copolymers all have affinity for the silica surface. In all cases there is a small difference between the segmental adsorption energies of the two blocks, giving rise to non-selective adsorption of the block copolymers. For the two types of block copolymers used in this study, the adsorbed amount as a function of block copolymer composition shows a shallow maximum; at this maximum the longest block is also the more strongly adsorbing block. The same trend is found for the hydrodynamid layer thickness. These findings differ from theoretical predictions concerning selective adsorption, where a pronounced maximum is found for a short anchor block. With numerical self-consistent field calculations we demonstrate that the same trends as in our experimental findings can be predicted by theory. In non-selective adsorption of diblock copolymers, with a small difference between the adsorption energies of the blocks, both blocks compete for the same adsorption sites on the surface. When the blocks are incompatible they try to avoid each other, which promotes an anchor-buoy structure. These factors then give rise to a maximum in the adsorbed amount as a function of the block copolymer composition. At this maximum the longest block is also the more strongly adsorbing block. The adsorbed layer has the typical anchor-buoy structure which is necessary for an effective steric stabilisation, but this structure is less pronounced than for selective adsorption.
The kinetics of adsorption of diblock copolymers can be very slow if the polymers form micelles in solution. In Chapter 4 we compare the experimental adsorption rates on silica and titania with the theoretical flux of copolymer molecules towards the surface for four poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) diblock copolymers with the same block length ratio but different molar masses. In aqueous solution these block copolymers form large polydisperse micelles with a very low critica l micellisation concentration (lower than 2 mg 1-1).
On both surfaces the adsorption behaviour is governed by the anchoring of the hydrophobic siloxane blocks The adsorption kinetics are affected by the exchange rate of free polymer molecules between micelles and solution. For the three smallest molar masses the exchange rate is fast compared to the time a micelle needs to diffuse across the diffusive layer. Before the micelles arrive at the surface they have already broken up into free polymers. Because the cmc is very low, the experimental adsorption rate is determined by the diffusion of micelles towards the surface. For the longest polymer this is not the case: the exchange of polymer molecules between micelles and solution is now relatively slow. As the micelles do not adsorb directly, the adsorption rate is retarded by the slow exchange process. We were able to make an estimate of the micellar relaxation time, i.e., the time a micelle needs to break up. For the largest polymer the relaxation time is of the order of a few tens of seconds. The other polymers have a micellar relaxation time that is shorter than roughly one second.
The adsorption increases linearly as a function of time, up to very high adsorbed amounts where it reaches a plateau. Such high adsorbed amount is expected for strongly (and selectively) adsorbing diblock copolymers with a relatively short anchor block. The adsorbed amount on silica is considerably higher than on titania. The reason is probably that the hydrophobic block is more strongly anchored to a silica surface than to titania, so that the density of the adsorbed layer can become higher on silica.
In Chapter 5 we investigate the interfacial behaviour of graft or comb copolymers. We compare the adsorption of graft copolymers with an adsorbing backbone and nonadsorbing side chains to the reverse situation of adsorbing side chains and a nonadsorbing backbone. Two high- molar-mass poly(acryl amide)-graft-poly(ethylene oxide) copolymers with different side chain densities were used in this study.
On titania only the backbone of these polymers adsorbs and the side chains do not. The adsorbed amount is then about the same as that found for the homopolymer without side chains. On the other hand, on silica the side chains adsorb and the backbone does have no affinity for the surface. For both polymer samples we observe a maximum in the adsorbed amount as a function of time ("overshoot"), after which the adsorbed amount decreases and a plateau is reached. The plateau adsorbed amount on silica is much higher than on titania and also much higher than for both types of homopolymers. Upon adsorption the graft copolymers initially adopt a conformation in which only part of the side chains are adsorbed. Following the overshoot, the graft copolymers show a decrease in the total adsorbed amount. The overshoot depends on the polymer concentration, which suggests that it is not caused by conformational changes in the adsorbed layer but by an exchange process between surface and solution.
Differences in graft distribution and graft density in the polymer sample are probably responsible for the displacement of adsorbed chains by polymer molecules from solution. The average number of grafts per molecule is rather low in our polymer samples. On statistical grounds there is probably an appreciable polydispersity in graft distribution and in graft density. Molecules in which the grafts are clustered to some extend can displace molecules with more regularly separated grafts, and molecules with a high graft density can displace those with a lower number of side chains. The newly arriving molecules can then adsorb in a flatter conformation with a lower adsorbed amount as the extra loss in conformational entropy is compensated by the gain in adsorption energy.
The effect of the polymers used in Chapters 3 to 5 on the stability of an aqueous silicium oxide dispersion is described in Chapter 6. The time-dependent increase of the average hydrodynamic radius of silicium oxide aggregates in the presence of electrolyte was measured. The increase of this radius with time is a measure of the aggregation rate of the dispersion. The effect of polymers on the stability of a dispersion was studied by adding polymer to the dispersion and recording the effect in the aggregation rate
Comparison of the aggregation rate of this "protected" silica with that of uncovered silica particles gives then an indication of the steric stabilisation by the adsorbing polymers.
Four different series of diblock and graft copolymers were used in these stability measurements. For two series of non-selectively adsorbing diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2-oxazoline)poly(ethyiene oxide), we find a good correlation between the adsorbed amount and the stabilising effect. A higher adsorbed amount provides a better steric stabilisation. Nevertheless, for these polymers the adsorbed amounts are not high enough (up to about 1.2 mg M -2) to protect the dispersion completely against aggregation. A series of amphiphilic diblock copolymers of poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) with very high adsorbed amounts (between 3.5 and 8 mg M -2) give excellent steric stabilisation of the dispersion. Adsorbed layers of the two graft copolymers of poly(acryl amide)-poly(ethylene oxide), with a non-adsorbing backbone and adsorbing side chains, are also effective in preventing the silica from aggregating. Even though the adsorbed amount of these graft copolymers is only around 1.3 mg M -2, which is much lower than that of the amphiphilic polymers, aggregation is completely prevented.
The best steric stabilisation is found for those systems in which either the surface or the solvent is selective. In practical aqueous systems, however, it is difficult to synthesise diblock copolymers in which both blocks are soluble and where only one of the blocks has affinity for the surface. We have shown that copolymers with a different architecture, graft copolymers, also can provide good steric stabilisation and may be a good alternative to diblock copolymers. Very good steric stabilisers are amphiphilic diblock copolymers in a selective solvent. However, it is important that the hydrophobic blocks are flexible enough for fast adsorption kinetics and that they completely wet the surface. Which copolymer should be chosen for the steric stabilisation of a practical colloidal system depends largely on the nature of the particles and the solvent, and on the availability of suitable copolymers.
The influence of spreading particles on the stability of thin liquid films
Bisperink, C.G.J. - \ 1997
Agricultural University. Promotor(en): A. Prins; H.J. Bos. - S.l. : Bisperink - ISBN 9789054857174 - 214
oppervlakten - grensvlak - dispersie - gassen - schuim - oppervlakteverschijnselen - surfaces - interface - dispersion - gases - foams - surface phenomena
The influence of spreading particles on the stability of thin liquid films was investigated. Due to the spreading of a particle, i.e. an oil droplet, over a surface of a thin liquid film the latter becomes thinner and may rupture. The following steps in the whole process were distinguished: 1) transport of the particle to the film surface, 2) dewetting of the particle ensuring physical contact between the particle surface and the film surface, 3) spreading of the particle over the film surface and 4) movement of the film bulk liquid induced by the surface movement due to spreading material.
An attempt was made to develop a theory that describes the spreading process quantitatively. It describes the film thinning process as a result of the liquid drag due to the surface motion initiated by the spreading material by using the parameters film thickness, droplet radius, liquid bulk viscosity, liquid bulk density and the surface rheological properties of the oil droplet and the film liquid.
Model systems of foaming liquid and lipid material were used to study this spreading process. The latter was done on a relative macroscopic scale over bulk surfaces which is different compared to the dimensions and conditions which are valid for spreading particles on a foam film. It was assumed that the developed theory could be applied to both dimensions. The experimental results pointed in this direction. This was verified by the experimental results of introducing small spreading emulsion droplets on thin liquid films. A clear correlation between the above mentioned parameters and film rupture initiated by the spreading droplets was found.
Polymer adsorption theory : universal aspects and intricacies
Linden, C.C. van der - \ 1995
Agricultural University. Promotor(en): G.J. Fleer; F.A.M. Leermakers. - S.l. : Van der Linden - ISBN 9789054854203 - 106
oppervlakten - grensvlak - polymeren - oppervlakteverschijnselen - surfaces - interface - polymers - surface phenomena
The work presented in this thesis is based on the theory for polymer adsorption by Scheutjens and Fleer (SF). Roughly, the thesis can be divided into two parts: the first two chapters consider the original theory from a new viewpoint, attempting to find universal laws and to establish connections with analytical theories. The last three chapters are devoted to extensions of the theory to more intricate systems.
In chapter 1 polymer adsorption from dilute solution is studied. We try to find the universal behaviour in the volume fraction profile as predicted by De Gennes from scaling arguments. In this analysis, three regimes are distinguished: close to the surface a proximal regime , which is dominated by the numerous contacts between polymer and surface, next to that a central regime , where the volume fraction profile decays as a power law which is independent of solution concentration and polymer chain length, and finally a distal regime with an exponential decay towards the bulk volume fraction. With the SF theory these regimes can indeed be found provided the polymer chains are sufficiently long (more than, say, 5000 segments). However, the exponent in the power law regime does depend on solution concentration and polymer chain length. Extrapolation to infinite chain length yields the proper mean-field exponent. Although in general mean-field theories (like the SF theory) can yield incorrect exponents, they tend to predict the proper trends, so that it can be expected that a chain length dependence is actually present. In *-solvents, where the mean-field treatment is thought to be exact because the second virial coefficient vanishes, an additional regime is found in between the central and distal regime. Its origin is, as yet, unclear.
The volume fraction profile is also the main topic in chapter 2, which discusses polymers adsorbing from a semi-dilute solution in a good solvent. In a semi- dilute solution the correlation length is independent of chain length, and it is found that this correlation length and the adsorption energy are the only parameters determining the volume fraction profile. Thus, in contrast to the case of dilute solutions in chapter 1, the profile for adsorption from semi-dilute solutions is independent of the polymer chain length. The free energy equation derived by SF is shown to be equivalent to that obtained in analytical mean-field theories if it is assumed that all segments of a polymer chain are distributed within the system in a similar way. Such an assumption is called a ground-state approximation. This ground-state approximation can also be used to extract the adsorbed volume fraction profile (comprising only the polymer chains touching the surface) from the overall profile. This has been done by Johner et al . Their results compare well with SF calculations when the bulk concentration is high and the adsorption energy low, but the agreement is much less when this is not the case, possibly due to the larger influence of tails under these conditions. When a bidisperse polymer mixture adsorbs from a semi-dilute solution the overall profile is not affected, even though the individual components may show a very different profile.
In chapter 3 we leave the case of simple flexible homopolymers and consider the influence of partial rigidity within the chain. Rigid polymers possess less conformational entropy, and hence adsorb more easily than flexible polymers. Chain stiffness is modelled by excluding direct backfolding and defining an energy difference between a straight and a bent conformation of two consecutive bonds, where the straight conformation is more favourable. When all parts of the polymer are equally stiff, a persistence length can be defined, which increases with the energy difference. Using this persistence length, the radius of gyration of a stiff polymer in solution can be rescaled to a flexible one with a smaller number of segments. However, it turns out that this procedure does not work out well for adsorption from dilute solution: the scaling laws in the central regime as found in chapter 1 are altered. The critical adsorption energy decreases with increasing persistence length, in full agreement with an equation formulated by Birshtein, Zhulina and Skvortsov. The situation gets complicated when only part of the polymer is stiff. As the stiffer par's lose less entropy upon adsorption, they adsorb preferentially. This effect leads to copolymer adsorption behaviour, even when there is no difference in interaction energy between the stiff and the flexible moieties.
Entropic effects play a major role also in chapter 4, where the adsorption of comb polymers is considered. Comb polymers consist of a backbone and a (large) number of teeth, hence they have a large number of chain ends per molecule. These ends prefer to protrude into the solution to form dangling tails. As a result, combs tend to adsorb in a conformation where the backbone is preferentially on the surface and the teeth stick out. This leads to relatively thin adsorbed layers, and if the distance between the branch points of the comb is small compared to the tooth length a depletion zone develops adjacent to the adsorbed layer. For comb copolymers it is found that if the teeth adsorb preferentially over the backbone segments the critical adsorption energy is lower than in the case where the backbone adsorbs, even though both types of molecules have the same number of adsorbing segments. At the point of desorption only a few segments are on the surface, and a polymer in which only the tooth segments adsorb loses less entropy than a polymer adsorbing with its backbone.
Finally, in chapter 5 we consider chemical surface heterogeneity by incorporating in the chain statistics a probability that a surface site has a particular adsorption energy. The surface can be constructed such that, on average, no energetic interaction between the polymer and the surface is present. Nevertheless, adsorption can take place on such a surface, provided "adsorbing sites" (sites with a favourable adsorption energy) are grouped together. The distribution of adsorbing sites determines largely the adsorption behaviour. If the driving force for adsorption is high, more polymer adsorbs on a surface with an equal distribution of adsorbing sites, as more of the available surface can be used. On the other hand, at low adsorption energy, it is more favourable to have the adsorbing sites group together, so that little of the non-adsorbing sites are in contact with the polymer.
In conclusion, universal behaviour is found only in the case of flexible, linear homopolymers adsorbing from a semi-dilute solution in a good solvent. In all other cases studied (dilute solutions, chain rigidity, chain branching and surface heterogeneity) the structure is more intricate. Although the meanfield character of the Scheutjens-Fleer theory is definitely a serious approximation, it does enable the modelling of a large variety of equilibrium systems, even at high concentrations, providing an abundance of detailed information. It is worthwhile to continue to check its assumptions and predictions with other theories and obviously with experiment. The volume fraction profile determines the properties of the system and is also very sensitive to the approximations used in the model. Therefore, precise and unambiguous measurements of the density profile remain of the utmost importance.
Adsorption and wetting : experiments, thermodynamics and molecular aspects
Schlangen, L.J.M. - \ 1995
Agricultural University. Promotor(en): J. Lyklema; L.K. Koopal. - S.l. : Schlangen - ISBN 9789054854005 - 145
oppervlakten - grensvlak - alkanen - koolwaterstoffen - oppervlakteverschijnselen - surfaces - interface - alkanes - hydrocarbons - surface phenomena
Adsorption and wetting are related phenomena. In order to improve knowledge of both and their relations, experiments, thermodynamics and a theoretical interpretation have been connected, starring n-alkanes.
Starting from the Gibbs adsorption equation thermodynamic relations between vapour adsorption and wetting are derived. The surface pressure of a film, formed by vapour adsorption on a solid surface, is calculated by integrating the vapour adsorption isotherm. The surface pressure at the saturated vapour pressure determines, together with the interfacial tension of the liquid, the difference between the interfacial tension of a clean solid and a solid- liquid interface. Moreover, the surface pressure is related to the spreading tension and contact angle in a solid-liquid-vapour system. The thermodynamic equations derived are generally valid and the approach covers wetting on both flat and powdered solids. From the individual surface pressure values of two immiscible liquids, wetting and displacement in a solid-liquid-liquid system can be assessed. The procedure is illustrated for a silica-water-octane system. Silica is one of the most abundant minerals on earth and the oil/water wettability of silica can be considered a model for oil displacement in reservoir rocks.
By combining the Young equation with the Gibbs adsorption equation, the contact angle and the work of adhesion of an aqueous electrolyte solution on a charged solid is investigated as a function of the solid surface charge density or the electrolyte concentration. In the case of partial wetting, the solid-solution-vapour contact angle is a maximum at the point of zero charge of the solid. The contact angle decreases and the work of adhesion increases with increasing absolute value of the surface charge. The derived equations are used to study the wettability of silica under changing electrolyte conditions. The surface charge density of silica Aerosil OX-50 at a number of indifferent KCl concentrations, ranging from 0.01 M to 1M, is determined as a function of pH by potentiometric titrations. The silica surface charge increases with increasing ionic strength and increasing pH. At its point of zero charge ( p H 0 ≈ 3) silica is already completely wetted by water. Charging of the surface results in an even better water wettability although this can not be observed experimentally. At pH = 9 and 1 M KCl the silica surface charge equals -0.25 C/m 2. Compared to the uncharged silica, this surface charge decreases the silica-water interfacial tension by 22 mJ/m 2. Under usual conditions the electrolyte adsorption at the solid-vapour interface will be less than at the solid-water interface. With respect to an uncharged silica ( pH =3), the silica surface charge of -0.25 C/m 2decreases the silica-(water) vapour interfacial tension by maximally 22 mJ/m 2whereas it increases the work of adhesion by maximally 22 mJ/m 2. By combining the present approach with theoretical equations describing the adsorption of charge determining ions on solids with different kinds and amounts of chargable groups the wettabililty of such solids as a function of their charging behaviour can be described theoretically. This remains a task for the future.
The vapour adsorption of different n-alkanes, cyclohexane, toluene and water on bare and methylated pyrogenic silica (Aerosil OX-50) has been studied gravimetrically. Linear adsorption isotherms of the n-alkanes and of cyclohexane on both substrates are found until high relative vapour pressures. The same holds for toluene on methylated silica. The linearity of the isotherms indicates relatively weak lateral interactions between adsorbed molecules. On bare silica, the adsorption of the n-alkanes studied (C7-C9) is, expressed in moles/m 2, independent of the chain length. The adsorption strongly increases after the coverage corresponding to a monolayer of alkanes, oriented perpendicular to the surface, has been reached (at p/p 0 ≈0.8). Methylation of the silica decreases the adsorption of all adsorptives studied. Until just before saturation the octane adsorption on methylated silica is below that of a monolayer parallel to the surface. The shape of these adsorption isotherms indicates that on bare silica n-alkanes predominantly adsorb end-on, perpendicular to the surface, whereas on methylated silica, the adsorption is rather parallel. From the adsorption data surface pressure isotherms are constructed and the work of adhesion is obtained. The work of adhesion reveals that the Lifshits-van der Waals part of the silica surface tension is reduced from 44 mJ/m 2for bare (pyrogenic) silica to 30 mJ/m 2for methylated silica. The adsorption data are also converted to disjoining pressure isotherms. At low film thicknesses, these can be described by an exponential short-range interaction. The classical macroscopic models are not very suited for the description of such thin films for which the molecular organization and the discrete character of the adsorbed layer are extremely important. However, thin adsorbed layers can be described on the basis of microscopic models for adsorption. Also surface pressures of simple systems can be obtained from classical adsorption equations (e.g., Langmuir, Volmer, BET, Polyani). However, for chain molecules like n-alkanes these models are inadequate as they are unable to describe the structure of the molecules and their adsorbed layers. This problem can be overcome by using, for instance, a more recent self-consistent-field (SCF) theory, orginially developed by Scheutjens and Fleer and extended by Leermakers; and others.
The SCF theory is applied for the description of chain molecular fluids and their interfaces. Hereto a fluid is considered as a mixture of chains and monomeric vacancies. The latter account for the free volume in the system. Intermolecular interactions are described in terms of Flory- Huggins (FH) parameters. For the homologous series of linear alkanes, these parameters are generalized and assessed from a fit to vapour pressure data. In the SCF lattice-fluid theory, each alkane is described as a chain of segments with a volume of 0.027 nm 3each. The segment- segment interactions (for which the FH parameter is zero by definition) are reflected in a non zero FH interaction parameter for a chain segment-vacancy contact χAO .Under the conditions mentioned χ AO equals 580/ T ( T in K) for all n-alkanes. With these parameters n-alkane bulk properties such as the vapour pressure, density, critical point and heat of vaporization can be obtained together with structural and thermodynamic properties of the liquidvapour ( LV ) interface. The calculations reveal that chain ends are the major constituents on the vapour side of the (alkane) LV interface. For longer chains and lower temperatures the (relative) preference of the chain ends to protrude into the vapour phase is more pronounced. The calculated variation of the n-alkane LV interfacial tension (γ) with temperature and chain length agrees quantitatively with experimental data. If the theory is applied for temperatures below the (experimental) n-alkane freezing points, positive dγ/d T values occur and a maximum in the LV interfacial tension is found at T/T C ≈0.12, irrespective of the chain length of the molecule. In experimental studies close to the n-alkane freezing points similar observations have been made. However, a comparion of these experimental observations with our theoretical predictions should be performed with some reservation as the theory describes a frozen phase as an isotropic supercooled fluid.
The lattice fluid theory description of the n-alkane interfacial properties may be improved by considering chain-flexibility constraints, such as trans-gauche conformations and/or by distinghuishing (the parameters of the) CH 3 and CH 2 segments. It is rather straightforward to extend the present theory to more complex systems such as fluid mixtures, or fluids (vapour and liquid) at solid surfaces. The interfacial tensions of such interfaces can be inferred from the theory so that the work of adhesion and contact angles on these interfaces can be investigated as a function of temperature and chain length of the (wetting) liquid. Some of these aspects are elaborated in the last Chapter.
The adsorption, structure and thermodynamics of (aliphatic) chain molecular fluids at rigid surfaces and at solids with thermally grafted (aliphatic) chains is also investigated. Vapour adsorption isotherms, inclusive the meta- and unstable regions, of an octameric fluid on various substrates are calculated. The octameric molecules are modelled as B-A 6 -B chains where A represents a CH 2 segment and B a CH 3 segment. On a bare solid, the influence of adsorption energy differences between the A and B segments of the chain molecule is investigated together with the influence of the chain flexibility. For semi-flexible chains with high chain end-adsorption energies the shape of the calculated isotherm qualitatively agrees with the linear vapour adsorption isotherms measured for n-alkanes on bare silica. With the theory adsorption isotherms resembling the ones measured for n-alkanes on methylated silica can be obtained. This requires semi-flexible chains with interaction parameters that favour a rather parallel adsorption of the chain-molecules with respect to the surface. A reduction of the chain flexibility, for instance by applying the RIS scheme, increases the tendency of the adsorbed molecules to line up. In general, this increases the adsorbed amounts when the interaction parameters favour end-on adsorption whereas this reduces the adsorbed amounts when the interaction parameters are in favour of parallel adsorption. On a poorly wetted rigid solid, a decreasing contact angle was calculated for increasing chain length of the (aliphatic) wetting liquid An . The contact angles and the (Zisman) critical surface tension for wetting decrease with increasing temperature. When the temperature approaches the critical temperature of the wetting fluid, complete wetting occurs. Furthermore, it is established that the temperature dependence of the contact angle mainly results from the influence of the temperature on the liquid-vapour and solid-liquid interfacial tensions.
On solids with grafted chains, octamer ( A8 ) adsorption isotherms and contact angles are calculated for different grafting densities and grafted chain lengths. Grafting aliphatic chains on a very poorly wetted (bare) solid decreases the contact angle of the octameric liquid. On such a solid, the contact angle as a function of the grafting density passes through a minimum. The rise beyond the minimum has an entropic origin. Grafting of chains on a completely wetting (bare) solid eventually results in a finite contact angle; higher grafting densities give rise to higher contact angles. When longer chains are grafted, lower contact angles result for both substrates. The calculations provide insight into the wettability of a substrate by chain-molecular fluids on a molecular level. Partial wetting of chain molecules can be explained from an autophobic effect: due to the ordering (anisotropy) of the molecules present in the thinnest adsorbed layer that can form at saturation, molecules of the isotropic liquid are repelled. The liquid does not spread on the thin film and droplet formation results. The calculations reveal that the partial wetting of chain molecular liquids on grafted solids is largely due to the enrichment of the grafted layer by middle segments of the liquid molecules.
In this thesis the thermodynamics and molecular aspects of adsorption and wetting have been investigated and coupled by means of vapour adsorption isotherms and a lattice fluid theory. At present a reasonable (semi) quantitative agreement between theory and experiments has been achieved. For the (near) future, some other investigations based on the present theory are challenging. Firstly, a better quantitative agreement with experimental data is feasible by optimizing the description of the aliphatic molecules, for instance by incorporating differences between end and middle segments and their mutual interactions, or, in our case, their interactions with a vacancy. Secondly, the theory is able to describe random, heterogeneous surfaces and rough substrates so that contact angles of chain molecular liquids on such substrates can be inferred and compared to theories such as developed by Cassie and Wenzel. Moreover, the theory can be extended to a twodimensional SCF approximation instead of the one dimension we used in this work. This renders calculation of the (density) contour plot of a droplet feasible. By comparing the contact angle of this contour plot with the equilibrium contact angle, calculated in onedimensional SCF, the effect of the drop size, its curvature and line tension on the contact angle can be studied. Finally, a "dynamic" version of the SCF theory is currently being developed in the department of Physical and Colloid Chemistry. In the future, this theory will be suited to investigate the dynamics of evaporation of chain molecular liquids as well as the dynamics of their adsorption, spreading and contact angles on solid substrates.
Biocatalysis in non-conventional media : kinetic and thermodynamic aspects
Vermuë, M. - \ 1995
Agricultural University. Promotor(en): J. Tramper. - S.l. : S.n. - ISBN 9789054854623 - 177
biokatalyse - enzymen - moleculaire structuur - grenslaag - oppervlakteverschijnselen - biocatalysis - enzymes - molecular conformation - boundary layer - surface phenomena
During the past decade biocatalysis in non-conventional media has gained a lot of interest. Especially in the field of bio-organic synthesis, where poorly water-soluble substrates and products are involved, these media are very attractive.
Non-conventional media generally consist of an apolar solvent phase and an aqueous phase. In this thesis, mixtures of water with water-miscible organic solvents, or water- immiscible organic solvents or (near-)supercritical solvents are described. The conventional aqueous phase contains the cellular or enzymic biocatalyst. The aqueous phase can vary from a dilute aqueous solution, with a thermodynamic water activity a w close to 1, to a dried enzyme particle with only a monolayer of adsorbed water molecules (a w < 1).
In non-conventional media biocatalytic processes are governed by the presence of a phase boundary when two phases are involved. This phase boundary not only influences the rate of the bioconversion (kinetics), but also the yield of the reaction (thermodynamic equilibrium). In this thesis, several factors are described which affect the (kinetics), and thermodynamics of biocatalytic porcessen in non-conventional media.
Chapter 2 gives an overview of the recent developments in the field of medium engineering for biocatalysis in non-conventional media. In this chapter a few basic design rules for the rational design are formulated. These rules may serve as useful tools for optimization of biocatalytic processes in non-conventional media.
A typical example of a non-conventional reaction medium is the mixture of water and water-immiscible organic solvent. Especially for this type of reaction media the liquid-impelled loop reactor has been developed. This reactor has been used for the bioconversion of tetralin, a very toxic apolar compound. In Chapter 3 the general strategy for the selection of a suitable solvent for the bioconversion of such toxic apolar compounds in the liquid-impelled loop reactor is given, where the tetralin conversion is used as a typical example. The water-immiscible solvents should be non-toxic and nonbiodegradable. Additionally, they should reduce the toxicity of the apolar substrate and they must be practical for use in the liquid-impelled loop reactor. All the steps in the selection procedure proved to be essential. Among the 57 solvents tested, only FC-40 proofs to be suitable for bioconversion of tetralin in the liquid-impelled loop reactor. In addition, the cellular biocatalyst needs to be immobilized, to reduce emulsion formation inside the bioreactor.
For the bioconversion of tetralin in the liquid-impelled loop reactor oxygen is needed. Chapter 4 describes the mass transfer of tetralin and oxygen in the liquidimpelled loop reactor from the apolar solvent phase to the aqueous phase, where the bioconversion occurs. It is found that in case of mass-transfer limitation, tetralin is the rate-limiting substrate and not oxygen.
One of the selection criteria of a suitable solvent for bioconversion of apolar substates is its non-toxicity for the biocatalyst. The log Poctanol , which describes the hydrophobicity of the solvent, is a good measure for the toxicity of the solvent in a twoliquid phase system. The toxicity of a water-immiscible solvent for cellular biocatalyst is caused by two factors, i.e. the presence of a phase boundery (phase toxicity) and by the solvent molecules that are dissolved in the aqueous phase (molecular toxicity). Chapter 5 describes these effects separately. When the solvent concentration in the membrane of the cellular biocatalyst reaches a critical concentration, the solvent becomes toxic. The toxic concentration in the membrane is constant and independent of the solvent used. It is directly related via the partition coefficient over the membrane and water, to the solvent concentration in the aqueous phase. This is in turn directly related to the log Poctanol of the solvent. If the critical membrane concentration of a certain microorganism is known, the toxicity of any solvent can be predicted with the
Apart from the log Poctanol , also the Hildebrand solubility parameter δcan be used as a measure of the hydrophobicity of the solvent. In Chapter 6 this parameter has been used successfully as an indicator of the solubility of apolar compounds in near-supercritical carbon dioxide (SCCO 2 ). In addition, the effect of this parameter on the transesterification rate of Lypozyme in this non-aqueous reaction medium has been studied. The change in δof near-supercritical carbon dioxide hardly influences the reaction rate. The water content of the medium influences the kinetics much more.
Water not only affects the kinetics of a synthetic reaction, but it also affects the equilibrium yield of these reactions. When the thermodynamic water activity a w is decreased, water-dependent side-reactions such as in transesterification reactions are suppressed (Chapter 6). In esterification reactions, a shift in equilibrium towards synthesis is expected upon decreasing the a w .
Chapter 7 describes a new method to control the a w during esterification reactions. With this a w -control method the a w can be maintained at an optimal value, at which the biocatalyst still shows sufficient activity while a high thermodynamic product yield can be obtained.
This thesis actually covers two central themes in biocatalysis in non-conventional media: kinetics and thermodynamics. In Chapter 8 a general discussion highlights how thermodynamics can be used as a basic tool to reveal the processes that govern biocatalysis in non-conventional media.
Physics of breadmaking
Kokelaar, J.J. - \ 1994
Agricultural University. Promotor(en): A. Prins; T. van Vliet. - S.l. : Kokelaar - ISBN 9789054852223 - 129
broodbereiding - bakkwaliteit - deeg - vloeistofmechanica - reologie - visco-elasticiteit - schuim - schuimen - oppervlakteverschijnselen - breadmaking - baking quality - doughs - fluid mechanics - rheology - viscoelasticity - foams - foaming - surface phenomena
Bread dough is a foam and the stability of the gas bubbles towards disproportionation and coalescence during the breadmaking process determines for a large part the final appearance of the baked bread with respect to crumb structure and loaf volume. Gas bubble behaviour in bread dough is determined by both surface and bulk rheological properties of dough (components). These properties were studied and their relevance to breadmaking was established.
Surface dilational moduli of different dough components were determined. It appears that wheat lipids and added surfactants as SSL and DATEM can retard disproportionation to a large extent if these components are present in the right concentration and modification. Wheat proteins will hardly retard this foam instability mechanism.
Dynamic measurements and biaxial extension tests on wheat and rye flour as well as wheat gluten doughs were performed. Both flour and gluten doughs show strain hardening at 20 and at 55°C. Wheat cultivars with good breadmaking performance exhibit stronger strain hardening properties than poor baking ones and rye. Next to strain hardening, biaxial stress and extensibility are important parameters determining bread quality.
During mixing both the surface tension and the viscosity of the dough determine the (minimum) radii of the entrapped gas bubbles. Directly after mixing surface properties dominate primarily gas bubble behaviour by retarding disproportionation, especially if surfactants like SSL or DATEM are added. Already during first proof bulk properties, especially biaxial stress, extensibility and strain hardening, start to dominate gas bubble stability and this remains as such during almost the remainder of the breadmaking process. In the final stage of oven rise surface properties may contribute to the stability of some dough films that have become very thin.