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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    Foam properties of proteins, low molecular weight surfactants and their complexes
    Lech, F.J. - \ 2016
    Wageningen University. Promotor(en): Harry Gruppen; Peter Wierenga; Marcel Meinders. - Wageningen : Wageningen University - ISBN 9789462576247 - 122
    surfactants - proteins - bovine serum albumin - beta-lactoglobulin - lysozyme - foams - chemical properties - stability - mixtures - food chemistry - oppervlaktespanningsverlagende stoffen - eiwitten - runderserumalbumine - bèta-lactoglobuline - lysozym - schuim - chemische eigenschappen - stabiliteit - mengsels - voedselchemie

    This thesis shows the effects that the addition of low molecular weight surfactants (LWMS) to proteins has on the foam stability of the mixture. For this, the bulk, interfacial, thin liquid films and foam properties are determined for different protein-LWMS mixtures at different molar ratios (MR). It was shown that the MR as well as the charge of the protein and LMWS determine the foam stability of the mixtures. For all mixtures it was found that the proteins have a select number of high affinity binding sites. So, the concentration of free LMWS in the solution is 0 until a critical MR (MRcr), at which all high affinity binding sites are saturated. Above this MRcr, part of the LMWS binds to low affinity binding sites of the proteins. The low affinity binding sites have a binding ratio < 1, which determines the concentration of bound and free LMWS. For similarly charged protein-LMWS mixtures (i.e. b-lactoglobulin (BLG) and sodium dodecyl sulphate (SDS) and bovine serum albumin (BSA) and SDS at pH 7) the foam stability typically decreases from the foam stability of the pure protein solution (MR 0) until MRcr is reached. At MR > MRcr the foam stability is dominated by the amount of free LMWS. For oppositely charged protein-LMWS mixtures, the binding of the LMWS to the proteins can be described in a similar way, although the number of high affinity sites and low affinity binding ratio are different. There is also a regime of MRs in which the protein-LMWS complexes form large aggregates. These aggregates were in some cases found to increase foam stability (lysozyme (LYS) and SDS and BLG-SDS at pH 3), while in another case (BLG and cetyltrimethylammonium bromide (CTAB)) they lead to decreased foam stability. Still, in all cases it was found that above MRD the aggregates dissociate and the foam stability becomes dominated by free surfactants, equivalent to what was observed for similarly charged protein-LMWS mixtures.

    A multi-scale model was developed to describe the stability of thin liquid films in terms of rupture time and thickness. Initially, the model was used to predict the stability of surfactant free films of water and electrolyte solutions. Later, it was used to predict the foam stability in LYS-SDS mixtures. For that purpose, the model was combined with a foam drainage model to provide theoretical estimations of foam stability. This model is the basis to understand coalescence of bubbles in foam. Finally, the concept of the critical MRs and the free LMWS was introduced. Using this, the foam properties of protein-LMWS mixtures can partly be predicted by relative charge of the components and the binding to both high and low affinity binding sites.

    Microbubble stability and applications in food
    Rovers, T.A.M. - \ 2015
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Marcel Meinders; Guido Sala. - Wageningen : Wageningen University - ISBN 9789462574755 - 138
    microbubbles - eiwit - stabiliteit - karakterisering - voedsel - voedseladditieven - oppervlaktespanningsverlagende stoffen - zuurbehandeling - reologische eigenschappen - sensorische evaluatie - tribologie - druk - verwarming - koelen - microbubbles - protein - stability - characterization - food - food additives - surfactants - acid treatment - rheological properties - sensory evaluation - tribology - pressure - heating - cooling

    Aeration of food is considered to be a good method to create a texture and mouthfeel of food products that is liked by the consumer. However, traditional foams are not stable for a prolonged time. Microbubbles are air bubbles covered with a shell that slows down disproportionation significantly and arrests coalescence. Protein stabilized microbubbles are seen as a promising new food ingredient for encapsulation, to replace fat, to create new textures, and to improve sensorial properties of foods. In order to explore the possible functionalities of microbubbles in food systems, a good understanding is required regarding the formation of protein stabilized microbubbles as well as their stability in environments and at conditions encountered in food products. The aim of this research was to investigate the key parameters for applications of microbubbles in food systems. In Chapter 1 an introduction to this topic is given.

    In Chapter 2, the effect of the microbubble preparation parameters on the microbubble characteristics, like the microbubble yield, size and stability, was investigated. The protein Bovine Serum Albumin (BSA) and the method sonication was used to manufacture the microbubbles. The manufactured number and stability of microbubbles was highest when they were prepared at a pH around 5 to 6, just above the isoelectric point, and at an ionic strength of 1.0 M. This can be related to the protein coverage at the air/water interface of air bubbles formed during sonication. At a pH close to the isoelectric point the BSA molecules is in its native configuration. Also the repulsion between the proteins is minimized at these pH values and ionic strength. Both the native configuration and the limited repulsion between the proteins result in an optimal protein coverage during the first part of sonication. Also a high protein concentration contributes to a higher surface coverage. The surface coverage is proportional to the protein concentration up to a concentration of 7.5% after which an increase in protein concentration did not lead to a substantial increase in the number of microbubble . In the second part of sonication the protein layer around the air bubble becomes thicker and stronger by heat induced protein-protein interactions. We found that and at a preheating temperature of 55-60°C, about 5 °C below the BSA denaturation temperature, and a final solution temperature of 60-65°C most microbubbles were obtained, while at higher temperatures mainly protein aggregates and (almost) no microbubbles are formed. This suggests that at temperature of around 60°C to 65°C protein aggregated mostly at the air-water interface creating a multi-layered shell, while at higher temperature, they also aggregated in bulk. These aggregates cannot form microbubbles. We found that optimal preparation parameters strongly depend on the protein batch. We hypothesize that the differences in microbubble formation between the protein batches is due to (small) differences in the protein molecular and denaturation properties that determine the temperature at which the molecules start to interact at the air-water interface. Microbubbles made with different protein concentration and preheating temperatures shrunk in time to a radius between 300 nm and 350 nm, after which the size remained constant during further storage. We argue that the driving force for the shrinkage was the Laplace pressure, resulting in an air flux from the bubbles to the solution. We argue that the constant final size can be explained by a thickening of the microbubble shell as a result of the microbubble shrinkage, thereby withstanding the Laplace pressure.

    In Chapter 3 and Chapter 4, microbubble stability at environments and conditions representative for food products were studies. In Chapter 3 we investigated the stability upon addition of surfactants and acid, When surfactants or acid were added, the microbubbles disappeared in three subsequent steps. The release of air from the microbubble can be well described with the two-parameter Weibull process. This suggests two processes are responsible for the release of air: 1) a shell-weakening process and 2) a random fracture of the weakened shell. After the air has been released from the microbubble the third process is identified in the microbubble disintegration: 3) the shell disintegrated completely into nanometer-sized particles. The probability of fracture was exponentially proportional to the concentration of acid and surfactant, meaning that a lower average breaking time and a higher decay rate were observed at higher surfactant or acid concentrations. For different surfactants, different decay rates were found. The disintegration of the shell into monomeric proteins upon addition of acid or surfactants shows that the interactions in the shell are non-covalent and most probably hydrophobic. After surfactant addition, there was a significant time gap between complete microbubble decay (release of air) and complete shell disintegration, while after acid addition the time at which the complete disintegration of the shell was observed coincided with the time of complete microbubble decay.

    In Chapter 4 the stability of the microbubbles upon pressure treatment, upon fast cooling after heating and at different storage temperatures was studied. The microbubble stability significantly decreased when microbubbles were pressurized above 1 bar overpressure for 15 seconds or heated above 50°C for 2 minutes. Above those pressures the microbubbles became unstable by buckling. Buckling occurred above a critical pressure. This critical pressure is determined by the shell elastic modulus, the thickness of the shell, and the size of the microbubble. Addition of crosslinkers like glutaraldehyde and tannic acid increased the shell elastic modulus. It was shown that microbubbles were stable against all tested temperatures (up to 120°C) and overpressures (4.7 bar) after they were reinforced by crosslinkers. From the average breaking time at different storage temperatures, we deduced that the activation energy to rupture molecular bonds in the microbubbles shell is 27 kT.

    In Chapter 5, we investigated the effect of microbubbles on the rheological, tribological sensorial properties of model food systems and we compared this effect to the effect on food systems with emulsion droplets and without an added colloid. We investigated the effect in three model food systems, namely fluids with and without added thickener and a mixed gelatine-agar gel. In a sensory test panellists were asked whether they could discriminate between samples containing microbubbles, emulsion droplets or no added colloid. Emulsions could be sensorially well distinguished from the other two samples, while the microbubble dispersion could not be discriminated from the protein solution. Thus, we concluded that at a volume fraction of 5% of these BSA covered microbubbles were not comparable to oil-in-water emulsions. The good discrimination of emulsion might be ascribed to the fact that emulsion had a lower friction force (measured at shear rates form 10 mm/s to 80 mm/s) than that microbubbles dispersions and protein solutions. Upon mixing emulsions and microbubble dispersions the friction value approximated that of emulsions. This effect was already noticed at only 1.25% (v/v) oil, indicating that microbubbles had not a significant contributions to the friction of these samples. Also microbubble dispersions with and without protein aggregates were compared. The microbubble dispersions with and without thickener containing protein aggregates had a higher viscosity than the those samples without protein aggregates. Protein aggregates in the gelled microbubble sample yielded a higher Young’s modulus and fracture stress. The differences between the gelled samples could be well perceived by the panellists. We attribute this mainly to the fracture properties of the gel. In general we concluded that microbubbles, given their size of ~ 1 mm and volume fraction of 5%, did not contribute to a specific mouthfeel.

    Finally in Chapter 6, the results presented in the previous chapters are discussed and put in perspective of the general knowledge on microbubbles production, stability, and applications in food. We described the main mechanisms leading to microbubble formation and stability. We showed that the production parameters significantly influence the interactions in the microbubble shell, and the those interactions highly determine the stability of the microbubbles under several conditions. We reported about limitations of sonication as a method to produce microbubbles suitable for food applications and we provided some ways to overcome these limitations. The use of microbubbles in food systems has been explored and we clearly see possible applications for microbubbles in food. We reported about directions for possible further research.

    In this work we made significant progress in understanding the interactions in the microbubble shell and their relation to microbubble stability. We also advanced in comprehension towards possible applications of microbubbles in food.

    Anaerobic degradation of anionic surfactants by denitrifying bacteria
    Paulo, A. - \ 2014
    Wageningen University. Promotor(en): Fons Stams, co-promotor(en): P.A. García-Encina; Caroline Plugge. - Wageningen : Wageningen University - ISBN 9789462571242 - 166
    oppervlaktespanningsverlagende stoffen - degradatie - anaërobe microbiologie - anaërobe behandeling - denitrificerende bacteriën - afvalwaterbehandeling - rioolwaterzuivering - pseudomonas syringae pv. pisi - surfactants - degradation - anaerobic microbiology - anaerobic treatment - denitrifying bacteria - waste water treatment - sewage treatment - pseudomonas syringae pv. pisi
    De verwijdering van organische stof alsook van stikstof en fosfor wordt in RWZI vaak bewerkstelligd middels een anaëroob-anoxisch-aëroob (A2/O) proces. Met behulp van het A2/O proces kunnen oppervlakte-actieve stoffen al in het anaërobe dan wel anoxische compartiment afgebroken worden. In dit proefschrift wordt de isolatie van Pseudomonas stutzeri stam SN1 en Pseudomonas nitroreducens stam SN2 uit actief slib van een RWZI met een A2/O proces beschreven.
    Molecular assembly, interfacial rheology and foaming properties of oligofructose fatty acid esters
    Kempen, S.E.H.J. van - \ 2013
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis; Henk Schols. - S.l. : s.n. - ISBN 9789461737328 - 238
    vetzure esters - oppervlaktespanningsverlagende stoffen - estervorming - oppervlaktereologie - schuimen - fatty acid esters - surfactants - esterification - surface rheology - foaming

    Aerated food products consist of air bubbles that are surrounded by a matrix that can be either liquid or solid. Due to the large number of air bubbles that are generally present in aerated products, these systems contain a large interfacial area. Therefore, the properties of the interfaces are considered to contribute significantly to the macroscopic properties of the system. The properties of these interfaces are largely determined by the type of surfactant that adsorbs. Two major types of surfactants that are used within the food industry are proteins and low molecular weight (LMW) surfactants. Proteins are macromolecules consisting of hydrophilic and hydrophobic patches that adsorb at the interface, where they lower the surface tension and can unfold to create a two-dimensional network that can provide a high modulus. In contrast, LMW surfactants are molecules with a well-defined hydrophilic and hydrophobic part. They can form more compact surface layers than proteins, leading to lower surface tensions. They generally do not provide the interface with a high modulus, instead they stabilize the interface through the Gibbs-Marangoni mechanism that relies on rapid diffusion of surfactants after deformations of the interface. A molecule that can lower the surface tension considerably, like a LMW surfactant, and at the same time provide a high modulus, like a protein, has the potential to be an excellent foam stabilizer. In this thesis we focus on a series of molecules that obey these criteria: oligofructose fatty acid esters. We address the influence of changes in chemical fine structure (fatty acid chain length and degree of saturation, degree of esterification and size of the hydrophilic group) on the functional properties.

    These esters are synthesized by esterification of fatty acids to oligofructose, which is a mixture of oligomers with different degrees of polymerization. As we show in chapter 2, reasonable yields are obtained when using lipase as the catalyst in a mixture of DMSO and ButOH. The conversion into mono-esters increased with increasing fatty acid chain length and is consistent with the preference of the enzyme for more hydrophobic substrates. The crude reaction product consisted of a mixture of unreacted oligofructose and fatty acids, the main reaction products mono-esters and small amounts of di-esters. The crude product was fractionated using RP-SPE. MALDI-TOF MS and (2D) NMR were used to confirm the structure and purity of the esters; >90% for mono-esters and >80% for di-esters.

    Similar to typical LMW surfactants, the oligofructose esters formed spherical micelles in the bulk after a certain critical concentration. As we show in chapter 3, the CAC depended on the hydrophobicity of the molecules. The efficiency also increased with increasing hydrophobicity and the effectiveness was similar. The area occupied by a single molecule at the interface was determined by fitting the CAC curves with the Gibbs adsorption model and measured directly using ellipsometry. The area occupied at the interface was larger for oligofructose mono-esters compared to sucrose esters. Furthermore, oligofructose di-esters occupied slightly more area than sucrose esters. All esters occupied significantly more area than a single fatty acid chain. This shows that the oligofructose group dominates the area occupied at the interface.

    The rheological properties, as studied in chapter 4, were determined using a traditional approach, where the dependency of the surface dilatational modulus on surface pressure and frequency was determined, and using a novel approach, where we show how the surface dilatational modulus is dependent on deformation amplitude and temperature. Furthermore, we show how Lissajous plots of surface pressure versus deformation may be used to gain information about the correlation between surface rheological properties and interfacial microstructure. Sucrose esters behaved like typical LMW surfactants, with low surface dilatational moduli, scaling exponents in the frequency dependency close to 0.5, and fairly viscous Lissajous plots without significant asymmetries. In contrast, oligofructose mono-esters formed interfaces with high surface dilatational moduli, low scaling exponents in the frequency dependency and asymmetric Lissajous plot with strain hardening during compression and strain softening during expansion. We conclude that the oligofructose mono-esters form a two-dimensional soft glass. The oligofructose di-esters behaved like typical LMW surfactants at high surface pressures, showing that the presence of the second fatty acid chain prevent the formation of the glass by the oligofructose part.

    In chapter 5 we focus on the difference in functionality between the crude reaction product, the individual components that are present in the crude product and mixes of these products. Unreacted fatty acids migrated to the interface only in very small amount, due to the low solubility in the bulk. The addition of mono-esters slightly improved the amount of fatty acid that could migrate to the interface. Oligofructose was not surface active and its addition to the mono-ester only diluted the mono-ester which did not lead to significant changes in functional properties because the concentration of mono-ester was still close to the CMC. When mono-esters and di-esters were mixed, the rheological results showed that the ratio between mono-ester and di-ester was very important for the rheological profile. In both cases the results suggest the presence of islands of glass phase formed by the mono-esters surrounded by a viscous phase formed by the di-esters. When the surface concentration of mono-esters was high, the glassy patches dominated the interface, leading to a high modulus, low frequency dependency and Lissajous plots with a high degree of asymmetry. When the surface concentration of mono-esters decreased, the lower connectivity between the glassy patches lead to a low modulus, intermediate frequency dependency, and Lissajous plots with moderate asymmetry.

    To study the potential of oligofructose esters as food grade surfactants it is important to consider that many food products contain ingredients with the potential to be surface active. Therefore, in chapter 6 we have studied the functional properties of an oligofructose mono-ester in the presence of whey protein isolate, a commonly used food protein. Except for at the highest protein concentration, the surface was dominated by the oligofructose ester. The stabilization mechanisms of oligofructose ester and WPI were mutually exclusive, leading to interfaces with a low surface dilatational modulus. Since the foaming properties were not negatively affected, we conclude that the Gibbs-Marangoni mechanism occurred. Only at the highest protein concentration, the surface concentration of WPI was sufficiently high to interfere with this mechanism, leading to a significant decrease in foam stability. Oligofructose esters were also able to displace a fully developed WPI network.

    In chapter 7 we discuss the foaming properties of the esters. We show that only esters of intermediate hydrophobicity are able to form foams with small bubbles and a uniform bubble size distribution that lead to high foam stability. The affinity of esters with shorter fatty acid chains, up to 8 carbon atoms, for the interface was quite low as a result of the relatively hydrophilic nature of the molecules. Therefore, they were not effective foam stabilizers. The most hydrophobic components (mono-ester with a chain length of 18 carbon atoms and di-ester with a chain length of 12 carbon atoms) were too slow to migrate to the interface. Therefore, also these components were poor foam stabilizers. We show that the surface tension at short time scales is the most accurate predictor of foam stability. However, despite similar initial surface tension values, oligofructose esters lead to higher foam stability. This could be attributed to the oligofructose part that forms a two-dimensional glass phase and provides mechanical stability to the foam films.

    In the general discussion that is presented in chapter 8 we integrate the results from the different chapters. One of the factors that is persistent throughout the different chapters is the rheological profile of the interfaces. We have shown that by using amplitude sweeps and Lissajous plots, a lot more information on the interfacial microstructure can be extracted from rheological data than by using more conventional methods. In the last part of the general discussion improvements to the synthesis are discussed, as the optimization of the synthesis was not considered in this thesis. Furthermore, improvements for the functional experiments and additional applications were identified.

    Revolution op fairway kan wereld op zijn kop zetten
    Oostindie, K. ; Dekker, L.W. ; Ritsema, C.J. ; Wesseling, J.G. - \ 2010
    Greenkeeper 21 (2010)2. - ISSN 1386-2499 - p. 30 - 33.
    golfbanen - bodemwater - zandgronden - infiltratie - oppervlaktespanningsverlagende stoffen - sensors - utrechtse heuvelrug - golf courses - soil water - sandy soils - infiltration - surfactants - sensors - utrechtse heuvelrug
    Op golfbaan ‘De Pan’ in Bosch en Duin onderzochten we de effecten van het toedienen van de surfactant Revolution op de bevochtiging van een zwakke helling in een fairway met behulp van een groot aantal vochtsensoren. Ook onderzochten we of door de surfactant het ontstaan van waterafstotendheid in de bovengrond en de vorming van (voor het milieu nadelige) preferente stroming kon worden voorkomen. Bovendien werd aandacht besteed aan de omgekeerde wereld op een fairway, waar een hogere zandrug juist groener en beter bevochtigbaar was dan de lagere delen
    Brushes and particles
    Vos, W.M. de - \ 2009
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Arie de Keizer; Mieke Kleijn. - [S.l. : S.n. - ISBN 9789085854524 - 264
    polymeren - oppervlaktespanningsverlagende stoffen - oppervlakte-interacties - fysische chemie - polymers - surfactants - surface interactions - physical chemistry
    Effects of three surfactants on soil wetting and turf performance of a fairway at the Dutch golf course de Pan
    Oostindie, K. ; Dekker, L.W. - \ 2009
    Wageningen : Alterra (Alterra-report 1819) - 84
    golfbanen - bodemwater - sensors - waterafstotende gronden - oppervlaktespanningsverlagende stoffen - utrechtse heuvelrug - golf courses - soil water - sensors - water repellent soils - surfactants - utrechtse heuvelrug
    This study reports about the applications of soil surfactants to reduce the occurrence of water repellency and to improve the soil wetting of the fairways 5, 7, 11, and 18 of golf course De Pan, located at Bosch en Duin near Utrecht, The Netherlands. The sandy soil of the fairways exhibits a water repellent behavior resulting in a lot of localized dry spots during dry periods in spring and summer. The influence of the treatments on the wetting of the soil was studied by measuring the volumetric water content with a hand-held Time Domain Reflectometry (TDR) probe
    Colloids from oppositely charged polymers: reversibility and surface activity
    Hofs, P.S. - \ 2009
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Arie de Keizer. - [S.l.] : S.n. - ISBN 9789085853107 - 128
    polymeren - colloïdale eigenschappen - oppervlaktespanningsverlagende stoffen - micellen - polymers - colloidal properties - surfactants - micelles
    The research described in this thesis concerns the formation, solution properties, and adsorption of polyelectrolyte complexes composed of at least one diblock copolymer with a neutral and a charged block and either an oppositely charged homopolyelectrolyte or a diblock copolymer, with a neutral block and an oppositely charged polyelectrolyte block. Upon mixing the aqueous solutions of the different polymers, the oppositely charged polyelectrolytes associate, forming a polyelectrolyte complex. Polyelectrolyte complex micelles – called complex coacervate core micelles (C3Ms) in this thesis – are the main focus of this thesis, but the formation of smaller aggregates, soluble complex particles, is also investigated. The salt concentration, pH, and the chemical structure of the polyelectrolytes are important variables in the formation of these polyelectrolyte complexes.
    In chapter 2 C3Ms were made from multiple polymer species; a diblock copolymer with a polyelectrolyte block and a neutral block, poly(acrylic acid)-block-poly(acryl amide), an oppositely charged polyelectrolyte, poly(N,N-dimethyl aminoethylamide), and a second diblock copolymer species with a charged block and a neutral block, poly(N,N-dimethyl aminoethylamide)-block-poly(glyceryl methacrylate). The polyelectrolyte block of the second diblock copolymer species had charged blocks that were oppositely charged to that of the first diblock copolymer species and whose neutral block was different from that of the first diblock copolymer. The effect of systematically varying the ratio of the homopolyelectrolyte and second diblock copolymer (based on the number of chargeable groups), while keeping the mixing fraction f+ (that is the number of positively chargeable groups, divided by the total number of chargeable groups) constant, was studied with light scattering. It was shown that the size of the resulting C3Ms decreased with increasing percentage of the second diblock copolymer, from 25 nm hydrodynamic radius, to 16 nm. Using a simple geometrical model and the light scattering intensities, the aggregation numbers were estimated to be in the range of 20-70 polymers.
    In chapter 3 the used diblock copolymer, poly([4-(2-aminoethylthio)-butylene] hydrochloride)-block-poly(ethylene oxide), has a polyelectrolyte part with a rather hydrophobic backbone which slows down the formation of the aggregates and the subsequent rearrangements. It was mixed with the oppositely charged poly(acrylic acid). Using light scattering and cryogenic transmission electron microscopy, it was shown that the complexes formed at f+ = 0.3 are initially very large (> 140 nm) and network like (as there is relatively little neutral polymer to stop the growth of the complexes), and rearrange relatively quickly, compared to the complexes formed at f+ = 0.5 and 0.7 (80 nm), towards small micellar complexes. The very large transient complexes formed at f+ = 0.3 are called highly aggregated polyelectrolyte complexes (HAPECs). The complexes formed at f+ = 0.5 are apparently most stable; that is, their size remains the same in time. It was concluded that there are at least three factors which influence the rearrangement rate of polyelectrolyte complexes; (1) high neutral blocks content, (2) excess charge, and (3) the chemistry of the polyelectrolytes. Increasing the salt concentration has previously been determined to speed up the rate of rearrangements as well. Furthermore, the radius of the complexes at f+ = 0.5 (80nm) is too large for the complexes to have the typical core-corona structure. Apparently, these large complexes are HAPECs as well. However, with different preparation procedures micelles can be obtained; if the HAPECs are forced to disassemble by changing the pH to an extreme value (either 11 or 3) and are subsequently re-assembled by changing the pH back to normal (7), the resulting C3Ms have a radius of about 15 nm. This is probably the state of minimum free energy, the stable state, whereas the highly aggregated complexes are in a metastable state (as they do not spontaneously rearrange in time).
    In chapter 4 complex coacervate core micro-emulsions (C3-μEs) were obtained by mixing solutions of anionic polyelectrolytes (poly(acrylic acid)) and diblock copolymers with an anionic polyelectrolyte block and a neutral block (poly(acrylic acid)-block-poly(acryl amide)) with solutions of a cationic polyelectrolyte (poly(N,N-dimethyl aminoethylamide)). By varying the fraction of the anionic polyelectrolyte and anionic diblock copolymer species, while keeping f+ constant, C3-μEs with radii varying from about 15 to 100 nm were prepared. Basically, these are C3Ms of which the core is swollen with extra polyelectrolyte complex, composed of oppositely charged homopolyelectrolytes. The solvent was shown to have a pronounced effect upon the size of the obtained complexes; in NaNO3 larger complexes were obtained which are in a metastable state. In phosphate buffer (a salt known to weaken the attractive forces between the used polyelectrolytes), smaller complexes were obtained, which are probably in the stable state. The geometrical model introduced in chapter 2 was extended and predicted a linear growth of the C3-μEs. The experimentally observed growth was however, non-linear, probably due to a transition of the neutral polymers in the corona from more star-like to more crew-cut behaviour (shown by self consistent field calculations).
    In chapter 5 the ability of a layer of adsorbed C3Ms with a more glass-like core (composed of poly([4-(2-aminoethylthio)-butylene] hydrochloride)-block-poly(ethylene oxide) and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)-block-poly(ethylene oxide)), to prevent protein adsorption to either silica or cross-linked 1,2 polybutadiene was investigated. With atomic force microscopy it was shown that the layer consists of closely packed adsorbed complex coacervate core micelles. Protein adsorption to the coated surfaces was generally reduced by > 80 %.
    The different forces and many variable parameters of the investigated system cause the time scales on which SCPs and C3Ms rearrange to span a very wide range; they can be both reversible and irreversible systems.
    Polymer vs. surfactant : competitive adsorption at the solid-liquid interface
    Postmus, B.R. - \ 2008
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers. - [S.l. : S.n. - ISBN 9789085049289 - 143
    polymeren - oppervlaktespanningsverlagende stoffen - adsorptie - grensvlak - polymers - surfactants - adsorption - interface
    The research described in this thesis focuses on the competitive adsorption of nonionic polymer and nonionic surfactant on a silica surface. These type of systems are interesting from both an academical and a technological viewpoint. Our academic interest stems simply from the observation that we had a hard time predicting the (adsorption) behaviour of the system beforehand. The technological relevance of our study can be attributed to the observation that technological applications are often complex mixtures containing a large variety of additives. The interactions between all these different components, such as the formation of mixed aggregates or co-adsorption, are quite complex. For applications, these interactions are very important since the properties of a mixture on a microscopical scale can be used to manipulate the macroscopical behaviour. Or, in the case of undesirable macroscopic behaviour, a detailed knowledge about the microscopic interactions can be used to improve on the situation.

    We have restricted ourselves to relatively simple complex mixtures, i.e. we have chosen a well-defined model system consisting of homodisperse components. This model system is an aqueous mixture of the nonionic polymer PEO with the nonionic surfactant CnEm. To study the adsorption behaviour of this mixture, we have chosen to use a flat silica surface as a model surface. The CnEm surfactants adsorb (on a hydrophilic surface such as silica) with their head groups. Because the head groups consist entirely of EO segments, the binding mechanism of the surfactants to the silica is exactly the same as the PEO binding mechanism, namely H-bonding. By evaluating the competitive adsorption of the system, we are effectively investigating the subtle effects of layer structure. By making small changes to the choice of surfactant architecture, polymer length or solvent quality, large changes in layer structure can be induced.

    Reflectometry was used to look at the competitive adsorption from mixtures containing PEO and CnEm. There are several methods to test this competitive adsorption. In the case of simultaneous adsorption, the polymer and surfactant are allowed to adsorb from a mixture. It is also possible to study adsorption sequentially, i.e. first adsorb component A, and then sequentially try to displace component A with component B. We decided to start by doing sequential adsorption experiments, because these are easier to control. In such an experiment, the PEO is allowed to adsorb onto the surface from a solution with 5 mg PEO/L. Care was taken to insure that the layer was in its steady state. Next, the flow of PEO solution was replaced by the background solution, and subsequently by a solution containing only surfactants. The concentration of the surfactant solution was 110-4 mol/L for all surfactants except for C12E3, where solubility problems demanded the use of a lower concentration c = 610-5. Still, all surfactant solutions had a concentration higher than the CMC. The results of these experiments can basically be grouped in two categories. Upon changing to the surfactant solution, the adsorbed amount would either increase sharply, or the adsorbed amount would remain constant. In the first case where the adsorbed amount would increase until the amount that the surfactant would also reach from a single component solution. Furthermore, subsequent rinsing of this layer would result in a total dissolution of the layer, and hence, the adsorbed amount would go to zero. Since this is typical surfactant behaviour, we can conclude that the surfactant displaces the polymer as it adsorbs.

    To better understand the experimental observations, we have developed an SCF model. In this model, it is possible to calculate the charge on the silica surface as a function of the pH and the ionic strength. This yields titration curves that can be compared with experimental titration curves. Our calculated results correspond quite well with literature data.

    One can also use the model to make predictions about the adsorption of PEO on our silica surface. It is possible to go to concentrations much lower than those that are experimentally accessible. We have made predictions about the response of the adsorbed polymer layer upon changes in ionic strength and pH. The results show that PEO adsorption is relatively insensitive for the ionic strength at pH ≈ 7, but at pH ≈ 10, the ions can displace the polymer quite well. This type of behaviour is also found experimentally. Every time that we perform a calculation (and we do find a solution), we obtain the mean field free energy and the most likely conformation of the system. By looking at the profiles of the most likely conformation, i.e. plotting the volume fraction of a species versus the distance from the surface, we can see that the adsorbed polymer inhibits the adsorption of salt. Hence, the polymer and the salt are in competition for adsorption.

    The behaviour of CnEm surfactants can also be evaluated with the model. Here we use exactly the same parameters that we used for the PEO. Again, we started by evaluating the surfactant bulk behaviour. Instead of investigating the first occurrence of a micelle, we have defined a more experimentally relevant CMC. We have evaluated that concentration where the volume fraction of micelles is approximately equal to the volume fraction of unimers. Based upon this criterion we have calculated the CMC and the corresponding micellar size for a number of surfactant architectures and for a number of ionic strengths. We have also evaluated surfactant adsorption isotherms. These calculated adsorption isotherms feature a first order transition at the CSAC. By evaluating the behaviour of the CSAC, we have found that the CSAC shifts to a higher concentration when the pH or the ionic strength is increased. We identified conditions for which the CSAC > CMC, which effectively implies that the surfactant does not adsorb anymore. We compared these predicted results to data measured using a reflectometer, and we find that the model predicts the experimental results quite well.

    The next step is to use the model to try and reproduce the displacement results. We have defined systems that include both PEO and CnEm, at some pH and ionic strength. To determine which component adsorbs from a mixture, we evaluate the response of the CnEm to the competing polymer. The surfactant starts adsorbing at some concentration (CSAC). If the surfactant concentration is lower than the CSAC, then the PEO will adsorb (we assume that the pH and ionic strength are such that the PEO is capable of adsorbing). For surfactant concentrations higher than the CSAC but lower than the CMC, the surfactant will preferentially adsorb. In the case of CSAC > CMC, the surfactant will not adsorb. Typically, the polymer will adsorb in this case, however, one can think of situations (high pH and high ionic strength) where the polymer will also stay in solution.

    Using the method described above, we can model the competitive adsorption of PEO and CnEm. We can evaluate the response of the surfactant to competing species, such as PEO of length N. By identifying for every surfactant architecture that polymer length N where CSAC = CMC, we can make predictions about the adsorption from mixtures.

    Interactions between biosurfactant-producing Pseudomonas and Phytophthora species
    Tran, H. - \ 2007
    Wageningen University. Promotor(en): Pierre de Wit, co-promotor(en): Jos Raaijmakers. - [S.l.] : S.n. - ISBN 9789085047445 - 136
    plantenziekteverwekkende schimmels - phytophthora - biologische bestrijding - pseudomonas - oppervlaktespanningsverlagende stoffen - gastheer parasiet relaties - oömycota - plant pathogenic fungi - phytophthora - biological control - pseudomonas - surfactants - host parasite relationships - oomycota
    Fluorescent Pseudomonas bacteria produce a wide variety of antimicrobial metabolites, including soap-like compounds referred to as biosurfactants. The results of this thesis showed that biosurfactant-producing Pseudomonas bacteria are effective in controlling Phytophthora foot rot disease of black pepper in Vietnam and promote root and shoot development of the ‘King of Spices’. Biosurfactant-producing P. fluorescens strain SS101 was also effective in controlling tomato late blight caused by Phytophthora infestans. Current and future studies focus on how to implement these biocontrol agents in an integrated management practice to control Phytophthora diseases. The broad-spectrum activity of these antagonistic Pseudomonas bacteria and their biosurfactants also provide new opportunities to apply these agents for the protection and growth promotion of other crops.
    Wetting films stabilized by block-copolymers
    Eliseeva, O.V. - \ 2006
    Wageningen University. Promotor(en): Martien Cohen Stuart; N.A.M. Besseling. - [S.l. ] : S.n. - ISBN 9789085044949 - 120
    film - afdeklagen - bevochtigen - polymeren - oppervlaktespanningsverlagende stoffen - film - coatings - wetting - polymers - surfactants
    Thin aqueous films formed on a solid surface play an important role in adhesion, spreading, and colloidal stability. These phenomena are all relevant for paint systems. Measuring surface forces in these films is an experimental challenge, and over the years several techniques have been developed to measure the interaction forces as a function of the thickness of the film, the so-called disjoining pressure isotherms. A thin film balance technique (TFB) in combination with an ellipsometer offers the possibility to study the surface forces in aqueous films formed at a silica surface.

    The aim of this project was twofold. Firstly, we wanted to set up the technique and test it by investigating interaction forces in thin aqueous films formed between an air interface and a solid interface (air/aqueous film/silica). Secondly, we intended to apply this technique for investigating interaction forces in aqueous films formed between an oilinterfaceand a solid interface (oil/ aqueous film/ silica). As happens so often in research, we were 'immersed' in the first system, and as a result of it, we did not reach the second aim. Hence, this thesis deals with the experimental investigations of the interaction forces in thin aqueous films formed between an air phase and a solid phase. A non-ionic polymeric surfactant is used as a stabilizer of these films. In addition, we have also used colloidal probe atomic force microscopy (CP-AFM) in order to see if phenomena similar to those in wetting films might occur in the interactions between two silica surfaces, immersed in solutions of the surfactant.

    In chapter 1 an introduction is given to wetting films, to the surfactant used and to the technique used in the present study. A short overview of the interaction forces in wetting films and the drainage of these films is presented. As foam films, which are aqueous films between two air phases (air/ aqueous film/ air), have much more extensively been studied than wetting films, we also discussed drainage of foam films, as well as and an interesting finding in foam films stabilized by similar surfactants.

    In chapter 2 the solution behaviour of the surfactant and its adsorption on silica-water and air-water interfaces is studied using light scattering, surface tensiometry and ellipsometry. Interaction forces in wetting films stabilized by the surfactant are measured and are interpreted in terms of a disjoining pressure as a function of a thickness of the film. Parameters of the systems are the concentration of the surfactant and the concentration of NaCl. The main findings are that addition of NaCl lowers the cmc, diminishes adsorption of the surfactant on silica, and increases the thickness of the wetting film stabilized by the surfactant.

    In chapter 3 we investigate the thinning of wetting films formed from aqueous solution of the surfactant on silica using a TFB and an ellipsometer. In addition, imaging ellipsometry is used to visualize the film structures at subsequent stages of their development. The main findings are that the thick films observed in chapter 2 are metastable. The time required for the formation of steady films strongly increases with the concentration of electrolyte. For sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified: after initial quick thinning, further thinning slows down enormously. The interpretation we give of the observed features is that these typical kinetic regimes are the result of coupled dependencies of the bulk and interfacial properties of the surfactant on salt concentration.

    In chapter 4 the time-dependent thickness of wetting films formed from aqueous solutions of NaCl on a silica surface modified with the surfactant is revisited, because of a rather surprising finding: the thickness as a function of time could go through a maximum. It turned out that that the thickness of the films depends strongly on whether or not the surfactant was applied over the entire substrate or only at a spot coinciding with the investigated wetting film. In the former case, the thickness shows a transient thickness maximum, in the latter case, it equilibrates quickly. A possible interpretation of the transient thicknesses is given, and the equilibrium values of the thickness are discussed.

    In chapter 5 the effects of pH and different additives (NaCl, Na 2 SO 4 , NaSCN and urea) on the adsorption of the surfactant at the interfaces of wetting films, on film drainage and on the interaction forces in these films are examined using an ellipsometer and a TFB. These additives reduce the adsorption of the surfactant on silica and all retard the drainage of the films stabilized by the surfactant. Moreover, the film destabilizes at high pH values if 0.1 M NaCl is present. The reduction of the adsorbed amount is correlated to a dramatic slow down of film drainage. We show that these effects must be attributed to bridging attraction between a densely covered air-water interface and a very sparsely covered silica-water interface.

    In chapter 6 the interactions between two silica surfaces, immersed in solutions of the surfactant is studied by CP-AFM. The main findings are that (i) there are long ranged attractive forces between bare hydrophobic and hydrophilic surfaces in aqueous solution; and that (ii) a weak adhesion occurs between a hydrophobic and a hydrophilic surface in the presence of the surfactant in 0.1 M NaCl. Results (i) can be attributed to capillary bridging and (ii) to observation bridging of the surfactant between a densely covered hydrophobic surface and a sparsely covered hydrophilic surface, thus confirming the effect found in chapter 5.

    Fate of linear alkylbenzene sulfonate (LAS) in activated sludge plants
    Temmink, B.G. ; Klapwijk, A. - \ 2004
    Water Research 38 (2004)4. - ISSN 0043-1354 - p. 903 - 912.
    afvalwaterbehandeling - oppervlaktespanningsverlagende stoffen - biodegradatie - monitoring - geactiveerd slib - zuiveringsinstallaties - waste water treatment - surfactants - biodegradation - monitoring - activated sludge - purification plants - waste-water treatment - surfactant monitoring program - biodegradation kinetics - environment - chemicals - behavior - culture - removal - sewage
    Monitoring data were collected in a pilot-scale municipal activated sludge plant to assess the fate of the C12-homologue of linear alkyl benzene sulfonate (LAS-C12). The pilot-plant was operated at influent LAS-C12 concentrations between 2 and 12 mg/l and at sludge retention times of 10 and 27 days
    Monitoring data were collected in a pilot-scale municipal activated sludge plant to assess the fate of the C-12-homologue of linear alkyl benzene sulfonate (LAS-C-12). The pilot-plant was operated at influent LAS-C-12 concentrations between 2 and 12 mg l(-1) and at sludge retention times of 10 and 27 days. Effluent and waste sludge concentrations varied between 5 and 10 mug l(-1) and between 37 and 69 mug g(-1) VSS, respectively. In the sludge samples only 2-8% was present as dissolved LAS-C-12, whereas the remaining 92-98% was found to be adsorbed to the sludge. In spite of this high degree of sorption, more than 99% of the LAS-C-12 load was removed by biodegradation, showing that not only the soluble fraction but also the adsorbed fraction of LAS-C-12 is readily available for biodegradation. Sorption and biodegradation of LAS-C-12 were also investigated separately. Sorption was an extremely fast and reversible process and could be described by a linear isotherm with a partition coefficient of 3.21 g(-1) volatile suspended solids. From the results of biodegradation kinetic tests it was concluded that primary biodegradation of LAS-C-12 cannot be described by a (growth) Monod model, but a secondary utilisation model should be used instead. The apparent affinity of the sludge to biodegrade LAS-C-12 increased when the sludge was loaded with higher influent concentrations of LAS-C-12. (C) 2003 Elsevier Ltd. All rights reserved.
    Influence of surfactant applications on the wettability of a dune sand with grass cover: long-term effect of Primerr604 and short-term effect of ACA 1897
    Oostindie, K. ; Dekker, L.W. ; Ritsema, C.J. - \ 2003
    Wageningen : Alterra (Alterra-report 659) - 54
    duinzand - neerslag - infiltratie - waterafstotende gronden - oppervlaktespanningsverlagende stoffen - bodemwater - dehydratie - dune sand - precipitation - infiltration - water repellent soils - surfactants - soil water - dehydration
    Effects of surfactant applications and irrigations on the wetting of a dune sand with grass cover
    Oostindie, K. ; Dekker, L.W. ; Ritsema, C.J. - \ 2002
    Wageningen : Alterra (Alterra-rapport 540) - 88
    waterafstotende gronden - oppervlaktespanningsverlagende stoffen - tijdsdomeinreflectometrie - bodemwatergehalte - duinzand - duingraslanden - irrigatie - bodemfysica - bodemvocht - hydrofobie - water repellent soils - surfactants - time domain reflectometry - soil water content - dune sand - dune grasslands - irrigation
    Extractable substances (anionic surfactants) from membrane filters induce morphological changes in the green alga Scenedesmus Obliquus (Chlorophyceae)
    Lürling, M. ; Beekman, W. - \ 2002
    Environmental Toxicology and Chemistry 21 (2002)6. - ISSN 0730-7268 - p. 1213 - 1218.
    toxiciteit - filters - scenedesmus - oppervlaktespanningsverlagende stoffen - oppervlaktewater - morfologie - kolonies - fytoplankton - aquatische ecosystemen - toxicity - filters - scenedesmus - surfactants - surface water - morphology - colonies - phytoplankton - aquatic ecosystems
    The effect of filtration of medium through different kinds of filters (glass fiber, mixed esters of cellulose and nitrocellulose) on the morphology in the green alga Scenedesmus obliquus was examined. Several compounds potentially released from membrane filters were further investigated, and among them, two anionic surfactants were found to be morphologically active. Exposure to the anionic surfactants resulted within 2 d in the transformation of unicellular populations of Scenedesmus in populations dominated by colonies. Growth rates between control and surfactant-exposed populations were identical, and the morphological effect occurred at surfactant concentrations far below the reported no-observed-effect concentration for growth inhibition, stressing the need for inclusion of morphological appearance of Scenedesmus in algal toxicity testing to improve the assessment of ecological risks.
    Effects of surfactant treatment on the wettability and wetting rate of a Sphagnum peat growing medium
    Dekker, L.W. ; Oostindie, K. ; Ritsema, C.J. - \ 2001
    Wageningen : Alterra (Alterra-rapport 80) - 26
    waterafstotende gronden - oppervlaktespanningsverlagende stoffen - groeimedia - bevochtigbaarheid - water repellent soils - surfactants - growing media - wettability
    This study evaluated the effectiveness of a new wetting agent, PsiMATRIC, on the wettability of a Sphagnum peat growing medium. A mixture of Sphagnum peat, perlite and vermiculite treated with 0.26 g of PsiMATRIC per litre was compared with the untreated blend. Water repellency tests (water drop penetration and alcohol percentage) and wetting rate measurements show that the wetting agent provokes a change from a hydrophobic to a hydrophilic character.
    Biodegradability and change of physical characteristics of particles during anaerobic digestion of domestic sewage
    Elmitwalli, T.A. ; Soellner, J. ; Keizer, A. de; Bruning, H. ; Zeeman, G. ; Lettinga, G. - \ 2001
    Water Research 35 (2001). - ISSN 0043-1354 - p. 1311 - 1317.
    rioolwater - afvalwaterbehandeling - anaërobe behandeling - biodegradatie - oppervlaktespanningsverlagende stoffen - sewage - waste water treatment - anaerobic treatment - biodegradation - surfactants
    Polymers and surfactants in solution and at interfaces : a model study on detergency
    Torn, B. - \ 2000
    Agricultural University. Promotor(en): J. Lyklema; L.K. Koopal; A. de Keizer. - S.l. : S.n. - ISBN 9789058082640 - 140
    polymeren - oppervlaktespanningsverlagende stoffen - adsorptie - polymers - surfactants - adsorption

    This thesis deals with detergency-related adsorption phenomena of (mixtures of) polymers and surfactants. Both types of molecules play an important role in the removal and subsequent stabilization of soil from a substrate. Starting with a model detergency system consisting of polymers, surfactants, soil and a substrate, a division is made into a set of sub-systems, each focusing on the interactions of two or more of these model components.

    The first chapter gives a short introduction on the typical behavior of polymers and surfactants in solution and at interfaces, and touches upon the physicochemical principles of detergency. In a washing process it is important to prevent the redeposition of soil, which in an earlier stage has been removed from a substrate. A way to keep particles dispersed in solution is to cover them with a thick polymer layer providing electrostatic and/or steric stabilization. The adsorption of the uncharged polymer poly(vinylpyrrolidone) (PVP) on Na-kaolinite has been studied in chapter 2. The surface of this clay mineral is patchwise heterogeneous with respect to its charge and chemical composition. In order to reveal these charge characteristics, potentiometric acid-base titrations were performed on samples at different concentrations of sodium chloride. An interpretation of the proton adsorption/desorption in terms of the contributions of the individual surface types, i.e. edges and plates, has been given. At the latter type, protons are strongly favored over sodium ions. Striking similarities were observed between the proton adsorption and the PVP adsorption experiments. PVP readily adsorbs high affinity on at least part of the kaolinite surface. Studying the effect of the pH, the electrolyte concentration, and the presence of multivalent ions on the amount adsorbed at the plateau has given further insight into the adsorption mechanisms. Increasing the pH or the electrolyte concentration leads to a decrease in PVP adsorption. A model is proposed in which PVP adsorbs on edges and basal planes by different mechanisms. The adsorption of PVP on the edges is strongly pH dependent, but that on the plates only weakly. Specifically adsorbed protons at the plates act as anchor sites for PVP segments. Multivalent ions do not influence the proposed adsorption mechanism directly, but primarily change the surface area accessible for PVP.

    Before studying adsorption of a polymer-surfactant mixture, information on the interaction between the polymer and the surfactant in solution is indispensable. Chapter 3 covers the interaction between the anionic surfactant sodium dodecylbenzenesulphonate (SDBS) and the uncharged polymer poly(vinylpyrrolidone) (PVP) by titration microcalorimetry. Since hydrophobic attractions are typically dependent on temperature, which is in general not the case for other types of interaction, measurements carried out at different temperatures have yielded information on the nature of the associations. The interaction enthalpy of mixed PVP/SDBS systems clearly showed a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the process. The exothermic region features the inverse of hydrophobic bonding behaviour. In our opinion, this is due to conformational changes of the polyelectrolyte complexes. With increasing amount of surfactants bound to the chain, electrostatic repulsion of neighbouring surfactant head groups tends to expand the complexes, whereas hydrophobic interactions do the opposite. Beyond a certain coverage, the coulombic repulsion forces the chains to swell. This is accompanied by losing hydrophobic inter- and intrachain linking. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.

    The results and the knowledge obtained with this study has helped to understand the mixed adsorption of PVP and SDBS on kaolinite, which is the subject of chapter 4. Both components adsorb from their mixture on the clay. This process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. In the absence of PVP, SDBS adsorbs on the clay by electrostatic and hydrophobic interactions. When polymers are present, the adsorbed amount of SDBS is at 10 -2 M NaCl mainly determined by charge compensation on the edges.

    Under different conditions PVP shows similar behaviour as a function of the surfactant concentration. With increasing SDBS concentration three subsequent regions in the PVP adsorption can be distinguished: initially a small increase, followed by a strong decrease, which finally flattens off to a plateau. These regions are related to the surface affinity of the species actually present in solution. They reflect the changing character of the charge of the polymer-surfactant complexes with increasing surfactant concentration. At low surfactant content, the polymer chains are not or hardly charged, and they adsorb on the clay by hydrogen bonding and hydrophobic interactions. At high surfactant concentrations, the adsorption of polymer-surfactant complexes is dominated by coulombic attraction. There is experimental evidence for the presence of mixed surface aggregates at the edges. The composition of these complexes differs from that in solution and is controlled by the surface charge. With increasing electrolyte concentration, this difference becomes smaller.

    After a detailed look at the solution side of the washing process, we have to focus on the substrate. In order to carry out fundamental studies, a flat and well-defined surface was needed which was a good mimic for cotton. To that end, a cellulose surface was developed which was able to function as a model for cotton. Chapter 5 describes the preparation of thick cellulose films. The method is based on the attachment of hydrophobized cellulose on a wafer and subsequent chemical regeneration to cellulose. With the spincoating technique, reproducible, rapidly prepared, and flat cellulose surfaces can be obtained. These are characterized by their thickness, roughness, swelling behaviour, stability, charge, and wetting and adsorption properties.

    So far, all studies concerned equilibrium aspects. However, in a washing process, the dynamics of processes, such as adsorption, removal, and stabilization, are very important. Kinetic and equilibrium aspects of nonionic surfactant adsorption on cellulose surfaces just described, are studied in a stagnation point flow cell (chapter 6). Nonionic surfactants readily adsorb on cellulose, thereby showing three distinct regions. At low surface coverages, molecules adsorb more or less in a flat state, with a contribution from both the head group and the tail. At increased concentrations, lateral attraction between surfactant molecules is dominant, leading to the formation of half-micelles at the surface. In line with the results of chapter 5, the adsorption features of cellulose are in between those for a hydrophilic and a hydrophobic surface.

    The kinetics of nonionic surfactant adsorption depends on the composition of the surfactant. Below the CMC, the initial adsorption rate is determined by monomer diffusion. Above the CMC, the magnitudes of the micellar dissociation rate and the micellar diffusion coefficient, should be compared to that of the monomer diffusion coefficient. If the micellar properties are sufficiently large, micelles acts as monomer-suppliers. This was observed for the most hydrophilic surfactant under study. The desorption rate depends on the surface coverage. Initially, it is controlled by monomer detachment. The desorption rate coefficients of different surfactants scaled with the CMC, suggesting an analogy between the surface aggregates to those formed in solution.

    The set-up of the cellulose surfaces in a stagnation point flow cell can be used for a variety of adsorbates and serve as a model for (re)deposition studies.

    Proteins and protein/surfactant mixtures at interfaces in motion
    Boerboom, F.J.G. - \ 2000
    Agricultural University. Promotor(en): A. Prins; M.A. Cohen Stuart. - S.l. : S.n. - ISBN 9789058082817 - 213
    eiwitten - oppervlakten - oppervlaktespanningsverlagende stoffen - eigenschappen - surfaces - surfactants - foaming - properties

    The research described in this thesis covers a number of aspects of the relation between surface properties and foaming properties of proteins, low molecular surfactants and mixtures thereof. This work is the result of a question of the industrial partners if it is possible to understand the foaming properties of protein hydrolysates. As there are many aspects of the surface properties that can be responsible for the foaming behaviour a number of problems were defined by which we can obtain a better understanding of the relation between surface properties and foam formation and stability in relation to the type of surface active substance.

    In this thesis an important question a priori has been: How can we understand the foaming properties from the properties of the surface active species. We presumed that the molecular properties cannot be translated forthwith to foaming properties but that a number of translation steps are necessary. First of all the molecular properties need to be translated into surface properties which manifest themselves in mechanical properties of surfaces and films. In addition there is a relation between these mechanical properties and the foaming properties.

    An important consideration a priori was that the way in which the surface is deformed is important for finding the relevant relation between the mechanical properties and the foaming properties. Here we made a distinction in two types of deformation being deformations caused by forces applied parallel and perpendicular to the surface. A force that is applied perpendicular to the surface (pressure) generally leads to a homogeneous deformation of the surface. The properties of the surface change with time or time scale but at the surface there are no differences with respect to surface tension. surface concentration or relative rate of expansion. Forces applied perpendicular to the surface generally lead solely to enlargement or reduction in surface area. Forces applied parallel to the surface (shear forces) can also lead to enlargement or reduction in surface area. These forces are generally due to viscous friction with the surface. In addition to a change in surface area these forces also cause a redistribution of surface active material over the surface. Hence the surface concentration. the surface tension as well as the relative rate of expansion vary over the surface. This leads to a surface tension gradient that is necessarily equal to the viscous drag at the surface. It is striking that in literature only homogeneous deformations are studied in detail. This fact can be attributed to the difficult experimental accessibility of surfaces subjected to shear forces. Nevertheless shear forces may play a role in the foaming properties such as in drainage and bubble break-up. An important part of this thesis will be devoted to the relation between viscous friction and surface motion.

    A device which enables the quantification of the relation between viscous drag and motion of the surface in relation to the surface properties is the overflowing cylinder. This device consists of an inner cylinder surrounded by an outer cylinder with a larger diameter. In this inner cylinder liquid is pumped up which flows over the rim into the space between the inner and outer cylinder. At the top surface we find a continuously expanding surface of which the relative expansion rate remains approximately constant over the surface in the vicinity of the centre of the cylinder. The expansion rate of the surface can be influenced by changing the length of the falling film. Hence within certain limits the expansion rate at the top surface c5n be varied. If we consider the falling film at the leading edge of the inner cylinder the falling motion of the liquid in the film causes the deformation of the surface parallel to the surface. If we would be able to measure the properties of the falling film we could learn how the surface properties vary with distance. However the surface of the falling film is experimentally not accessible. Therefore in this thesis the changes in surface properties of the falling film have been studied by measuring the surface tension of the top and bottom surface at a fixed place. This provides a reasonable measure of the surface tension difference over the falling film of the overflowing cylinder.

    In order to interpret this difference, the conditions at the falling film have been approximated by means of simple hydrodynamic theory. From this approximation we could conclude that there is a relation between the relative expansion rate, the length of the falling film and the surface tension difference. The surface of the falling film is propelled by the falling motion of the liquid which causes a surface tension gradient at the surface as a consequence of the viscous drag. If we compare the calculations with experimental data we can find that this approach is in agreement with the experiments. Hence the difference in surface tension over the falling film can be considered to be a measure of the resistance to deformation of the surface to forces applied parallel to the surface.

    The surface tension gradients which could be generated by means of different surface active species appeared to differ significantly from each other. Especially the difference between low molecular surfactants and proteins could be shown to be large. This can be ascribed to the sensitivity of the surface tension of these substances to expansion and compression of the surface. Adsorbed layers of proteins have a high surface tension in expansion. This can be explained by the time required for unfolding at a surface of these substances. In compression low surface tensions are found for proteins due to the slow desorption of proteins. The surface tension of low molecular surfactants is less sensitive to compression and expansion. In expansion the relatively short diffusion length causes the surface tension to deviate much less from the equilibrium surface tension. In compression these substances desorb easily causing the surface tension to be close to the equilibrium surface tension as well. Hence the surface tension gradient that can be generated by proteins is much larger the for low molecular surfactants. If mixtures of low molecular surfactants and proteins (Tween 20 andβ-casein) are considered it is found that in expansion the surface tension is influenced by both species. In compression however the surface tension is around the equilibrium surface tension of the low molecular surfactant. From this we can conclude that the low molecular surfactant determines the surface tension in compression. Most probably there the affinity of the surface active substance for the surface is important which causes the preferential desorption of proteins. This means that the surfaces of these mixtures have a low resistance to deformation by forces applied parallel to the surface.

    Unfolding behaviour of proteins at interfaces

    The most important class of surface active substances which have been studied in this thesis are proteins due to the similarities in structure and properties with protein hydrolysates. Proteins consist of 20 different amino acids which vary in residual group. Despite the similarity in the basic structure of these substances the difference in foaming properties between proteins is large. In literature this difference in foaming properties is ascribed to the difference in unfolding rate during adsorption at air/water interfaces of these polymers. There are no reliable data on the unfolding rate of proteins however. The overflowing cylinder technique can be used to determine the unfolding rates of proteins since the top surface is in a steady state while the relative expansion rate is finite. The relative expansion rate can be seen as a characteristic time scale of the surface.

    In order to measure the unfolding rate of the proteins, at the top surface, the surface tension, the relative rate of expansion and the surface concentration were determined using the Wilhelmy plate technique, laser Doppler anemometry and ellipsometry respectively. With these three parameters the surface can be characterised completely. The reasoning behind the characterisation is as follows: If proteins need time for the unfolding at an interface, the relative expansion rate determines the mean degree of unfolding of the proteins at the interface. As a function of the relative rate of expansion and at equal adsorbed amounts the surface tension will vary due to a difference in the mean degree of unfolding.

    If we would know the surface tension and surface concentration for different bulk concentrations as a function of the relative rate of unfolding then we would be able to establish the influence of the unfolding rate of the protein on the surface tension. Since this is difficult to determine on the basis of the raw date, a simple model was used to express this influence of unfolding in an unfolding and a refolding parameter. In this model the transport to and the unfolding at the top surface of an overflowing cylinder has been described. The unfolding and refolding has been described by means of a first order reaction. In essence we assume that the degree of unfolding can be seen as an average over many stages of unfolding over a large number of molecules. By dividing the interface into a large number of concentric rings and by carrying out the calculations for the transport and unfolding for each concentric ring the surface properties can be calculated as a function of the distance to the centre. By varying the relative rate of expansion as a function of the distance to the centre in the same way as takes place at the top surface of the overflowing cylinder also the surface tension gradient can be determined. Since there is a fixed relation between the surface tension gradient and the maximum relative rate of expansion, the maximum relative rate of expansion can be determined by means of iteration.

    In this thesis the unfolding behaviour of the proteins:β-casein,β-lactoglobulin, BSA and lysozyme has been determined experimentally. In literature these proteins have been characterised well with respect to adsorption and unfolding behaviour. The unfolding rates of these proteins differ several orders of magnitude.β-Casein andβ-lactoglobulin were shown to unfold most rapid. The characteristic time scales of unfolding of these proteins is in the order of a tenth forβ-casein to a few tenths forβ-lactoglobulin. Lysozyme hardly unfolds within the time scale of the experiment which indicates that the unfolding takes more than 100 seconds. The model was shown not to apply for BSA since the change in surface tension proceeds in two steps. Nevertheless it could be calculated that the unfolding of BSA takes place in a time scale in the order of 20 seconds.

    The model and the overflowing cylinder technique have also been applied to mixtures of proteins and low molecular surfactants. The systemβ-casein/Tween 20 was chosen because in this system no complications are known such as aggregation in he bulk phase, or electrostatic interactions at the surface. At expanding interfaces these systems were shown to behave in a more or less additive manner. The surface tension of the mixture appeared to be lower than the addition of the decrease in surface tension of each species individually. This can be attributed to the fact that both species occupy part of the space at the interface. In static conditions it was found in literature that low molecular surfactants can displace proteins from the interface. This difference in behaviour is caused by the fact that in expansion the surface tension is controlled mainly by transport to and unfolding at the surface while in equilibrium time scale is irrelevant and the affinity of the substances to the surface determines the behaviour. Despite certain deviations the measured properties were indicated to be consistent with the calculations by the model. Higher order effects such as the presence of the surfactants in micelles and preferential adsorption were shown to have little effect of these mixtures at expanding surfaces.

    Foam formation and foam stability

    In order to quantify the experimental values and techniques of this research for practical situations, foaming experiments have been performed for a number of relevant systems. In these experiments, the bubble size and the drainage rate of the foams have been determined. Subsequently the results of these experiments were related to mechanical surface properties.

    The experiments indicated that the surface tension difference over the falling film in the overflowing cylinder correlated with the bubble size and the rate of drainage. For the formation of foam this means that not only the amount of energy supplied is important for the bubble size but that also characteristic properties of the surface being the maximal viscous drag that can be transferred is important for the break-up of foam bubbles. The explanation that can be given for this is that the shear stress generated at the surface provides the deformation that leads to an unstable shape of the bubbles which finally leads to break-up. Surface active species that enable the generation of a high shear stress therefore promote the generation of small bubbles.

    In addition it was demonstrated that there is a relation between drainage and viscous friction at the bubble surface. The rate of drainage expressed in the decrease of the characteristic film thickness decreases when a higher surface tension difference between top and bottom surface in the overflowing cylinder is present at a rather arbitrarily chosen relative rate of expansion of 1 s -1 .

    In chapter 6 the foaming properties of protein hydrolysates were discussed. The most important reason that protein hydrolysates have good foaming properties is that in these systems the low molecular components do not displace high molecular components. This causes the surface tension difference between a compressed and expanded surface to be high. This supports the creation of small bubbles and the resistance to drainage. It is possible that the good foamability of protein hydrolysates compared to proteins is caused by the lower surface tension in equilibrium.

    In general it can be said that surface tension gradients play a larger role in foam formation and foam stability than the attention in literature would suggest. More attention for the properties which determine the resistance against deformation of surfaces, parallel to the surface would lead to a better insight in the reasons why different surface active substances exhibit different foaming behaviour.

    Brushes and soap : grafted polymers and their interactions with nanocolloids
    Currie, E.P.K. - \ 2000
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : s.n. - ISBN 9789080347069 - 209
    borstels - polymeren - colloïden - oppervlaktespanningsverlagende stoffen - brushes - polymers - colloids - surfactants - cum laude

    Layers of polymer chains end-attached to a grafting plane at high densities, so-called brushes, are a curious state of matter. The (average) monomer density within the brush is as high as in a semi-dilute polymer solution, resulting in a high osmotic pressure in the brush. Due to the grafting, however, this isotropic osmotic pressure results in an anisotropic stretching of the chains normal to the surface. This degree of stretching can be quite extensive; in this thesis PEO-chains of 700 monomers are considered which are stretched up to 20% of their total contour length, i.e. form a brush with a thickness of 50 nm, merely by the presence of similar grafted chains.

    It is evident that such extended polymer layers may strongly modify the properties of the grafting surface. To this end brushes are applied as, for instance, adsorption inhibitors or colloidal stabilisators. In this thesis we focus on the thermodynamic and structural properties of polymer brushes, both neutral and charged, and on their interactions with nanocolloids. A mean-field model is developed that describes the effect of complexes formed by polymer (or polyelectrolyte) chains and nanocolloids on the polymer conformation, and the phase behaviour of such mixtures. These two modes of investigation converge in the theoretical and experimental investigation of the interaction between neutral brushes and nanocolloids which may form complexes with the polymer chains in a bulk solution.

    A general introduction to brushes and polymer-nanocolloid complexes is presented in Chapter 1. The concepts underlying scaling and analyticalself-consistent-field (aSCF) models of brushes are briefly discussed, as are a number of technological applications of grafted polymers. The difficulties encountered in the preparation of a brush of a controlled chain length and grafting density are also considered.

    In Chapter 2 surface pressure isotherms of neutral, end-grafted chains that can adsorb to the grafting plane are modelled with the numerical Scheutjens-Fleer self-consistent-field (nSCF) model. These numerical results are compared to experimental isotherms of PS-PEO block copolymers irreversibly adsorbed at the air/water interface. Semi-quantitative agreement between the numerical and experimental isotherms is found. It is shown that for long chains the experimental and numerical isotherms obey the power law for the brush surface pressure as a function of the grafting density predicted by aSCF models.

    The predicted power law for the brush thickness is only obeyed when the experimental surface pressure isotherms also follow the aSCF power law. The adsorption/desorption transition of grafted polymers upon increasing grafting density is investigated numerically by considering the chemical potential of the grafted chains and its derivative with respect to the grafting density. It is shown that this adsorption/desorption transition is continuous, irrespective of the chain length and the adsorption strength. The behaviour of the chemical potential at large adsorption energies is reminiscent to that of a (mean-field) magnetic system approaching its critical point.

    The monomer density profiles of monodisperse and bimodal PEO-brushes are determined with neutron reflectivity and compared to profiles predicted by the nSCF model in Chapter 3. The monomer density distribution predicted by aSCF-models, namely a parabolic profile, is only found at a relatively high grafting density. At lower densities the contribution of a `tail' region at the edge of the brush to the reflectivity spectra is considerable. In this distal region, which originates from fluctuations of the extended chains, the density smoothly drops to zero. Good agreement is found between the experimental and nSCF density profiles. When short and long PEO-chains are mixed at relatively high grafting densitites a bimodal brush is formed. This biomodal density distribution is enhanced by unequal chain length ratio's and mixing ratio's at high grafting densities of such mixed layers. As expected on the basisof theoretical predictions, the long chains in the bimodal brush are additionally stretched by the presence of the shorter ones.

    In Chapter 4 the properties of annealed polyeclectrolyte brushes, consisting of grafted polyacrylic-acid (PAA) chains in contact with aqueous solution, are examined with surface pressure measurements, optical reflectivity and ellipsometry. When the ionic strength of the subphase is high and the pH relatively low, the predicted power law for the surface pressure as a function of the grafting density in the salted brush (SB) regime is found. At low ionic strength and pH, however, the PAA-chains are found to adsorb at the air/water interface.

    Due to such adsorption the predicted osmotic brush regime is not observed at the air/water interface. A novel manner to prepare brushes on a solid substrate, namely Langmuir-Blodgett deposition of PS-PAA block copolymers from an air/water interface on a hydrophobic modified silicon wafer and subsequent thermal annealing, is developed. Using this technique the average degree of dissociation of grafted PAA chains as a function of pH is measured with reflectometry. It is shown that dense grafting of the PAA-chains shifts the titration curves significantly to higher pH, as predicted by scaling models and numerical studies.

    The thickness of the PAA brushes on hydrophobic modified silicon wafers is measured with ellipsometry as a function of pH, ionic strength and grafting density. At a pH not far from the monomeric pKa, the brush thickness is theoretically predicted to initially increase with increasing ionic strength and to decrease again at high ionic strength. This non-monotonic behaviour of the brush thickness is now observed experimentally for the first time.

    The initial increase in brush thickness with increasing ionic strength is, however, experimentally less pronounced than predicted by theory.

    An analytical mean-field theory for long polymer chains that form complexes with nanocolloids is developed in the following chapters. In Chapter 5 the complexation between single polymer chains in a good solvent and surfactants in micellar aggregates is considered, using a Flory-like approach. It is shown that the number of complexed micelles on a polymer chain continuously increases with increasing surfactant concentration, in agreement with experimental evidence. The size of the coil can monotonously increase, decrease, or have a maximum as a function of the surfactant concentration. Comparison with experimental data for PEO-gels complexed with SDS shows a reasonable agreement between the predicted dependence of the gel volume on the ionic strength and experiments.

    In Chapter 6 semi-dilute solutions of complexed chains are considered. Osmotic interactions are found to strongly influence the degree of complexation in a semi-dilute solution. The degree of loading of the chains by nanocolloids decreases with increasing monomer density when the osmotic interactions between complexed particles are strong compared to those between bare monomers. If, however, the complex-monomer osmotic interactions are strong compared to both the complex-complex and monomer-monomer, phase separation into a relatively dilute phase consisting of highly loaded chains coexisting with a relatively dense phase of bare chains may occur. Such phase separation is promoted when the solvent quality decreases. If the solution is below the Theta-temperature of the bare polymer, a first-order phase transition from a bare, collapsed globule to a swollen coil with increasing particle density is predicted.

    Such a first-order phase transition is reported experimentally for collapsed polymer globules with increasing surfactant concentration. An analytical self-consistent-field theory for polymer brushes, in the presence of particles capable of complexation is presented in Chapter 7. As a monomer density gradient is present in a brush, the density of complexed particles is also predicted to vary across the brush. Roughly speaking, the complexes are predominantly located in the distal region of the brush, where the average monomer density is low. In the proximal region of the brush, close to the grafting plane, the density of complexed particles is low. Microphase separation may occur in the brush under the same conditions for which macroscopic phase separation occurs in a bulk solution.

    The overall number of complexed particles is predicted to have a maximum as a function of the grafting density. The height of the brush is found to either increase monotonously with increasing grafting density, or have a local maximum and minimum. The adsorption of the protein BSA on hydrophobic silicon wafers covered with grafted PS-PEO-chains is experimentally examined in Chapter 8. The amount of adsorbed BSA is measured with reflectometry at several grafting densities and different PEO chain lengths.

    Conventional models for the interaction between a brush and adsorbing proteins predict the adsorbed amount to decrease with increasing grafting density and chain length as the interaction between PEO and BSA in the bulk is purely repulsive. However, it is observed that the adsorbed amount has a maximum as a function of the grafting density for long chains, whereas it decreases monotonously in the case of short chains. This maximum is qualitatively understood with our aSCF model presented in Chapter 7 and indicates that some (unknown) attraction between grafted PEO and BSA may exist.

    Finally, in Chapter 9, our theoretical model is extended to complexation of polyelectrolyte chains with oppositely charged nanocolloids. In a given system (particle size, charge densities of the chain and particle) the ionic strength is the main parameter which controls complexation. At high ionic strength the attractive electrostatic interactions are suppressed and the degree of complexation is negligible. As the ionic strength decreases the attractive electrostatic interactions induce complexation. The transition from a bare polyelectrolyte to a complexed chain is predicted to be either continuous or abrupt, depending on the ratio of the charge densities and the Hamaker constant of the particles. In the former case the complex remains soluble, in the latter a non-soluble coacervate is formed. Both kinds of loading processes have been reported in the literature.

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