Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    High-resolution mass spectrometry for the analysis of interfacial kinetics of organic surface reactions
    Sen, Rickdeb - \ 2017
    Wageningen University. Promotor(en): H. Zuilhof. - Wageningen : Wageningen University - ISBN 9789463436243 - 308
    surface chemistry - unimolecular films - chemical reactions - analytical methods - mass spectrometry - oppervlaktechemie - unimoleculaire films - chemische reacties - analytische methoden - massaspectrometrie

    In this thesis, XPS and DART–HRMS have been used in close conjugation to supplement each other, since the latter is a relatively new addition to surface chemist’s repertoire that – after development – needed a firm comparison to build up a reputation of its own. The strength of our approach has been underlined by the high correlation between these two independent analytical techniques. Central to our approach has been the formation of mixed monolayers in case of aluminum oxide substrates. As presented in Chapters 2, 3 and 4, we have succeeded in the rapid formation of range stable, covalently bound mixed monolayers. The subsequent development of a general and fast analytical technique to determine the interfacial reaction kinetics, including the activation parameters DH‡ and DS‡, provided unparalleled insights. We have developed a “MS–ionizable tag” technique, which has been applied for the analysis of surface–bound organic reactions, to the best of our knowledge, for the first time.

    The Strain–Promoted Alkyne–Azide Cycloaddition (SPAAC) reaction was chosen as a model reaction given the fact that its kinetics had been well–studied in solution. As shown in Chapter 2, the microenvironment around the reactive surface group was carefully controlled by the length of the inert alkyl chains surrounding it. We observed a few interesting trends which could be of great interest to future surface chemists. First, the SPAAC reaction – which is a click reaction in solution – does not retain this nature on the surface (It does not proceed to full conversion and converges sluggishly to around 37% yield after significant temporal passage). A partially accessible microenvironment, where the motion of reactive groups is slightly restricted, was found to provide a high rate with the highest surface yield. In contrast, a freely accessible reactive moiety afforded a lower surface yield albeit with the highest overall rate. Finally, a buried microenvironment led to the highest overall rate albeit with a lower surface yield. As a corollary, for the surface–bound SPAAC reaction we can compare the partially accessible microenvironment to a marathon runner who is able to run further but at a pace slower than a sprinter (free microenvironment). This provides the surface chemist with a handle for tuning the monolayer as per her/his reaction goals.

    Harnessing the valuable insights gained from the SPAAC reaction, our concept of ionizable MS tag coupled with DART–HRMS was further extended to a more novel and yet unstudied interfacial reaction in Chapter 3. The Strain–Promoted Oxidation–Controlled cycloalkyne–1,2–Quinone (SPOCQ) cycloaddition was applied for the first time on a surface and afforded a quantitative yield for a free microenvironment in under 4 h. It is to be noted here, that for the first time a 100% (quantitative) metal–free click reaction was observed at a surface. This proved that our approach of engineering the microenvironment around the reactive site provides a distinct edge needed to attain quantitative yields. Quinones are hard to synthesize/store/use in solution given their high propensity to polymerize. However, we demonstrated that on the surface, quinones can be easily generated and stored over–extended period of time by a facile periodate oxidation. Auto–polymerization of surface–bound quinones is precluded by their tether and enforced distal separation by surrounding inert alkyl chains (3:1 ratio). The wider application of this interesting mixture has been further rigorously demonstrated in later chapters too. The bioorthogonality of the SPOCQ reaction coupled with its higher speed and its quantitative yields on the surface are definitely its most salient features.

    After studying strain–promoted click reactions on the surface (culminating for SPOCQ in quantitative conversion within 4 h), the question arose if DART–HRMS could also be used to reproducibly and precisely determine a different class of cycloadditions, for which we selected the interfacial inverse electron demand Diels–Alder (IEDDA) reaction as this reaction was reported to be really fast –at least for click reactions– in solution. This was studied in Chapter 4 extensively and we surpassed our previous kinetic record (SPOCQ) by obtaining a quantitative yield in a mere 15 min. The other interesting observation of this study was that reversing the reaction counterparts on the surface produced a discernible reaction rate difference. We found that one of the reactants when tethered in a particular stereochemistry (exo– form) gave the highest surface coverage (100%) within the shortest amount of time. This was also the first time that the effect of diastereomerism on interfacial reaction rates was studied.

    In Chapter 5, covalent modification of native non–activated mica has been carried out utilizing catechol linkers. Previous studies for mica modification produced poorly defined polymeric structures on the surface or required extensive and tedious organic synthesis. We have addressed both these issues head–on in this thesis. Well–defined and characterized ultrathin layers were constructed on mica using a catechol–based molecule involving a two–step synthesis. Mica being atomically flat provides an ideal surface upon which to study various phenomena by AFM and other forms of microscopy. However, most research until now was restricted to simply drop–casting the pre–fabricated moieties followed by studying their final structures. Our method now allows for the step–wise formation and characterization of these very interesting structures. Along with it, we also performed several click attachment chemistries on these ultrathin layers which can be harnessed by surface chemists to put various functional and structurally complex moieties on the surface. This opens the pathway for the attachment of more complex architectures on the surface with higher functionality along with the ability to study their formation in a step–wise controlled fashion.

    Overall, this thesis wishes to understand organic surface chemistry and several of its intricate mysteries. It clearly outlines several modification techniques and unravels interfacial kinetics of several interesting “metal–free click reactions”. It strives to rationalize the activation parameters in conjunction with classical organic chemistry and gives details on how surrounding “inert” alkyl chains can play a profound role in reaction rates. Lastly, we have striven to and achieved rapid and quantitative reactions on the surface by virtue of optimization of this microenvironment. Personally I believe, we have treaded on a road seldom traveled and unraveled a new understanding about molecular interactions on the ever–interesting and an infinitely–complex surface.

    Bioinspired nanopatterned surfaces via colloidal templating; a pathway for tuning wetting and adhesion
    Akerboom, Sabine - \ 2016
    Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Marleen Kamperman. - Wageningen : Wageningen University - ISBN 9789462578470 - 198
    surface chemistry - surfaces - particles - water - nanotechnology - unimolecular films - adhesion - colloidal properties - oppervlaktechemie - oppervlakten - deeltjes - water - nanotechnologie - unimoleculaire films - adhesie - colloïdale eigenschappen

    We can learn from nature that, next to chemistry, surface structures can be used for tuning different functions of surfaces. In this thesis we present a novel fabrication method using colloidal templating on the air/water interface. Two distinct ways to obtain nanopatterned surfaces are described, namely (i) addition of PDMS on top of the colloidal monolayer and (ii) synthesis of polypyrrole around the particles of the monolayer. An increase in adhesion is found for the nanopatterned PDMS surfaces, and the contact angle of water on the nanopatterned polypyrrole surface is increased.

    Surface functionalization and analysis thereof by ambient mass spectrometry
    Manova, R.K. - \ 2014
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Teris van Beek. - Wageningen : Wageningen University - ISBN 9789462571570 - 214
    biosensoren - detectie - biomarkers - allergenen - oppervlaktechemie - analytische methoden - synthese - unimoleculaire films - biosensors - detection - biomarkers - allergens - surface chemistry - analytical methods - synthesis - unimolecular films

    A challenge in the global healthcare is the lack of suitable diagnostic tools for early disease detection. One possible solution is the use of biosensors in diagnostic tests. By definition, a biosensor is a bioanalytical device that detects the presence of a compound (analyte) in the sample. The detection relies on the specific interactions between the ligands that are attached onto the biosensor surface and the analytes in the sample.

    This PhD dissertation is focused on developing an optimal protocol for attachment of ligands onto the biosensing surface. A step-wise approach was established for the versatile and reproducible modification and functionalization of a silicon nitride-based biosensor. This approach included the application of bioorthogonal copper-free reactions as a useful tool for oriented attachment of biomolecules. Additionally, a novel surface sensitive analytical method was developed for the identification of covalently bound molecules in monolayers. The method, which is fast and easy to apply, uses DART ionization coupled to a high-resolution mass spectrometer. The nm-thin layers were analysed, and interpretation rules for the obtained mass spectra were formulated. The method was applied in the identification of commercially available nm-thin coatings and biochips.

    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.

    Colloids in ultra-low dielectric media : surface forces and self-assembly
    Banerjee, S. - \ 2013
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Mieke Kleijn. - S.l. : s.n. - ISBN 9789461736857 - 257
    schoonmaken - was - kooldioxide - oppervlaktechemie - oppervlakte-interacties - oplosmiddelen - cleaning - laundry - carbon dioxide - surface chemistry - surface interactions - solvents

    This thesis aims at gaining fundamental insight on colloidal interactions in two types of apolar media, namely, liquid CO2 and (as a model for liquid CO2) n-hexane. The other components playing major roles are surfactants and water. The background of the work was to address the challenges met in the use of liquid CO2 as a dry-cleaning solvent, in particular the insufficient removal of particulate soil.

    Since the dielectric constant of liquid CO2 is extremely low (1.6 at 60 bar and 10 °C), it has a low individual Hamaker constant. This in turn leads to a many orders of magnitude higher van der Waals force between interacting surfaces through liquid CO2 as opposed to traditionally employed solvents like perchloroethylene (PERC), which are toxic and environmentally unfriendly. Both the absence of charge on surfaces and the high Van der Waals force mean that a high solvodynamic force (high Reynolds number) is required to dislodge particles from fabrics.

    The situation becomes worse in the presence of water (without a suitable surfactant), which is a minor component in any dry-cleaning formulation. Our atomic force microscopy results indicate that water-mediated capillary bridges can lead to higher adhesion forces between the interacting surfaces. The roughness and softness of the surfaces were found to affect the kinetics, magnitude and range of the interaction force.

    Further, we have shown that using suitable surfactants these forces can be reduced. Following a systematic selection approach based on the hypothesis that a hydrocarbon surfactant for liquid CO2 should have a low molecular weight and a branched t-butyl tail in its alkyl part, Igepal CA520 was chosen. This surfactant has an ethylene oxide (EO) chain as a headgroup (CiEOj type surfactant). The surfactant solubility was tested first in the model solvent, followed by measuring its cloud point in the liquid CO2 system, which showed that the surfactant is soluble at ~ 50 bar and 5 - 10 °C (CO2 dry-cleaning conditions). Furthermore, we found that Igepal CA520 was surface active at the water - liquid CO2 interface. Igepal CA520 was further tested in a pilot scale dry-cleaning apparatus, where it showed marked improvement in detergency of particulate soil.

    The interfacial behaviour of the surfactant - water - liquid CO2 system was also studied using the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We showed that the interfacial tension of bare water - CO2 interface decreases with increasing pressure and becomes invariant of pressure beyond the saturated vapour pressure. The water contact angle on a hydrophilic surface in CO2 increases with increasing pressure. The first phenomenon was explained from the increasing Gibbs excess of CO2 at the water - vapour interface. The increase in contact angle was shown to result from the adsorption of CO2 on the -OH populated surfaces with increasing pressure. The model further predicted complete wetting of the water - vapour interface by a CO2 layer, in line with the fact that the system conditions were chosen not far from criticality.

    The model was further extended to describe and predict the interfacial and bulk properties of the liquid CO2/surfactant/water system. The experimental water - CO2 interfacial tension data and the SF-SCF modeling of the Igepal/water/liquid CO2 system indicated that Igepal adsorbs at the water - liquid CO2 interface. The model also predicted the formation of reverse micelles both at the three-phase (water/liquid CO2/gaseous CO2) coexistence (at P/Psat = 1 ) and for P/Psat > 1. With increasing pressure the critical reverse micellar concentration (CRMC) increases and the aggregation number at the CRMC decreases. A higher pressure leads to a stronger stopping mechanism for reverse micellization due to the better solvation (better solvency power of liquid CO2) of the surfactant tails by CO2.

    Apart from the bulk phase behaviour, the presence of the surfactant gives rise to interesting wetting phenomena at the water - vapour interface. Partial wetting by CO2 was noted, followed by a re-entrant wetting transition as the surfactant concentration in the bulk water phase was increased.

    The theoretical phase behaviour was validated by small angle X-ray scattering experiments (SAXS) on Igepal/water/liquid CO2 ternary systems. The SAXS results indicated conclusively the presence of self-assembled structures. The role of water in driving the self-assembly has been mapped and it was concluded that water acts as a mesogen (promoter of liquid crystals) in CiEOj type surfactant - liquid CO2 systems. In the absence of water, at a particular range of surfactant concentrations, the system contains isotropic reverse micellar mesophases (often termed as L2) and with the addition of water L2 undergoes a phase transition to a lamellar phase, Lα. The lamellar repeat distance increases with increasing water content. Following these findings, a partial phase diagram in liquid CO2 has been generated. Comparing the phase diagrams of Igepal CA520/water in liquid CO2 and in n-hexane it is clear that water plays similar roles in the two systems. Based on this we also conclude that n-hexane is a good model for liquid CO2.

    The model was used to arrive at design guidelines for surfactants for liquid CO2. It is interesting to note that the stability window for reverse micelles in liquid CO2 is rather narrow with respect to the two key Flory-Huggins interaction parameters, namely the χC3Dand χOW, characterizing the interaction between methyl groups in the surfactant tail and CO2, and between the head group oxygen and water, respectively. The first interaction parameter dominates the stopping mechanism for micellization, while the latter determines the driving force for this process. The fact that this window is narrow essentially points out the difficulties involved in designing amphiphiles for liquid CO2. The design criteria emerging from modeling are based on the numerical results that for smaller molecules (< C10), branching is important and for longer molecules (C10 and above), CH3 type interactions are more important over branching.

    Apart from CO2 dry-cleaning, the knowledge gained in this thesis can be beneficial to many other environmentally friendly industrial processes involving liquid CO2, such as enhanced oil recovery and extraction of polar compounds. The outcome of this thesis can also be extended to alleviate the problems associated with the geological storage of CO2 at high pressure under the ocean floor (deep saline aquifers).

    Oriented antibodies as versatile detection element in biosensors
    Trilling, A.K. - \ 2013
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Jules Beekwilder. - S.l. : s.n. - ISBN 9789461735683 - 168
    biosensoren - oppervlakten - oppervlaktechemie - oriëntatie - antilichamen - lama's - antistoftesten - mycobacterium tuberculosis - unimoleculaire films - immobilisatie - biosensors - surfaces - surface chemistry - orientation - antibodies - llamas - antibody testing - mycobacterium tuberculosis - unimolecular films - immobilization

    The aim of this thesis is to explore orientation of detection elements on biosensor

    surfaces. To this end, different strategies were combined such as surface chemistry and protein functionalization, with the aim to generate a platform for oriented immobilization of antibodies

    in biosensors. Chapter 2 investigates the formation of organic monolayers on

    oxide-free copper. Detailed studies were performed to characterize the monolayers

    and proof its quality. Apart from being the first oxide-free monolayers on copper

    reported thus far, further functionalization was successfully investigated.

    Chapter 3 gives an overview about approaches used to orient antibodies on surfaces.

    It also summarizes methods used to characterize the orientation of immobilized

    antibodies in a more direct manner.

    In chapter 4 a set of detection elements for tuberculosis bacteria is described. These

    are variable domains of llama heavy-chain antibodies, known as VHH proteins. A

    number of VHHs, selected by phage display, were expressed by Escherichia coli bacteria

    and characterized for binding towards Mycobacterium tuberculosis bacteria.

    Specificity of VHHs was investigated and the antigen was identified.

    In chapter 5 the impact of orientation on the analyte binding capacity was studied

    by SPR as model biosensor. Established techniques (NH2 coupling, biotyinylation)

    were used to immobilize VHHs, and a comparison between oriented and random

    immobilized VHHs was made. The effect of molecular weight, epitope number and

    affinity of the target analyte was investigated.

    In chapter 6, a novel coupling chemistry was used to immobilize VHHs, and in this case the same chemistry could be used for oriented and random immobilization. VHHs were engineered and functionalized with a non-natural amino acid to bear either one or five azide groups. Azide groups served as unique chemical handles on the VHHs and were used to click proteins onto a cyclooctyne-modified surface in an oriented and random approach. Spectacular effects on biosensor sensitivity were observed when VHHs were immobilized in an oriented manner.

    Finally, in chapter 7, the main results of this thesis are summarized and remaining

    problems as well as ideas for future research are discussed.

    Mechanic studies of monolayer formation on H-Si(111) surfaces
    Rijksen, B.M.G. - \ 2012
    Wageningen University. Promotor(en): Han Zuilhof. - S.l. : s.n. - ISBN 9789461732002 - 167
    oppervlaktechemie - unimoleculaire films - alkenen - alkynen - waterstof - silicium - surface chemistry - unimolecular films - alkenes - alkynes - hydrogen - silicon

    Covalently attached organic monolayers on silicon surfaces form thermally and chemically stable platforms for (bio)functionalization of the surface. Recent advances in monolayer formation – yielding increases in monolayer quality and the complete exclusion of oxygen at modified surfaces – have paved the way for the future development of biosensors, photovoltaics, and molecular electronic devices. Despite these successful innovations in monolayer formation (including patterning and functionalization) over the last two decades, the actual knowledge of the processes at the silicon surface still lags behind. Yet, a good understanding of the mechanisms of initiation and propagation will help in finding new tunable parameters for further speed-up, and new strategies for attaching interesting biomolecules to the silicon surface.

    In the search for even faster synthetic methods to build monolayers of even higher quality, this thesis presented a combined systematic experimental and theoretical study of the mechanisms underlying monolayer formation.

    A detailed overview of the current knowledge regarding the mechanisms that underlie monolayer formation onto hydrogen-terminated silicon (H-Si) is presented in Chapter 2. The focus of this chapter is mainly directed to H-Si(111) and H-Si(100) surfaces, where silyl radicals play a key role in the formation of Si-C bonds that link the monolayers to the surface. These radicals also readily react with oxygen leading to oxidation of the surface. Several initiation mechanisms that induce the formation of these radicals are discussed, along with supporting theoretical and experimental modeling studies.

    The radical cation initiation mechanism is studied in more detail in Chapter 3. This chapter describes how radical cations of low molecular-weight silicon model compounds, which were synthesized to represent the H-Si surface, were obtained by photo-induced electron transfer. The stability and the nature of the radical cations were investigated with lifetime and secondary electron transfer studies. This chapter shows that radical cation initiation at the silicon surface is feasible. However, given the differences in reactivity between oxygen-centered and carbon-centered nucleophiles, this reaction is likely to only play a significant role in the initiation steps of monolayer formation, and not in the propagation of the Si-C bond formation.

    Chapter 4, describes the experimental and theoretical study of the radical chain mechanism, and in particular the effect of stabilization of the b-carbon radical. The radical reactivity was studied by performing competition reactions of precursors (alkenes, alkynes, etc.) with the tris(trimethylsilyl)silyl radical, and via high-level theoretical calculations on a theoretical Si4-model to obtain the activation barrier and overall free energy changes.

    Based on the insights obtained in the research described in Chapters 3 and 4, Chapter 5 describes in a combined experimental and theoretical study a significant improvement of surface coverage and speed-up of monolayer formation on H-Si.

    Modification of inorganic surface with 1-alkenes and 1-alkynes
    Maat, J. ter - \ 2012
    Wageningen University. Promotor(en): Han Zuilhof. - S.l. : s.n. - ISBN 9789461732019 - 160
    oppervlaktechemie - alkenen - alkynen - surface chemistry - alkenes - alkynes

    Surface modification is important because it allows the tuning of surface properties, thereby enabling new applications of a material. It can change physical properties such as wettability and friction, but can also introduce chemical functionalities and binding specificity. Several techniques are available to modify the surface. Of these, organic monolayers have the advantages that they are easily tunable, fairly stable, and do not change the structural properties of the surface.

    In the last two decades studies on the coupling of unsaturated hydrocarbons to inorganic surfaces have emerged. These compounds (also referred to as 1-alkenes and 1-alkynes) form covalently coupled monolayers on a variety of substrates, which is shortly reviewed in Chapter 1. This class of surface modification also forms the basis of the studies described in the following chapters.

    In Chapter 2, a method for the direct patterning of 1-alkynes onto hydrogen-terminated silicon is presented. The method combines microcontact printing with visible light illumination through the stamp. Since the surface modification is clearly enhanced by the illumination, the method was named light-enhanced microcontact printing (LE-µCP). It results in the local formation of an alkenyl monolayer at the areas where the stamp is in contact with the surface. The method is compatible with functional inks and also allows the preparation of chemically heterogeneous surfaces by backfilling of the uncontacted areas with a second functional ink.

    In Chapter 3, a method is introduced to photochemically modify fused silica substrates with 1-alkenes. This yields highly hydrophobic surfaces with high thermal stability, whereas the adsorbed layer provides proper chemical passivation of the underlying surface. The alkenes initially bind to the surface hydroxyl groups in Markovnikov fashion, but at prolonged reaction times oligomerization takes place. Since the reaction is photochemically initiated, it enables the use of photolithography to constructively pattern the silica surfaces. Because of this, the newly developed method forms a valuable addition to the existing modification methods.

    The method developed in Chapter 3 is applied in Chapter 4 to locally furnish silica surfaces with a functional linker. This has allowed the selective attachment of single-stranded DNA onto the modified areas. In addition to plain surfaces, the surface reaction is also demonstrated on onto curved, enclosed surfaces, i.e. the inner surface of a microchannel. The surface-bound DNA has been selectively and reversibly hybridized with the complementary DNA. These experiments show that ~ 7 ´1011 fluorescently labeled DNA molecules can be hybridized per cm2. By furnishing target compounds with the complementary DNA strand, this hybridization approach allows the selective, localized binding of proteins, antibodies and other biomolecules to the surface.

    In Chapter 5, a new method for the organic modification of porous anodic alumina (PAA) is presented, which is based on the reaction of terminal alkynes with the alumina surface. The reaction results in the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. These monolayers are well-defined and consist of an oxidation product of the 1-alkyne, i.e. its a-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures. Modification with 1,15-hexadecadiyne results in a surface with available alkyne endgroups, which can be used for further surface chemistry.

    In Chapter 6, the biofunctionalization of PAA is explored. To this aim, lactosyl-terminated surfaces are prepared and the subsequent adsorption of peanut agglutinin (PNA) is studied. The PNA binds selectively and reversibly to these surfaces. Moreover, PNA adsorption is higher on surfaces that expose the b-lactoside than on those that display the a-anomer, which is attributed to surface-associated steric hindrance. The adsorption of the pathogens Neisseria gonorrhoeae and Candida albicans onto the lactosylated PAA surfaces is also investigated. Whereas quantification of N. gonorrhoeae adsorption is hindered by high background staining, C. albicans shows increased colonization onto lactosylated surfaces. Thus, this chapter shows that aluminum oxide surfaces can be modified to induce selective adsorption of proteins and microorganisms.

    The studies in this thesis show that there is much to be explored in the surface modification of inorganic surfaces. Future studies could focus on the mechanism of the coupling reaction, but also on the reactivity of 1-alkenes and 1-alkynes towards other relevant inorganic materials. In addition, the surface modification with living cells and biofilms is still largely unexplored and may be a research topic of prime interest for the coming years!

    Covalent functionalization of silicon nitride surfaces for anti-biofouling and bioselective capture
    Nguyen, A.T. - \ 2011
    Wageningen University. Promotor(en): Cees van Rijn; Han Zuilhof, co-promotor(en): Jos Paulusse. - [s.l.] : S.n. - ISBN 9789461730084 - 141
    microfiltratie - ongewenste aangroei van levende (micro)organismen - aangroeiwerende middelen - oppervlaktechemie - microfiltration - biofouling - antifouling agents - surface chemistry

    Microsieves – microengineered membranes – have been introduced in microfiltration technology as a new generation of inorganic membranes. The thin membranes are made of silicon nitride (SixN4), which gives the membranes outstanding features, such as chemical inertness and high mechanical strength. Microsieves have very well-defined pore size and pore shape, with an extremely homogeneous size distribution and high porosity. As a result, high-flux performance and excellent selectivity may be achieved. However, biofouling issues exert limitations on the application of microsieves in filtration and diagnostics. Surface functionalization was found to be a feasible way to minimize biofouling, but also to achieve biorecognition in microbiological applications. The aim of this thesis is to improve microsieve performance in biological applications by means of surface functionalization with organic coatings for protein repellence and selective capture of microorganisms.
    In this thesis, SixN4 surfaces were functionalized with organic monolayers via stable Si C and N-C linkages. Coatings to render SixN4 surfaces protein repellent were studied in depth by two approaches: grafting of ethylene oxide monolayers onto the surface (Chapter 2); and grafting of zwitterionic polymers from the surface (Chapter 3). UV induced surface modification with oligo(ethylene oxide) chains with three (EO3) and six (EO6) units and the detailed characterization of these modified surfaces are described in Chapter 2. Successful attachment of EO3 and EO6 on SixN4 surfaces was achieved. The modified surfaces exhibit excellent protein repellence in bovine serum albumin (BSA) solution (~ 94%), but only moderate (~ 80%) protein repulsion was observed in fibrinogen (FIB) solution. This observation motivated the study towards grafting zwitterionic polymer brushes from SixN4 surfaces for improved protein repellence. A new method to grow zwitterionic polymers from monolayers containing tertiary bromides, via atom transfer radical polymerization (ATRP) was developed. The zwitterionic polymer coated surfaces showed excellent protein repellence in FIB solution (> 99%), while exhibiting very stable performance in PBS during one week, i.e., unchanged thickness, no hydrolysis of the polymers occurred and protein repellence in FIB solution remained constant.
    The use of microsieves as detection platform for microorganisms was explored in Chapter 4. Microorganisms can be caught by microsieves whose pore sizes are smaller than the microorganisms while allowing an easy flow-through of other components. However, detection capacity of microsieves is severely hampered by fouling issues. To avoid this problem, the use of microsieves with pore sizes larger than the microorganisms, in combination with immobilized antibodies was investigated in Chapter 4. Anti Salmonella antibodies were immobilized onto epoxide monolayers on microsieve surfaces by reaction with the primary amines present in the antibody. The antibody-coated microsieves showed excellent detection of Salmonella with high sensitivity and selectivity, significantly improving detection efficiency in crude biological samples, and reducing analysis times.
    The capture efficiency of Salmonella in milk samples was, however, found to be lower than that achieved in buffered solution. Most likely, this is due to nonspecific adsorption of milk proteins on the antibody-coated microsieves. In addition, the use of a blocking solution before incubation with microorganism solution remained an essential step in order to avoid the occurrence of interfering background fluorescence. In order to minimize these problems, the incorporation of antibodies on top of protein-repellent zwitterionic polymers coated on SixN4 surfaces was studied in Chapter 5. Anti-Salmonella antibodies were immobilized on zwitterionic polymer brushes coated SixN4 surfaces through the bromide moieties retained at the end of the polymer chain after ATRP. Antibody-functionalized zwitterionic polymers adsorbed only minimal amounts of FIB, indicating excellent protein repellence of the modified surfaces. Moreover, anti-Salmonella antibodies immobilized onto zwitterionic surfaces exhibit highly selective capture and improved sensitivity, as compared to antibodies on epoxide coated surfaces. This achievement offers a new approach that enables highly sensitive and selective detection of microorganism, while minimizing nonspecific adsorption of proteins that are not of interest.
    In Chapter 6, an overview is given of the most important findings presented in the thesis. Recommendations, as well as additional ideas on how to bring this research into industrial application are discussed.

    Reduction of protein adsorption on surfaces coated with Complex Coacercate Core Micelles
    Brzozowska, A.M. - \ 2010
    Wageningen University. Promotor(en): Martien Cohen Stuart; Willem Norde, co-promotor(en): Arie de Keizer. - [S.l. : S.n. - ISBN 9789085856832 - 246
    micellen - afdeklagen - eiwitten - adsorptie - oppervlaktechemie - micelles - coatings - proteins - adsorption - surface chemistry
    The structure and formation of Ionomer Complexes (ICs) consisting of linear polyelectrolytes
    (C3Ms) has been extensively studied in the past years. Recently, these structures were also
    considered for several applications. This thesis deals with the possible application of ICs as a
    surface coating suppressing protein adsorption independent of the properties of the native
    surface. To reach this goal it was crucial to understand the principles governing the adsorption
    and the stability of the IC layers on solid surfaces, as well as their interactions with proteins.
    Therefore, this research was limited to well defined model substrates: silica (model hydrophilic
    surface), polystyrene (model hydrophobic surface), and polysulfone (a surface mimicking
    polymeric membrane material), and model proteins: β-lactoglobulin, bovine serum albumin,
    fibrinogen, and lysozyme. The ultimate goal, however, was to apply the coating on surfaces of
    membranes used in water purification to suppress biofilm growth. We have observed that
    reduction of protein adsorption by coating formed by regular C3Ms is not satisfactory due to
    relatively low density of the polymer brushes formed on the coated surface. We increased the
    grafting density, and hence significantly improved the reduction of protein adsorption, by
    introducing grafted block and grafted copolymers into the micelles. In our work we discuss an
    influence of various factors, i.e. physical-chemical properties of the native surfaces, lengths of the
    charged blocks, distribution of the grafts along the backbone of the copolymer, salt concentration
    etc., on the performance of the formed coatings. In the final part we focus on the mechanical
    stability of the coatings formed with ICs, and discuss their applicability as a membrane surface
    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
    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)
    Colloids and interfaces in life sciences
    Norde, W. - \ 2003
    New York; Basel : Marcel Dekker - ISBN 9780824709969 - 433
    colloïden - colloïdale eigenschappen - grensvlak - oppervlaktespanning - emulsies - schuim - reologische eigenschappen - studieboeken - oppervlaktechemie - colloids - colloidal properties - interface - surface tension - emulsions - foams - rheological properties - textbooks - surface chemistry
    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.
    Phosphate availability in the soil-root system : integration of oxide surface chemistry, transport and uptake
    Geelhoed, J.S. - \ 1998
    Agricultural University. Promotor(en): G.R. Findenegg; W.H. van Riemsdijk. - S.l. : Geelhoed - ISBN 9789054858621 - 177
    fosformeststoffen - rizosfeer - voedingsstoffenbeschikbaarheid - oppervlaktechemie - phosphorus fertilizers - rhizosphere - nutrient availability - surface chemistry

    A study is presented on the adsorption of phosphate on goethite, the interaction of phosphate with other adsorbing ions at the goethite surface, and the resulting availability of phosphate to plants. The plant-availability of sorbed phosphate was determined from phosphorus uptake of plants growing on an artificial substrate containing goethite with phosphate. Uptake can be predicted from zero sink behaviour of a growing root system, diffusion and mass flow in soil, and measured non linear adsorption of phosphate on goethite. With high phosphate loading of goethite, the equilibrium phosphate concentration in solution increased, which resulted in larger phosphate availability. Competitive interaction between phosphate and sulphate on goethite caused only a small decrease in phosphate adsorption at low pH, where sulphate adsorption is strongest, but a considerable increase in the phosphate concentration in solution.

    Experiments showed that phosphorus uptake by plants growing on suspensions of goethite in the presence of sulphate was seven times larger at pH 3.7 than at pH 5.5. Citrate competes more strongly with phosphate than sulphate, and shows strongest interaction at pH 4.5-5. On account of the non-linear adsorption behaviour of phosphate, the relative increase in phosphate in solution upon competition is much larger at low than at high phosphate loading of goethite. Therefore, competition results in an apparent lower affinity of phosphate sorption on goethite.

    Adsorption of the individual anions and competitive adsorption was described with the CD-MUSIC ion adsorption model, which is based on a detailed description of the adsorbing surface and the use of surface complexes identified in spectroscopic studies. The combination of the ion adsorption model with the uptake model is a powerful tool to predict the phosphate availability to plants. This was illustrated with a simulation study in which the effect of citrate exudation from roots on the uptake of phosphate was predicted.

    Adsorption and desorption of cellulose derivatives
    Hoogendam, C.W. - \ 1998
    Agricultural University. Promotor(en): B.H. Bijsterbosch; M.A. Cohen Stuart; A. de Keizer. - S.l. : Hoogendam - ISBN 9789054858812 - 149
    cellulose - carboxymethylcellulose - oppervlakte-interacties - oppervlaktechemie - cellulose - carboxymethylcellulose - surface interactions - surface chemistry

    Cellulose derivatives, in particular carboxymethyl cellulose (CMC) are used in many (industrial) applications. The aim of this work is to obtain insight into the adsorption mechanism of cellulose derivatives on solid-liquid interfaces.

    In chapter 1 of this thesis we discuss some applications of cellulose derivatives. Application of CMC in pelleting of iron ore and in papermaking and the role of adsorption are given in more detail. Further we present a short introduction in the adsorption of polyelectrolytes.

    A set of 20 CMC samples was used in this study. Samples with four different degrees of substitution (ds=0.75, 0.91, 0.99, and 1.25) were prepared by AKZO Nobel by reaction of cellulose with NaOH and sodium monochloro acetate (ClCH 2 COONa). Samples were subjected to a random cleavage reaction with hydrogen peroxide yielding samples with molar masses ranging from M w =30 to 10 3kg mol -1. Characterisation of the CMC samples by size exclusion chromatography in combination with multi-angle laser light scattering (SEC-MALLS) and potentiometric titrations has been described in chapter 2 . Size exclusion chromatography separates molecules according to their size.

    The radius of gyration (R g ) and the molar mass (M w ) of each eluted fraction are then obtained on-line by multi-angle laser light scattering (via a Zimm-plot), yielding information about the molecular mass distribution of each sample. SEC-MALLS characterisation has been carried out at pH=7 in 0.02 and 0.1 mol l -1NaNO 3 . It turns out that the distribution depends on the salt concentration and the method of extrapolation of the scattered intensity to zero scattering angle in the Zimm-plot (i.e. using a linear or a non-linear extrapolation). Such a non-linearity is often attributed to the presence of aggregates. Because a CMC solution is supposed to contain more aggregates at high electrolyte concentration it is expected that the molar mass distribution will be shifted to higher molar mass when obtained in 0.1 mol l -1NaNO 3 in comparison with 0.02 mol l -1NaNO 3 . However, the experiments show the opposite trend, indicating that the lack of coincidence of distributions obtained at both salt concentrations is probably not caused by the presence of aggregates.

    Because SEC-MALLS gives both the molar mass and the radius of gyration of each eluted fraction, it is a highly suitable experimental technique to obtain the relation between M and R g . We applied the electrostatic wormlike chain model as well as Odijk's theory concerning the dimension of a polyelectrolyte to analyse this relation (both M and R g were obtained from the non-linear extrapolation method). A meaningful parameter in these models is the persistence length (L p ) of a polymer, L p characterising the length scale on which a polymer may be considered as rigid. The persistence length of a polyelectrolyte has two additive contributions. The first is the intrinsic or bare persistence length (L p0 ) which characterises the stiffness of the polymer backbone, the second accounts for the stretching of the chain due to electrostatic repulsion (electrostatic persistence length L pe ). Using the electrostatic wormlike chain theory, L p0 is assessed at 16 nm, indicating that CMC can be considered as a semiflexible polymer. A somewhat lower value (12 nm) has been obtained from the theory of Odijk. The value of L p0 does not depend on ds. The difference in L p0 between both models arises from the fact that in the Odijk model the contribution of L pe to L p is higher as compared to the electrostatic wormlike chain model. Furthermore the Odijk model assumes the chain as infinitely long. The electrostatic wormlike chain theory gives a more complete description of a polyelectrolyte chain as it takes molecular properties (such as the length and the cross-section of the molecule) and the details of the electrostatics into account.

    Potentiometric titrations were used to characterise the dissociation behaviour of CMC as a function of the NaCl concentration and pH. From the titration data the cross-section (radius) of CMC was obtained. Considering CMC as a uniformly charged cylinder radii of 0.95 nm (ds=0.75) up to 1.15 nm (ds=1.25) were obtained. Applying Katchalsky's theory for the dissociation of a polyelectrolyte, L p0 could be also determined from the titration data. In comparison to the analysis of the SEC-MALLS data Katchalsky's model gives a lower value (L p0 =5.9 nm). The difference is probably related to an incorrect evaluation of the electrostatic energy in Katchalsky's model.

    In chapter 3 the kinetics of polyelectrolyte adsorption has been investigated theoretically. Analogous to Kramers' rate theory for chemical reactions a model is presented which is based on the assumption that a polyelectrolyte encounters a barrier in its motion towards an adsorbing surface. The barrier is composed of the resistance due to transport in solution and to the presence of an electrical field. As soon as one segment touches the surface the chain is assumed to be adsorbed, i.e. the resistance that a chain encounters in the process of spreading out is neglected.

    We consider the motion of a strong polyelectrolyte with only one segment positioned at the front of the moving chain, all other segments are lagging behind the front segment. At each distance the chain explores all possible configurations, i.e. one needs to calculate the partition function of a chain with one segment at z=z* and all other segments at z > z* (Q(z*)). Such a partition function is readily evaluated from the numerical procedure proposed by Scheutjens and Fleer. Using this self-consistent-field (SCF) lattice model the resistance of an entering polyelectrolyte chain is calculated as a function of the distance from the surface. It turns out that the profile of the potential energy felt by the moving chain shows a strong resemblance with the interaction curve of colloidal particles, i.e. we observed a resemblance between the attachment process and the classical DLVO theory.

    Summing the contributions over the entire trajectory yields the barrier for adsorption R b . The barrier is calculated as a function of the adsorbed amount, and the results are inserted in the equation for the rate of the adsorption process. Finally, integration at a fixed concentration of polyelectrolyte leads to the time dependent adsorption. Endpoints in the calculated time dependent adsorption refer to equilibrium at that particular polyelectrolyte concentration.

    Parameters that affect the height of the barrier are the net charge at the interface (i.e. the surface charge plus the charge of the adsorbed polyelectrolyte), the charge density of the chain and the electrolyte concentration. Consider the adsorption of a polyelectrolyte on an oppositely charged surface. As long as the surface charge is not compensated there is no electrostatic barrier for adsorption, i.e. the rate of adsorption is determined by the rate of transport in solution to the surface. The height of the barrier strongly decreases with the electrolyte concentration. Consequently, the time needed to reach adsorption equilibrium also strongly depends on the electrolyte concentration. For low electrolyte concentration (0.01 mol l -1) an extremely long time is needed (15s), at a moderate concentration (0.2 mol l -1) it takes about 10 s. Hence, compared to the time scale of an experiment (around 10 5 s) adsorption equilibrium will not be accomplished for low electrolyte concentrations.

    The adsorption of carboxymethyl cellulose on rutile (TiO 2 ) and hematite (α-Fe 2 O 3 ) is discussed in chapter 4 . Data were obtained by batch adsorption experiments (depletion method) and by reflectometry, the latter yielding information about the kinetics of the adsorption. Systematically, we examined the influence of pH (pH=3 to 11), electrolyte concentration (c NaCl = 0.01 to 1 mol l -1) , molar mass (M w =35 to 1200 kg mol -1) and degree of substitution (ds=0.75 to 1.25). Adsorption isotherms are of the high affinity type and have well-defined plateau values. Plateau values in the adsorption decrease with increasing pH and increase with salt concentration. The adsorbed amount depends neither on ds nor on M w , the latter indicating a (rather) flat conformation of adsorbed CMC. The non-dependence on ds is possibly related to the fact that counter ions in the proximity of the polyelectrolyte chain lower the effective charge in such a way that CMCs varying in ds can have an identical effective charge density.

    On both surfaces a strong hysteresis in the adsorption with respect to pH is observed: in the high pH range a substantially higher adsorbed amount can be obtained by initially adsorbing at low pH and subsequently increasing the pH than by measuring the adsorption directly at any specified pH value. Desorption of CMC only takes place after the pH is increased substantially, which indicates a (very) strong interaction between CMC and the surface. Strong binding is likely related to the formation of ion pairs between the carboxylic groups of CMC and positively charged surface groups. Furthermore the desorption becomes even more difficult due to the chain rigidity of the CMC backbone, i.e. several bonds to the surface need to be broken simultaneously.

    Both the dissociation of the OH groups of the mineral surfaces and of the carboxylic groups of CMC depends on pH and electrolyte concentration. Furthermore the adsorption of a weak polyelectrolyte on such variable charged surfaces induces additional charges on the surface as well as on the polyelectrolyte. These characteristics cause the adsorption of a weak polyelectrolyte on a mineral surface to be very complicated. The model as presented in chapter 3 is used to elucidate this kind of adsorption. We calculated the adsorption and the charge of the surface at 10 5 s (a time which is comparable to the duration of an experiment). Because the short-range interaction between CMC and the surface is strong, the charge of adsorbed CMC can exceed the surface charge. The amount of overcompensation (or excess adsorbed chargeσ exc ) depends on the possibility that molecules reach the surface, i.e. on the height of the barrier for adsorption. As this barrier is a function of the net charge at the interface,σat a fixed electrolyte concentration does not depend on the pH. Increasing the electrolyte concentration lowers the barrier which allows higherσ exc . The calculations in chapter 4 show that at pH values where a weak polyelectrolyte is fully dissociated (i.e. acts as a strong polyelectrolyte) the adsorbed amount decreases linear with pH. Our experiments are in qualitative agreement with these calculations. The shape of the calculated time dependent adsorption curves also shows qualitative agreement with reflectometry experiments.

    In chapter 5 we discuss the adsorption of hydroxyethyl cellulose and quaternary ammonium substituted HEC (QNHEC) on silica and titanium dioxide. The adsorption has been investigated as a function of pH (pH=2 to 12) and electrolyte concentration (c NaCl =0.01 and 0.5 mol l -1) by means of reflectometry.

    The adsorption of HEC on SiO 2 shows a strong resemblance with the adsorption of polyethylene oxide. The adsorption is constant up to pH=5 in both 0.01 and 0.5 mol l -1NaCl, albeit in the latter case the adsorption is higher. At pH > 5 the adsorption decreases, which is most pronounced at the high salt concentration, reaching the level of zero adsorption at pH≈9. On TiO 2 the adsorption decreases monotonously with pH in 0.01 mol l -1NaCl. At high salt concentration it is constant up to pH=10, beyond which it decreases rapidly. The adsorption of HEC on SiO 2 is facilitated by hydrogen bonding between HEC ether groups and Si-OH surface groups, whilst the mechanism on TiO 2 is probably an interaction between non-substituted glucose hydroxyl groups and Ti-OH surface groups. The latter involves a chemical reaction, which may account for the fact that the time dependent adsorption of HEC on TiO 2 lacks a region where the adsorption increases linearly in time (i.e. mass transport in the solution is not the rate determining step even when the adsorption is low).

    Just as for CMC, in the adsorption of QNHEC there is an electrostatic barrier for adsorption. We compared the time dependent adsorption of QNHEC with calculations obtained from the model presented in chapter 3. It appears that in the case of QNHEC equilibrium is very likely not reached in 0.01 mol l -1NaCl, whereas in 0.5 mol l -1NaCl equilibrium is reached. As the charge density of QNHEC is lower (0.4 charged groups per glucose unit) than for CMC, the electrostatic barrier is also lower. In 0.01 mol l -1NaCl both on SiO 2 and TiO 2 the adsorption increases linearly with pH up to pH=10. This linearity is interpreted in analogous to the CMC adsorption. The adsorption reaches a maximum at pH≈12, then it decreased rapidly. According to the classification of van de Steeg the adsorption of QNHEC on SiO 2 in 0.5 mol l -1NaCl is of the screening-enhanced type up to pH ≈10, whereas at higher pH it is of the screening-reduced type. On TiO 2 the adsorbed amount is low and does not depend on pH.

    In chapter 6 the diffusion of spherical silica particles (with radii ranging from 12 to 510 nm) in dilute CMC solutions (M w =180 to 1200 kg mol -1, c CMC =5 to 1000 mg l -1) was investigated by means of dynamic light scattering. From the diffusion coefficient the viscosity as experienced by these inert probes (the "microscopic" or effective viscosity) is obtained. The smallest particles experience a viscosity which is slightly higher than the solvent viscosity, which may be interpreted in terms of the motion of these particles hardly being affected by the presence of polymer. The effect of polymer on the motion of the probes increases with the size of the probes. However, the value of the viscosity as obtained from capillary viscosimetry (bulk viscosity) is still not reached for the largest sphere, albeit for CMC M w =180 kg mol -1the effective viscosity comes rather close to the bulk viscosity.

    The thickness of the CMC/HEC layer adsorbed on Fe 2 O 3 /SiO 2 is also investigated in chapter 6. The layer thickness as obtained using the bulk viscosity shows a maximum as a function of the polymer concentration. The origin of the maximum is a consequence of an incorrect choice of the viscosity. Using the viscosity as obtained from the inert probe diffusion the layer thickness increases monotonously with polymer concentration.

    The diffusion behaviour of the inert probes is discussed in terms of a model in which the particles are surrounded by a layer of polymer free solution. This layer is assumed to be equal to the thickness of the depletion layer. According to this model the thickness of the depletion layer decreases with the CMC concentration, at low concentration approaching the radius of gyration of CMC.

    Semi-flexible polymers near interfaces : equilibrium aspects and adsorption kinetics
    Eijk, M. van - \ 1998
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789054858546 - 99
    oppervlakte-interacties - polymeren - xanthan - adsorptie - oppervlaktechemie - surface interactions - polymers - xanthan - adsorption - surface chemistry

    The first chapter is about semi-flexible polymers at a liquid-liquid interface: self-consistent-field calculations. The adsorption of semi-flexible polymers at a liquid-liquid interface largely differs from that at a solid surface. The width of the interface is an additional length scale in the problem, making the system behaviour particularly rich. We consider two phase-separating monomeric liquids, C and D, and a polymer A N which dissolves equally well in both liquids. We study this system in a self-consistent-field model in the dilute regime. The stiffness of the polymer is controlled by the use of a rotational isomeric state approach. We show that the interfacial widthxi(determined by the interaction parameter between the two solvents), the persistence length q, and the chain length N are relevant parameters in the adsorption behaviour.

    A key observation is that, while keeping N 1/2/xiconstant, the adsorbed amount goes through a minimum with increasing q/xi. An initial increase of q/xi(q/xikleiner of gelijk aan1) effectively leads to a larger coil size, leading to a decrease of the adsorbed amount. However, when q/xiveel groter dan1, alignment of parts of the polymer within the interfacial region occurs due to the lack of entropic penalties. This alignment process induces an increase of the adsorbed amount. These observations also have implications for the ongoing discussion about the preferential adsorption in a mixture of flexible and stiff polymers. In this discussion one should consider the effects of the finite size of the interfacial region.

    The second chapter is about wetting by polymers of a liquid-liquid interface: effects of short-range interactions and of chain stiffness. The behaviour of both flexible and semi-flexible polymers near a liquid-liquid interface is investigated with the aid of the self-consistent-field theory as developed by Scheutjens and Fleer. Aternary system (A/B N C) is studied near the wetting transition. In a symmetric system, i.e.χ AB = χ BC = χ, a change in the interaction parameterχintroduces a wetting transition. The ratio of the interfacial widthxiof the binary A/C system and the coil size of the polymer determines the order of this transition. Beyond a certain chain length N C (at fixedxi) the wetting transition is of first order, whereas it is of second order for NC . The characteristics of the prewetting line, including the prewetting critical point, are discussed in some detail. The non-trivial N-dependence of the position of this critical point is analysed in terms of a crude thermodynamic model. For a semi-flexible polymer an increase of the chain stiffness at a certain value ofχis sufficient to introduce a wetting transition.

    Chapter 3 is about adsorption kinetics of the polysaccharide xanthan on ZrO 2 . The adsorption kinetics of the polysaccharide xanthan from aqueous solution on zirconium oxide were studied as a function of pH and ionic strength. The adsorption was monitored by reflectometry in astagnation-point flow setup. At intermediate pH and ionic strength, xanthan is present in a helical form and it can be viewed as a semi-flexible polymer under these conditions. By lowering the salt concentration or increasing the pH a helix-coil transition takes place. This transition is caused by the mutual electrostatic repulsion of the short side chains of xanthan. The so-formed coil can be considered as a Gaussian chain, with a large radius of gyration. The conformation of the polysaccharide is roughly reflected in its adsorption behaviour.

    It is, however, deduced that the electrostatic interaction between polymer and surface influences the stability ofthe helix. The adsorption process can be divided in two regimes. At low surface coverage the rate of adsorption is transport-limited, which in a stagnation-point flow leads to a linear time dependence of the adsorbed amount. The adsorption rate in this regime hardly changes with ionic strength or pH. The time range over which it holds, however, does, which can be understood in terms of electrostatic effects. At higher surface coverage two types of behaviour are observed. At low ionic strength and on a highly charged surface the adsorbed amount saturates abruptly. This kind of kinetics resemble those of flexible polymers. In this case the xanthan presumably adsorbs in a coil-like conformation, because the helix becomes unstable in the vicinity of the surface. At higher ionic strength and on a weakly charged surface, the adsorbed amount increases gradually over very long times. Under these conditions, the helix conformation is more stable so that we ascribe this slow process to tentatively rearrangement and alignment processes of the stiff chains on the surface.

    Chapter 4 is about the competition between transport and spreading in protein adsorption kinetics. The saturation adsorbed amount of polymers on solid surfaces is mostlyfound to be independent of the polymer transport rate, or flux J, to the surface. In most cases this is because the experimental rate of transport strongly deviates from the relaxation rate in the polymer layer. We studied the adsorption of both immunoglobulin G and savinase on SiO 2 from aqueous solution and found that the transport rate is an important parameter in the adsorption kinetics. The adsorption process can be viewed as an attachment to, followed by the spreading over the surface of a polymer molecule. In this way the adsorbed amount strongly depends on J if the time for transport to the surface is in the same range as the spreading time. Using an analytical "growing disk" model for the polymer adsorption, we are able to, at least qualitatively, describe the adsorption kinetics.

    Chapter 5 is about adsorption and spreading of polymers at plane interfaces; theory and molecular dynamics simulations. Nonequilibrium processes play a key role in the adsorption kinetics of macromolecules. It is expected that the competition between transport of polymer towards an interface and its subsequent spreading has a significant influence on the adsorbed amount. An increase of the transport rate can lead to an increase of the adsorbed amount, especially when the polymer has too little time to spread at the interface. In this study we present both molecular dynamics simulations and analytical calculations to describe some aspects of the adsorption kinetics. From MD simulations on a poly(ethylene oxide) chain in vacuum near a graphite surface, we conclude that the spreading process can, in first approximation, be described by either a simple exponential function or by first-order reaction kinetics. Combining these spreading models with the transport equations for two different geometries (stagnation-point flow and overflowing cylinder)we are able to derive analytical equations for the adsorption kinetics of polymers at solid-liquid and at liquid-fluid interfaces.

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