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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A rapid method to screen wild Solanum for resistance to early blight
Wolters, Pieter J. ; Vos, Lennard de; Bijsterbosch, Gerard ; Woudenberg, Joyce H.C. ; Visser, Richard G.F. ; Linden, Gerard van der; Vleeshouwers, Vivianne G.A.A. - \ 2019
European Journal of Plant Pathology 154 (2019)1. - ISSN 0929-1873 - p. 109 - 114.
A. grandis - A. protenta - A. solani - Alternaria - Disease assay - Wild potato

Early blight of potato and tomato is caused by Alternaria fungi and negatively impacts crop yields. Environmental factors and plant maturity influence disease development, which is usually kept under control by fungicide applications. Wild tuber-bearing Solanum section Petota species are a promising source of resistance to early blight that could be used to control the disease, for example by crossbreeding or modern breeding approaches. An efficient screening method is a first prerequisite for the identification of resistant genotypes in wild Solanum germplasm. Here, we describe a protocol that can be used to rapidly screen for resistance to early blight in wild Solanum collections. This protocol provides a good starting point for the identification of resistant genotypes and is a step towards breeding for resistance to early blight using wild Solanum species.

Functional characterization of cucumber (Cucumis sativus L.) Clade V MLO genes
Berg, J.A. ; Appiano, Michela ; Bijsterbosch, G. ; Visser, R.G.F. ; Schouten, H.J. ; Bai, Y. - \ 2017
cucumber - cucumis sativus - powdery mildew - MLO - susceptiblity genes - gene expression
Functional characterization of cucumber (Cucumis sativus L.) Clade V MLO genes
Berg, Jeroen A. ; Appiano, Michela ; Bijsterbosch, Gerard ; Visser, Richard G.F. ; Schouten, Henk J. ; Bai, Yuling - \ 2017
BMC Plant Biology 17 (2017)1. - ISSN 1471-2229
Cucumber (Cucumis sativus L.) - Gene expression - MLO - Powdery mildew - Susceptibility genes

Background: Powdery mildew (PM) causing fungi are well-known pathogens, infecting over 10.000 plant species, including the economically important crop cucumber (Cucumis sativus L.). Loss-of-function mutations in clade V MLO genes have previously been shown to lead to recessively inherited broad-spectrum resistance to PM in several species. In cucumber, one clade V MLO homolog (CsaMLO8) was previously identified as being a susceptibility factor to PM. Two other closely related homologs (CsaMLO1 and CsaMLO11) were found, but their function was not yet unravelled. Methods: CsaMLO1 and CsaMLO11 were cloned from cucumber and overexpressed in a tomato mlo mutant. The transcript abundances of all three CsaMLO genes in different cucumber tissues were quantified using qRT-PCR and RNA-seq, with and without inoculation with the cucumber PM fungus Podosphaera xanthii. Allelic variation of CsaMLO1 and CsaMLO11 was screened in silico in sequenced cucumber germplasm. Results: Heterologous overexpression of all three CsaMLO genes in the tomato mlo mutant restored susceptibility to PM caused by Oidium neolycopersici, albeit to a different extent: whereas overexpression of CsaMLO1 or CsaMLO8 completely restored susceptibility, overexpression of CsaMLO11 was only partially able to restore PM susceptibility. Furthermore, it was observed by qRT-PCR and RNA-seq that CsaMLO8 was significantly higher expressed in non-inoculated cucumber compared to the other two MLO genes. However, inoculation with P. xanthii led to upregulation of CsaMLO1, but not to upregulation of CsaMLO8 or CsaMLO11. Conclusions: Both CsaMLO1 and CsaMLO11 are functional susceptibility genes, although we conclude that based on the transcript abundance CsaMLO8 is probably the major clade V MLO gene in cucumber regarding providing susceptibility to PM. Potential loss-of-function mutations in CsaMLO1 and CsaMLO11 have not been identified. The generation and analysis of such mutants are interesting subjects for further investigation.

Effectoromics-based identification of cell surface receptors in potato
Domazakis, Emmanouil ; Lin, Xiao ; Aguilera-Galvez, Carolina ; Wouters, Doret ; Bijsterbosch, Gerard ; Wolters, Pieter J. ; Vleeshouwers, Vivianne G.A.A. - \ 2017
In: Plant Pattern Recognition Receptors / Shan, Libo, He, Ping, Humana Press Inc. (Methods in Molecular Biology ) - ISBN 9781493968596 - p. 337 - 353.
Agroinfiltration - Apoplastic effector - Effectoromics - Genetic mapping - Pattern recognition receptors (PRRs) - Protein infiltration - PVX agroinfection - Solanum - Yeast protein production

In modern resistance breeding, effectors have emerged as tools for accelerating and improving the identification of immune receptors. Effector-assisted breeding was pioneered for identifying resistance genes (R genes) against Phytophthora infestans in potato (Solanum tuberosum). Here we show that effectoromics approaches are also well suitable for identifying pathogen recognition receptors (PRRs) that recognize apoplastic effectors. To detect genotypes that recognize apoplastic proteins of P. infestans, routine agroinfiltration and potato virus X (PVX) agroinfection methods can be applied. In addition, protein infiltrations are feasible for assessing responses to apoplastic effectors and aid in confirming results obtained from the aforementioned methods. Protocols for the effectoromics pipeline are provided, starting from phenotyping for effector responses, up to genotyping and PRR gene identification.

Elicitin recognition confers enhanced resistance to Phytophthora infestans in potato
Du, J. ; Verzaux, E.C. ; Chaparro-Garcia, A. ; Bijsterbosch, G. ; Keizer, L.C.P. ; Zhou, J. ; Liebrand, T.W.H. ; Xie, C. ; Govers, F. ; Robatzek, S. ; Vossen, E.A.G. van der; Jacobsen, E. ; Visser, R.G.F. ; Kamoun, S. ; Vleeshouwers, V.G.A.A. - \ 2015
Nature Plants 1 (2015). - ISSN 2055-026X - 5 p.
Potato late blight, caused by the destructive Irish famine pathogen Phytophthora infestans, is a major threat to global food security1,2. All late blight resistance genes identified to date belong to the coiled-coil, nucleotide-binding, leucine-rich repeat class of intracellular immune receptors3. However, virulent races of the pathogen quickly evolved to evade recognition by these cytoplasmic immune receptors4. Here we demonstrate that the receptor-like protein ELR (elicitin response) from the wild potato Solanum microdontum mediates extracellular recognition of the elicitin domain, a molecular pattern that is conserved in Phytophthora species. ELR associates with the immune co-receptor BAK1/SERK3 and mediates broad-spectrum recognition of elicitin proteins from several Phytophthora species, including four diverse elicitins from P. infestans. Transfer of ELR into cultivated potato resulted in enhanced resistance to P. infestans. Pyramiding cell surface pattern recognition receptors with intracellular immune receptors could maximize the potential of generating a broader and potentially more durable resistance to this devastating plant pathogen.

Potato (Solanum tuberosum L.) is the most important non-grain food crop and a major source of calories for the world's poor5. Increasing potato production is critical to prevent global malnutrition and hunger in an era of expanding world population. Unfortunately, potato suffers from the devastating late blight disease, which is caused by the notorious oomycete pathogen Phytophthora infestans. To limit losses to late blight, potato breeders rely on fungicide treatment and breeding of disease resistance (R) genes, all of which identified thus far belong to the coiled-coil, nucleotide-binding, leucine-rich repeat (CC-NB-LRR) class of immune receptors. These intracellular proteins recognize pathogen avirulence (Avr) proteins of the RxLR class of effectors to mount defence responses. However, RxLR effectors display high evolutionary rates4, and as a result, P. infestans can rapidly circumvent recognition by intracellular R immune receptors, thereby limiting the development of sustainable and durable genetic resistance. Therefore, novel types of immune receptors that recognize a broader spectrum of pathogen molecules are needed.

To fend off pathogens, plants rely on two classes of immune receptors that either reside inside the plant cell (NB-LRRs) or on the cell surface. The first line of defence is initiated by surface receptors, also called pattern recognition receptors (PRRs). PRRs are characteristically receptor-like proteins (RLPs), such as Ve1/Ve26, Cfs7,8 and LeEIX19, or receptor-like kinases (RLKs), such as FLS2, EFR and XA2110. PRRs typically recognize conserved pathogen-associated molecular patterns (PAMPs)11. So far, only a relatively few cell surface receptors against agronomically important pathogens have been identified.

Elicitins are structurally conserved extracellular proteins in Phytophthora and Pythium pathogen species (Pfam PF00964)12,​13,​14. P. infestans contains six elicitin genes that are conserved among different strains13,15. Elicitins are recognized as oomycete PAMPs but their intrinsic function in oomycetes is to bind lipids. Some elicitins sequester sterols from plants, thereby fulfilling an important biological function in Phytophthora and Pythium species that cannot synthesize sterols12. Targeting such conserved ‘Achilles heel’ proteins of pathogens is expected to lead to a more broad-spectrum resistance.

To identify novel types of potential immune receptors against the potato late blight pathogen, we initiated the cloning of ELR, a gene that determines response to elicitins16. We screened a collection of wild Solanum germplasm by Potato virus X (PVX) agroinfection for responses to INF1, a secreted elicitin of P. infestans (Fig. 1a). Solanum microdontum genotype mcd360-1 consistently responded to INF1 with a cell death response (Fig. 1b). We crossed mcd360-1 with S. microdontum ssp. gigantophyllum gig714-1 that does not respond to INF1. The F1 population segregated for response to INF1 in a 1:1 ratio, which suggests that ELR is a single dominant gene.
The receptor-like protein ELR is providing a novel layer of resistance to Phytophthora infestans in potato
Du, J. ; Verzaux, E.C. ; Bijsterbosch, G. ; Chaparro-Garcia, A. ; Jacobsen, E. ; Visser, R.G.F. ; Kamoun, S. ; Vleeshouwers, V.G.A.A. - \ 2012
Qualitative and quantitative late blight resistance in the potato cultivar Sarpo Mira is determined by the perception of five distinct RXLR effectors
Rietman, H. ; Visser, R.G.F. ; Vossen, J.H. ; Bijsterbosch, G. ; Cano, L.M. ; Jacobsen, E. ; Kamoun, S. ; Lee, H.R. ; Vleeshouwers, V.G.A.A. - \ 2012
Molecular Plant-Microbe Interactions 25 (2012)7. - ISSN 0894-0282 - p. 910 - 919.
phytophthora-infestans - disease resistance - solanum-tuberosum - hypersensitive response - maximum-likelihood - general resistance - field-resistance - genome sequence - central mexico - toluca valley
Potato defends against Phytophthora infestans infection by resistance (R)-gene-based qualitative resistance as well as a quantitative field resistance. R genes are renowned to be rapidly overcome by this oomycete, and potato cultivars with a decent and durable resistance to current P. infestans populations are hardly available. However, potato cultivar Sarpo Mira has retained resistance in the field over several years. We dissected the resistance of 'Sarpo Mira' in a segregating population by matching the responses to P. infestans RXLR effectors with race-specific resistance to differential strains. The resistance is based on the combination of four pyramided qualitative R genes and a quantitative R gene that was associated with field resistance. The qualitative R genes include R3a, R3b, R4, and the newly identified Rpi-Smira1. The qualitative resistances matched responses to avirulence (AVR)3a, AVR3b, AVR4, and AVRSmira1 RXLR effectors and were overcome by particular P. infestans strains. The quantitative resistance was determined to be conferred by a novel gene, Rpi-Smira2. It was only detected under field conditions and was associated with responses to the RXLR effector AvrSmira2. We foresee that effector-based resistance breeding will facilitate selecting and combining qualitative and quantitative resistances that may lead to a more durable resistance to late blight.
Environmental cost-benefit analysis of alternative timing strategies in greenhouse gas abatement: A data envelopment analysis approach
Kuosmanen, T.K. ; Bijsterbosch, N. ; Dellink, R.B. - \ 2009
Ecological Economics 68 (2009)6. - ISSN 0921-8009 - p. 1633 - 1642.
eco-efficiency - shadow prices - model
Assessing the benefits of climate policies is complicated due to ancillary benefits: abatement of greenhouse gases also reduces local air pollution. The timing of the abatement measures influences both the economic costs and ancillary benefits. This paper conducts efficiency analysis of ten alternative timing strategies, taking into account the ancillary benefits. We apply the approach by Kuosmanen and Kortelainen [Valuing Environmental Factors in Cost-Benefit Analysis Using Data Envelopment Analysis, Ecological Economics 62 (2007), 56–65], which does not require prior valuation of the environmental impacts. The assessment is based on synthetic data from a dynamic applied general equilibrium model calibrated to The Netherlands. Our assessment shows that if one is only interested in GHG abatement at the lowest economic cost, then equal reduction of GHGs over time is preferred. If society is willing to pay a premium for higher ancillary benefits, an early mid-intensive reduction strategy is optimal.
StGA2ox1 is induced prior to stolon swelling and controls GA levels during potato tuber development
Kloosterman, B.A. ; Navarro, C. ; Bijsterbosch, G. ; Lange, Theo ; Prat, S. ; Visser, R.G.F. ; Bachem, C.W.B. - \ 2007
The Plant Journal 52 (2007)2. - ISSN 0960-7412 - p. 362 - 373.
differential gene-expression - gibberellin metabolism - phytochrome-b - solanum-tuberosum - transcript levels - 20-oxidase gene - in-vitro - tuberization - biosynthesis - antirrhinum
The formation and growth of a potato (Solanum tuberosum) tuber is a complex process regulated by different environmental signals and plant hormones. In particular, the action of gibberellins (GAs) has been implicated in different aspects of potato tuber formation. Here we report on the isolation and functional analysis of a potato GA 2-oxidase gene (StGA2ox1) and its role in tuber formation. StGA2ox1 is upregulated during the early stages of potato tuber development prior to visible swelling and is predominantly expressed in the subapical region of the stolon and growing tuber. 35S-over-expression transformants exhibit a dwarf phenotype, reduced stolon growth and earlier in vitro tuberization. Transgenic plants with reduced expression levels of StGA2ox1 showed normal plant growth, an altered stolon swelling phenotype and delayed in vitro tuberization. Tubers of the StGA2ox1 suppression clones contain increased levels of GA20, indicating altered GA metabolism. We propose a role for StGA2ox1 in early tuber initiation by modifying GA levels in the subapical stolon region at the onset of tuberization, thereby facilitating normal tuber development and growth.
Solvency effects in polymer layers studied by electrokinetics : PVME as homopolymer and in an anionic diblock copolymer adsorbed on SiO2
Laat, A.W.M. de; Bijsterbosch, H.D. ; Cohen Stuart, M.A. ; Fleer, G.J. ; Struijk, C.W. - \ 2000
Colloids and Surfaces. A: Physicochemical and Engineering Aspects 166 (2000). - ISSN 0927-7757 - p. 79 - 89.
The adsorption from aqueous solution of polyvinylmethylether (PVME) and poly(vinylmethylether)-block-poly(vinyloxy-4-butyric acid) (PVME-b-PVOBA) on SiO2 and the effect on the electrophoretic mobility of SiO2 particles was studied at different temperatures and pH values. The temperature range included the lower critical solution temperature (LCST) of PVME, above which the polymer becomes insoluble. The negatively charged block copolymer starts to form micelles a few degrees above the LCST of PVME. Upon adsorption of PVME below the LCST, the SiO2 particles showed a decreasing electrophoretic mobility with increasing PVME dosage. With increasing temperature, a sharp increase in the mobility occurs at the LCST of PVME. At temperatures above the LCST, the mobility is about the same as of the uncovered particles. The PVME-b-PVOBA block copolymer adsorbs on SiO2 with its PVME block, while the negatively charged PVOBA block protrudes into the solution. SiO2 dispersions with PVME-b-PVOBA at neutral pH and below the temperature where micelles are formed (Tmic) have a constant value for the mobility at a level below that of the bare particles, independent of polymer concentration. A large increase in mobility occurs at Tmic leading to mobilities which are higher than of the uncovered particles. Probably, surface micelles and a highly charged brush are formed. No change in the mobility occurs at the LCST of PVME, which is a few degrees below Tmic. The adsorption of PVME-b-PVOBA dominates the mobility of the SiO2 particles even at low pH values. The difference in effect on the mobility of SiO2 particles with PVME and PVME-b-PVOBA is ascribed to the presence of charged groups in the PVOBA block which are expected to be located near the slipping plane.
Non-equilibrium adsorption behavior carboxymethyl cellulose on mineral surfaces
Keizer, A. de; Hoogendam, C.W. ; Cohen Stuart, M.A. ; Bijsterbosch, B.H. - \ 1999
In: Book of Abstracts, 217th ACS National Meeting, Anaheim, Calif., March 21-25 Washington : American Chemical Society - p. 114 - 114.
The adsorption behavior of the CM-cellulose (CMC) on some inorg.oxide surfaces in aq.soln.has been studied systematically.CMC is a neg.charged polyelectrolyte with a persistence length (stiffness of the chain) of about 16 nm.The general trends are th
Effect of block and graft copolymers on the stability of colloidal silica
Bijsterbosch, H.D. ; Cohen Stuart, M.A. ; Fleer, G.J. - \ 1999
Journal of Colloid and Interface Science 210 (1999). - ISSN 0021-9797 - p. 37 - 42.
We use dynamic light scattering to measure the time-dependent increase of the average hydrodynamic radius of colloidal silica particles in the presence of salt. This increase appears to be linear in time up to twice the radius of unaggregated particles. The method is a very useful tool in monitoring the stabilizing effect of adsorbed polymers. Four different series of diblock and graft copolymers were used to stabilize an aqueous silica dispersion against aggregation by salt. For two series of nonselectively adsorbing diblock copolymers with relatively low adsorbed amounts, we found a clear correlation between the adsorbed amount and the stabilization: a higher adsorbed amount leads to slower aggregation and thus provides a better stabilization. This is rationalized in terms of retarded bridge formation. Excellent steric stabilization was obtained with a series of amphiphilic diblock copolymers and with two graft copolymers. All these polymers produce adsorbed layers with a clear "anchor-buoy" structure, either due to selective solvency (amphiphilic diblocks) or selective adsorption (graft copolymers). Such layers cannot form bridges so that the layers are purely repulsive.
Adsorption and desorption of cellulose derivatives
Hoogendam, C.W. - \ 1998
Agricultural University. Promotor(en): B.H. Bijsterbosch; Martien Cohen Stuart; A. de Keizer. - S.l. : Hoogendam - ISBN 9789054858812 - 149
cellulose - carboxymethylcellulose - oppervlakte-interacties - oppervlaktechemie - surface interactions - surface chemistry
<p>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.</p><p>In <strong>chapter 1</strong> 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.</p><p>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 <sub>2</sub> COONa). Samples were subjected to a random cleavage reaction with hydrogen peroxide yielding samples with molar masses ranging from M <sub>w</sub> =30 to 10 <sup>3</SUP>kg mol <sup>-1</SUP>. 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 <strong>chapter 2</strong> . <strong></strong> Size exclusion chromatography separates molecules according to their size.</p><p>The radius of gyration (R <sub>g</sub> ) and the molar mass (M <sub>w</sub> ) 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 <sup>-1</SUP>NaNO <sub>3</sub> . 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 <sup>-1</SUP>NaNO <sub>3</sub> in comparison with 0.02 mol l <sup>-1</SUP>NaNO <sub>3</sub> . 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.</p><p>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 <sub>g</sub> . 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 <sub>g</sub> were obtained from the non-linear extrapolation method). A meaningful parameter in these models is the persistence length (L <sub>p</sub> ) of a polymer, L <sub>p</sub> 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 <sub>p0</sub> ) 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 <sub>pe</sub> ). Using the electrostatic wormlike chain theory, L <sub>p0</sub> 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 <sub>p0</sub> does not depend on ds. The difference in L <sub>p0</sub> between both models arises from the fact that in the Odijk model the contribution of L <sub>pe</sub> to L <sub>p</sub> 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.</p><p>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 <sub>p0</sub> 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 <sub>p0</sub> =5.9 nm). The difference is probably related to an incorrect evaluation of the electrostatic energy in Katchalsky's model.</p><p>In <strong>chapter 3</strong> 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.</p><p>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 &gt; 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.</p><p>Summing the contributions over the entire trajectory yields the barrier for adsorption R <sub>b</sub> . 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.</p><p>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 <sup>-1</SUP>) an extremely long time is needed (<around 10 <sup>15</SUP>s), at a moderate concentration (0.2 mol l <sup>-1</SUP>) it takes about 10 s. Hence, compared to the time scale of an experiment (around 10 <sup>5</SUP> s) adsorption equilibrium will not be accomplished for low electrolyte concentrations.</p><p>The adsorption of carboxymethyl cellulose on rutile (TiO <sub>2</sub> ) and hematite (α-Fe <sub>2</sub> O <sub>3</sub> ) is discussed in <strong>chapter 4</strong> . 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 <sub>NaCl</sub> = 0.01 to 1 mol l <sup>-1</SUP>) , molar mass (M <sub>w</sub> =35 to 1200 kg mol <sup>-1</SUP>) 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 <sub>w</sub> , 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.</p><p>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.</p><p>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 <sup>5</SUP> 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 <sub>exc</sub> ) 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 <sub>exc</sub> . 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.</p><p>In <strong>chapter 5</strong> 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 <sub>NaCl</sub> =0.01 and 0.5 mol l <sup>-1</SUP>) by means of reflectometry.</p><p>The adsorption of HEC on SiO <sub>2</sub> 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 <sup>-1</SUP>NaCl, albeit in the latter case the adsorption is higher. At pH &gt; 5 the adsorption decreases, which is most pronounced at the high salt concentration, reaching the level of zero adsorption at pH≈9. On TiO <sub>2</sub> the adsorption decreases monotonously with pH in 0.01 mol l <sup>-1</SUP>NaCl. At high salt concentration it is constant up to pH=10, beyond which it decreases rapidly. The adsorption of HEC on SiO <sub>2</sub> is facilitated by hydrogen bonding between HEC ether groups and Si-OH surface groups, whilst the mechanism on TiO <sub>2</sub> 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 <sub>2</sub> 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).</p><p>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 <sup>-1</SUP>NaCl, whereas in 0.5 mol l <sup>-1</SUP>NaCl 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 <sup>-1</SUP>NaCl both on SiO <sub>2</sub> and TiO <sub>2</sub> 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 <sub>2</sub> in 0.5 mol l <sup>-1</SUP>NaCl is of the screening-enhanced type up to pH ≈10, whereas at higher pH it is of the screening-reduced type. On TiO <sub>2</sub> the adsorbed amount is low and does not depend on pH.</p><p>In <strong>chapter 6</strong> the diffusion of spherical silica particles (with radii ranging from 12 to 510 nm) in dilute CMC solutions (M <sub>w</sub> =180 to 1200 kg mol <sup>-1</SUP>, c <sub>CMC</sub> =5 to 1000 mg l <sup>-1</SUP>) 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 <sub>w</sub> =180 kg mol <sup>-1</SUP>the effective viscosity comes rather close to the bulk viscosity.</p><p>The thickness of the CMC/HEC layer adsorbed on Fe <sub>2</sub> O <sub>3</sub> /SiO <sub>2</sub> 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.</p><p>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.</p>
Copolymer adsorption and the effect on colloidal stability
Bijsterbosch, H.D. - \ 1998
Agricultural University. Promotor(en): Martien Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789054857907 - 139
adsorptie - membranen - colloïden - oppervlakteverschijnselen - adsorption - membranes - colloids - surface phenomena
The main aim of the work described in this thesis is to study the effect of different types of copolymers on the stability of aqueous oxide dispersions. Such dispersions are a major component in water-borne paints. In order to obtain a better insight in steric stabilisation we first investigated the relation between the adsorbed amount and layer thickness, and paid attention to the effect of the type of copolymer on the adsorbed amount. We also studied the adsorption kinetics as these are relevant for industrial purposes.<p>An introduction on steric stabilisation is given in Chapter 1. For block copolymers the solvent may be <em>non-selective</em> or <em>selective.</em> In a non-selective solvent both blocks are solvated and the polymer molecules are likely to be in a non-aggregated conformation. However, in a selective solvent the molecules form micelles in which the non-soluble blocks are clustered together, surrounded by a layer of solubilised chains. The adsorption kinetics are expected to be affected by the existence of such micelles. Another important feature for the adsorption of block copolymers is the selectivity of the surface. When only one of the blocks has affinity for the surface this will give rise to <em>selective adsorption.</em> On the other hand, the adsorption of a block copolymer in which both blocks have affinity for the surface is <em>non-selective.</em> The resultant polymer layer will differ for both cases. In thesis we studied selective and non-selective adsorption from a selective and a non-selective solvent. As the architecture of the copolymers is also relevant we paid attention to the adsorption of both block copolymers and graft copolymers.<p>In Chapter 2 we describe the properties of spread monolayers of polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers at the air-water interface. The surface pressure and the thickness of the layer were measured as a function of the adsorbed amount. The thickness was determined with neutron reflectivity measurements.<p>Upon compression of the polymer monolayer the surface pressure increases over the entire experimental range of compression. At low coverage the adsorbing PEO block forms a flat "pancake" structure at the surface. When the surface area per molecule is decreased the PEO is pushed out of the surface layer into the solution to form a "cigar" or "brush" structure, which is firmly anchored by the PS block. Some scaling analysis have suggested that this desorption occurs as a first-order surface phase transition. When the polymer layer is compressed further, so that the surface density σincreases, the chains stretch and the thickness H of the layer increases too. Theories predict that H scales as Nσ <sup>1/3</SUP>, where N is the number of monomers per polymer chain. This is confirmed by our results. However, our experimental data do not show the first-order surface phase transition between pancake and brush. Numerical self-consistent-field calculations also show a gradual transition rather than a first-order phase transition.<p>In Chapter 3 we present a study on the non-selective adsorption of two series of diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2oxazoline)- poly(ethylene oxide), from aqueous solution on a macroscopically flat silicium oxide surface. The adsorbed amounts in this study, and in that of Chapters 4 and 5, were measured with an optical reflectometer in an impinging jet flow cell. The hydrodynamic layer thickness was determined by dynamic light scattering.<p>The different blocks in the copolymers all have affinity for the silica surface. In all cases there is a small difference between the segmental adsorption energies of the two blocks, giving rise to non-selective adsorption of the block copolymers. For the two types of block copolymers used in this study, the adsorbed amount as a function of block copolymer composition shows a shallow maximum; at this maximum the longest block is also the more strongly adsorbing block. The same trend is found for the hydrodynamid layer thickness. These findings differ from theoretical predictions concerning selective adsorption, where a pronounced maximum is found for a short anchor block. With numerical self-consistent field calculations we demonstrate that the same trends as in our experimental findings can be predicted by theory. In non-selective adsorption of diblock copolymers, with a small difference between the adsorption energies of the blocks, both blocks compete for the same adsorption sites on the surface. When the blocks are incompatible they try to avoid each other, which promotes an anchor-buoy structure. These factors then give rise to a maximum in the adsorbed amount as a function of the block copolymer composition. At this maximum the longest block is also the more strongly adsorbing block. The adsorbed layer has the typical anchor-buoy structure which is necessary for an effective steric stabilisation, but this structure is less pronounced than for selective adsorption.<p>The kinetics of adsorption of diblock copolymers can be very slow if the polymers form micelles in solution. In Chapter 4 we compare the experimental adsorption rates on silica and titania with the theoretical flux of copolymer molecules towards the surface for four poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) diblock copolymers with the same block length ratio but different molar masses. In aqueous solution these block copolymers form large polydisperse micelles with a very low critica <strong>l</strong> micellisation concentration (lower than 2 mg 1-1).<p>On both surfaces the adsorption behaviour is governed by the anchoring of the hydrophobic siloxane blocks The adsorption kinetics are affected by the exchange rate of free polymer molecules between micelles and solution. For the three smallest molar masses the exchange rate is fast compared to the time a micelle needs to diffuse across the diffusive layer. Before the micelles arrive at the surface they have already broken up into free polymers. Because the cmc is very low, the experimental adsorption rate is determined by the diffusion of micelles towards the surface. For the longest polymer this is not the case: the exchange of polymer molecules between micelles and solution is now relatively slow. As the micelles do not adsorb directly, the adsorption rate is retarded by the slow exchange process. We were able to make an estimate of the micellar relaxation time, i.e., the time a micelle needs to break up. For the largest polymer the relaxation time is of the order of a few tens of seconds. The other polymers have a micellar relaxation time that is shorter than roughly one second.<p>The adsorption increases linearly as a function of time, up to very high adsorbed amounts where it reaches a plateau. Such high adsorbed amount is expected for strongly (and selectively) adsorbing diblock copolymers with a relatively short anchor block. The adsorbed amount on silica is considerably higher than on titania. The reason is probably that the hydrophobic block is more strongly anchored to a silica surface than to titania, so that the density of the adsorbed layer can become higher on silica.<p>In Chapter 5 we investigate the interfacial behaviour of graft or comb copolymers. We compare the adsorption of graft copolymers with an adsorbing backbone and nonadsorbing side chains to the reverse situation of adsorbing side chains and a nonadsorbing backbone. Two high- molar-mass poly(acryl amide)-graft-poly(ethylene oxide) copolymers with different side chain densities were used in this study.<p>On titania only the backbone of these polymers adsorbs and the side chains do not. The adsorbed amount is then about the same as that found for the homopolymer without side chains. On the other hand, on silica the side chains adsorb and the backbone does have no affinity for the surface. For both polymer samples we observe a maximum in the adsorbed amount as a function of time ("overshoot"), after which the adsorbed amount decreases and a plateau is reached. The plateau adsorbed amount on silica is much higher than on titania and also much higher than for both types of homopolymers. Upon adsorption the graft copolymers initially adopt a conformation in which only part of the side chains are adsorbed. Following the overshoot, the graft copolymers show a decrease in the total adsorbed amount. The overshoot depends on the polymer concentration, which suggests that it is not caused by conformational changes in the adsorbed layer but by an exchange process between surface and solution.<p>Differences in graft distribution and graft density in the polymer sample are probably responsible for the displacement of adsorbed chains by polymer molecules from solution. The average number of grafts per molecule is rather low in our polymer samples. On statistical grounds there is probably an appreciable polydispersity in graft distribution and in graft density. Molecules in which the grafts are clustered to some extend can displace molecules with more regularly separated grafts, and molecules with a high graft density can displace those with a lower number of side chains. The newly arriving molecules can then adsorb in a flatter conformation with a lower adsorbed amount as the extra loss in conformational entropy is compensated by the gain in adsorption energy.<p>The effect of the polymers used in Chapters 3 to 5 on the stability of an aqueous silicium oxide dispersion is described in Chapter 6. The time-dependent increase of the average hydrodynamic radius of silicium oxide aggregates in the presence of electrolyte was measured. The increase of this radius with time is a measure of the aggregation rate of the dispersion. The effect of polymers on the stability of a dispersion was studied by adding polymer to the dispersion and recording the effect in the aggregation rate<p>Comparison of the aggregation rate of this "protected" silica with that of uncovered silica particles gives then an indication of the steric stabilisation by the adsorbing polymers.<p>Four different series of diblock and graft copolymers were used in these stability measurements. For two series of non-selectively adsorbing diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2-oxazoline)poly(ethyiene oxide), we find a good correlation between the adsorbed amount and the stabilising effect. A higher adsorbed amount provides a better steric stabilisation. Nevertheless, for these polymers the adsorbed amounts are not high enough (up to about 1.2 mg M <sup>-2</SUP>) to protect the dispersion completely against aggregation. A series of amphiphilic diblock copolymers of poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) with very high adsorbed amounts (between 3.5 and 8 mg M <sup>-2</SUP>) give excellent steric stabilisation of the dispersion. Adsorbed layers of the two graft copolymers of poly(acryl amide)-poly(ethylene oxide), with a non-adsorbing backbone and adsorbing side chains, are also effective in preventing the silica from aggregating. Even though the adsorbed amount of these graft copolymers is only around 1.3 mg M <sup>-2</SUP>, which is much lower than that of the amphiphilic polymers, aggregation is completely prevented.<p>The best steric stabilisation is found for those systems in which either the surface or the solvent is selective. In practical aqueous systems, however, it is difficult to synthesise diblock copolymers in which both blocks are soluble and where only one of the blocks has affinity for the surface. We have shown that copolymers with a different architecture, graft copolymers, also can provide good steric stabilisation and may be a good alternative to diblock copolymers. Very good steric stabilisers are amphiphilic diblock copolymers in a selective solvent. However, it is important that the hydrophobic blocks are flexible enough for fast adsorption kinetics and that they completely wet the surface. Which copolymer should be chosen for the steric stabilisation of a practical colloidal system depends largely on the nature of the particles and the solvent, and on the availability of suitable copolymers.
Adsorption of cellulose derivatives on inorganic oxides.
Hoogendam, C.W. ; Derks, I. ; Keizer, A. de; Cohen Stuart, M.A. ; Bijsterbosch, B.H. - \ 1998
Colloids and Surfaces. A: Physicochemical and Engineering Aspects 144 (1998). - ISSN 0927-7757 - p. 245 - 258.
Adsorption of hydroxyethyl cellulose (HEC) and quaternary ammonium substituted HEC (QNHEC) on silica and titanium dioxide has been investigated as a function of pH and electrolyte (NaCl) concentration. Adsorbed amounts have been determined by means of reflectometry. Adsorption of HEC on SiO2 is constant up to pH=5. At higher pH the adsorption decreases, which is most pronounced at high (0.5 mol l−1) electrolyte concentration. The thickness of the adsorbed layer, determined by dynamic light scattering, is substantial. This indicates an adsorbed layer with extended conformation having loops and (few) long tails protruding into the solution. Adsorption on TiO2 in 0.01 mol l−1 NaCl decreases monotonically with increasing pH. In 0.5 mol l−1 NaCl the adsorption is constant up to pH=10, beyond which it decreases rapidly. Mechanisms of binding to both surfaces leading to the observed adsorption behaviour are proposed. Electrostatics dominate the adsorption of QNHEC in 0.01 mol l−1 NaCl on both surfaces. The adsorbed amount increases linearly with pH up to pH=10. A decrease is observed for pH>11. The linearity is interpreted in terms of a molecular condenser which is composed of the surface layer and the polyelectrolyte in the first layer near the surface. In 0.5 mol l−1 NaCl adsorption on SiO2 is constant up to pH=5. A maximum is observed at pH=10. On TiO2 the adsorption is low.
Nonselective adsorption of block copolymers and the effect of block incompatibility.
Bijsterbosch, H.D. ; Cohen Stuart, M.A. ; Fleer, G.J. ; Caeter, P. van; Goethals, E.J. - \ 1998
Macromolecules 31 (1998). - ISSN 0024-9297 - p. 7436 - 7444.
The adsorbed amount and the hydrodynamic layer thickness of two series of block copolymers and the corresponding homopolymers on silica were determined. We used diblock copolymers of poly(vinyl methyl ether) and poly(2-ethyl-2-oxazoline) and tri- and diblock copolymers of poly(2-methyl-2-oxazoline) and poly(ethylene oxide). The diblock copolymers of poly(ethylene oxide) and poly(2-methyl-2-oxazoline) were obtained by polymerization of 2-methyl-2-oxazoline initiated by the tosylate of poly(ethylene glycol) monomethyl ether. The difference between the adsorption energies of the segments is found to be small: the block copolymer adsorption is nonselective. The adsorbed amount as a function of block copolymer composition shows a maximum at a composition where the longest block is also the strongest adsorbing block. The adsorbed amounts and the layer thicknesses are relatively low. Similar results are obtained with numerical self-consistent field calculations for nonselective adsorption for the case when the different blocks are incompatible. The typical anchor-buoy structure of the adsorbed layer is maintained, albeit less explicit than that found for selective adsorption.
Effective viscosity of polymer solutions: relation to the determination of the depletion thickness and thickness of the adsorbed layer of cellulose derivatives.
Hoogendam, C.W. ; Peters, J.C.W. ; Tuinier, R. ; Keizer, A. de; Cohen Stuart, M.A. ; Bijsterbosch, B.H. - \ 1998
Journal of Colloid and Interface Science 207 (1998). - ISSN 0021-9797 - p. 309 - 316.
The diffusion of silica particles with radii ranging from 12 to 510 nm in dilute solutions of carboxymethyl cellulose (Mw= 180 to 1200 kg mol−1,cCMC= 5 to 1000 mg l−1) was investigated by means of dynamic light scattering at pH 5 in 0.01 mol l−1NaCl. The viscosity of the polymer solution as experienced by the silica probes (the "microscopic" or effective viscosity, eff) differs from the viscosity as determined by capillary viscometry (p). For small particles effnearly equals the viscosity of the solvent (0). The effective viscosity increases with the size of the probe particles and the polymer concentration but remains less than p. The effective viscosity is interpreted in terms of a model in which the particle is surrounded by a layer of polymer free solution ( = 0). The thickness of the polymer-free layer is assumed to be equal to the thickness of the depletion layer (d). Applying this model, a decrease in das a function of CMC concentration is observed. At low concentration dequals the radius of gyration. The hydrodynamic layer thickness (h) of cellulose derivatives (carboxymethyl cellulose and hydroxyethyl cellulose) adsorbed on inorganic oxide surfaces (-Fe2O3and SiO2) is also investigated by dynamic light scattering. Upon using pmaxima in hare found. However, these maxima are a consequence of an incorrect choice of the viscosity. When the viscosity is used as obtained from inert probe diffusion, no anomalies are observed.
Adsorption of graft copolymers onto silica and titania.
Bijsterbosch, H.D. ; Cohen Stuart, M.A. ; Fleer, G.J. - \ 1998
Macromolecules 31 (1998). - ISSN 0024-9297 - p. 8981 - 8987.
The adsorption of graft copolymers of poly(acrylamide) (PAAm, backbone) and poly(ethylene oxide) (PEO, side chains) from aqueous solution onto silica and titania was studied with reflectometry. Two high-molar-mass copolymers were used with different PEO graft densities (10 and 18% w/w PEO in copolymers G10 and G18, respectively). On titania only the PAAm backbone adsorbs and the PEO does not. This results in adsorbed amounts of 0.83 and 0.85 mg m-2, respectively, which is about the same as that for a PAAm homopolymer. On silica the situation is reversed: now the PEO side chains adsorb and the PAAm backbone does not. The adsorption as a function of time shows a maximum, before the stable plateau is reached. The adsorbed amount on silica is much higher than that on titania: in the final plateau it is 1.35 and 1.2 mg m-2 for G18 and G10, respectively. On silica the polymers form longer loops and tails so that more molecules can be accommodated at the surface. The overshoot on silica depends on the polymer concentration, suggesting that it is not caused by a conformational change of the adsorbed layer but by exchange with polymer molecules from solution. Differences in graft distribution and graft density in the polymer sample are probably responsible for the displacement. The average number of grafts per polymer is rather low. On statistical grounds there should be an appreciable polydispersity in graft distribution and in graft density. Molecules in which the grafts are clustered in a few groups can displace molecules with more regularly separated grafts, and molecules with a high graft density can displace those with a lower number of side chains. The newly arriving molecules can then adsorb in a flatter conformation with a lower adsorbed amount as the extra loss in conformational entropy is compensated by the gain in adsorption energy.
Adsorption kinetics of diblock copolymers from a micellar solution on silica and titania.
Bijsterbosch, H.D. ; Cohen Stuart, M.A. ; Fleer, G.J. - \ 1998
Macromolecules 31 (1998). - ISSN 0024-9297 - p. 9281 - 9294.
The solution and adsorption behavior of a series of diblock copolymers of hydrophobic poly(dimethyl siloxane) and hydrophilic poly(2-ethyl-2-oxazoline) was studied. These block copolymers formed large polydisperse micelles in an aqueous solution. The critical micelle concentration was lower than 2 mg L-1. The adsorption kinetics of these polymers onto macroscopically flat oxide surfaces was studied with reflectometry in stagnation point flow. Both blocks of the copolymers had affinity for silica, and only the hydrophobic block had affinity for the titania surface. Nevertheless, the adsorption curves on silica and titania had similar features. The adsorption kinetics was affected by the exchange rate between micelles and free polymers. For short polymer chains the exchange rate was fast compared with the time necessary for diffusion across the diffusive layer. Before the micelles arrived at the surface, they had already broken up into unimers. Because the critical micelle concentration was very low, the experimental adsorption rate was determined by the diffusion of micelles toward the surface. This was not the case for the longest polymer chain; the exchange between micelles and unimers was relatively slow. The micelles did not adsorb directly, and the adsorption rate was determined by the exchange of polymers between micelles and solution. For all polymer samples the adsorption increases linearly as a function of time, up to very high adsorbed amounts where it reaches a plateau. The adsorbed amount on silica is considerably higher than found for titania. The poly(dimethyl siloxane) was anchored more strongly to the silica surface than to titania; the density of the adsorbed layer could therefore become higher.
Persistence length of carboxymethyl cellulose as evaluated from size exclusion chromatography and potentiometric titrations.
Hoogendam, C.W. ; Keizer, A. de; Cohen Stuart, M.A. ; Bijsterbosch, B.H. ; Smit, J.A.M. ; Dijk, J.A.P.P. van; Horst, P.M. van der; Batelaan, J.G. - \ 1998
Macromolecules 31 (1998). - ISSN 0024-9297 - p. 6297 - 6309.
The intrinsic persistence length of carboxymethyl cellulose (CMC) is determined by size exclusion chromatography in combination with multiangle laser light scattering (SEC-MALLS) as well as from potentiometric titrations. Samples with degree of substitution (ds) ranging from 0.75 to 1.25 were investigated. The relation between molar mass M and radius of gyration Rg as obtained by SEC-MALLS is determined in 0.02, 0.1, and 0.2 mol L-1 NaNO3. Using the electrostatic wormlike chain theory a bare (intrinsic) persistence length Lp0 of CMC is assessed at 16 nm, irrespective of the degree of substitution. A somewhat lower value (12 nm) is obtained when Odijk's theory for the description of polyelectrolyte dimensions is applied. The difference between Lp0 assessed from both models is discussed briefly. Potentiometric titrations were carried out in NaCl solutions (ranging from 0.01 to 1 mol L-1). From the titrations the radius of the CMC backbone was obtained by application of the model of a uniformly charged cylinder. The radius amounts to 0.95 nm for CMC ds = 0.75, and increases to 1.15 nm for CMC with ds = 1.25. The pK for the intrinsic dissociation constant of the carboxyl groups (i.e., at zero degree of dissociation) amounted to 3.2. Lp0 was also deduced from potentiometric titrations. A model developed by Katchalsky and Lifson, which relates the dissociation behavior of a polyelectrolyte to the stiffness of its chain, was applied to CMC. From analyses of the potentiometric titrations an intrinsic persistence length of 6 nm was deduced. The difference between Lp0 assessed from SEC-MALLS and potentiometric titrations is discussed briefly.
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