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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.

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    Adsorption of charged block copolymers: effect on colloidal stability.
    Israëls, R. ; Leermakers, F.A.M. ; Fleer, G.J. - \ 1995
    Macromolecules 28 (1995). - ISSN 0024-9297 - p. 1626 - 1634.
    Charged polymeric brushes: structure and scaling relations.
    Israëls, R. ; Leermakers, F.A.M. ; Fleer, G.J. ; Zhulina, E.B. - \ 1994
    Macromolecules 27 (1994). - ISSN 0024-9297 - p. 3249 - 3261.
    On the theory of grafted weak polyacids.
    Israëls, R. ; Leermakers, F.A.M. ; Fleer, G.J. - \ 1994
    Macromolecules 27 (1994). - ISSN 0024-9297 - p. 3087 - 3093.
    Adsorption of charged diblock copolymers : effect on colloidal stability
    Israels, R. - \ 1994
    Agricultural University. Promotor(en): G.J. Fleer, co-promotor(en): F.A.M. Leermakers. - S.l. : Israels - ISBN 9789054852315 - 100
    adsorptie - sorptie - kunststoffen - industrie - colloïden - dispersie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - colloids - dispersion - macromolecular materials

    In this thesis we present Scheutjens-Fleer (SF) calculations on the adsorption of diblock copolymers. More specifically, we restrict ourselves to adsorption at uncharged surfaces, while the specific type of block copolymers we consider have one uncharged adsorbing "anchor" block and one non-adsorbing charged "buoy" block. We compare these systems with a more simple one, that of the charged brushes. A polymer brush is the structure that is formed when polymer molecules are attached by one end to a surface, with a density high enough so that the chains are obliged to stretch away from the interface. Complementary to the numerical computations, the scaling behaviour of these systems is discussed. We study the structure of the adsorbed layer, and try to answer ultimately the question what the effect of the adsorption is on colloidal stability.

    In the introductory Chapter 1 we explain the most important terms and discuss the relevance of this study. Furthermore, we introduce the SF model and compare it to two other approaches: Monte Carlo and Scaling. Finally, we briefly present the available information on the two systems under consideration, and compare them to a number of related systems.

    The body of this work is divided in two parts. In Chapters 2 and 3 we discuss charged brushes, systems that are simpler than diblock copolymer adsorption, but still exhibit similar characteristics. In the subsequent two chapters we then proceed to the adsorption of diblock copolymers (Chapter 4) and its effect on colloidal stability (Chapter 5).

    In Chapter 2 we present numerical results from the SF model for the structure and sealing behaviour of charged brushes and compare these with predictions of an analytical model on the same system. The relevant parameters are the chain length N , the average anchoring density σ, the average segmental charge αon the chains, and the salt concentration φ S .

    At high anchoring densities, three regimes of brush behaviour may be distinguished. In the salt-free case, the behaviour of the brush is dominated by electrostatic interactions if the charges are high (the so-called Osmotic Brush) or by non-electrostatic excluded volume interactions if the charges are low (the quasi-Neutral Brush regime). Upon adding salt a third regime can be found: the Salted Brush. The behaviour in this regime, although resulting from electrostatic interactions, is very similar to that in a neutral brush and can effectively be described using an electrostatic excluded volume parameter vel ≈ φ S-1α2. We find excellent agreement regarding structure as well as scaling relations between the two theories in these three (high anchoring density ) regimes. At extremely low anchoring densities, the agreement with the analytical theory is less good. This is due to the breakdown at low densities of the mean-field approximationpresently used in the numerical model.

    In between, at intermediate anchoring densities, the analytical theory predicts a very peculiar regime, where the thickness H scales as H ≈N-1α2. This so-called " Pincus Brush ", named after the author who originally described it, is not recovered with the numerical theory. For the wide range of parameters used, we find the Pincus regime is too small to be detected. This is probably true for any reasonable set of parameters.

    In Chapter 3 we consider the acid-base equilibrium of the charged brush segments, so that grafted weak polyacids may be studied. For these systems the charge of a brush segment depends on its local environment and on the pH in the solution. The scaling dependence of the thickness H on the salt concentration φ S for such a brush is very different from that for a conventional charged brush with constant charge density.

    In Chapter 4 we proceed to the adsorption of ionic diblock copolymers. One block, the "anchor", consists of N A uncharged adsorbing A segments, whereas the "buoy" block has N B segments which carry a fixed charge and are non-adsorbing. Upon adsorption these sorbed amount and layer thickness as a function of the block lengths N A and N B , the charge αe on the B segments, and the salt concentration φ S in each of the four regimes. The scaling relations axe checked using SF calculations.

    The existence of two regimes for uncharged diblock copolymer adsorption has been reported previously. We argue that those HU and LU regimes are closely related to the two regimes HC and LC we find for charged molecules. Scaling relations can be translated from the uncharged to the corresponding charged regimes by replacing the excluded volume parameter v of the buoy segments by an effective electrostatic excluded volume parameter ve = α 2S .

    In the LC regime the chain density σscales as σ α( N A /N B ) 3/2ve-1and the layer thickness H as H α ( N A /N B ) 1/2. The latter scaling is independent of ve . Using the SF model, these relations axe found to be valid for an adsorbed amount of A segments below 10% of monolayer coverage.

    In the HC regime the adsorption is dominated by the anchoring block and the scaling relation σ α1/ N A for the chain density is identical to that for uncharged molecules. The SF calculations show that this regime will not be reached in practical situations.

    Finally, we address in Chapter 5 the effect of the adsorption of charged diblock copolymers on colloidal stability. Using again a scaling as well as the SF approach, we focus on the LC regime and find that the adsorbed layer may cause a significant repulsive interaction between two surfaces, despite the very low adsorbed amounts. The magnitude of this repulsion is well within the range that could be mea, sured using a surface force apparatus. Moreover, we estimate that the repulsive interaction may be strong enough to induce kinetic stability, provided the particle radius is large enough. Upon lowering the salt concentration, however, a critical concentration φ S * is reached eventually, below which the repulsion is no longer strong enough to effect colloidal stability. The scaling analysis predicts that this critical concentration scales as:
    φ S * ≈ N 2/ RN A3

    where R is the radius of the particles and the other parameters have been defined above. Thus the repulsive interaction decreases when the relative importance of charge effects increases, i.e., with decreasing salt concentration, and increasing buoy block length or buoy block charge. This counterintuitive behaviour can be explained from the effect that electrostatic interactions have on the adsorbed amount: stronger interactions lead to a lower adsorbed amount, which, in turn, leads to a weaker repulsion. The SF calculations confirm these scaling predictions.

    The lipid bilayer membrane and its interactions with additives
    Meijer, L.A. - \ 1994
    Agricultural University. Promotor(en): J. Lyklema; F.A.M. Leermakers. - Wageningen : Meijer - ISBN 9789054853367 - 162
    membranen - transport - bio-energetica - elektrische eigenschappen - kunstmatige membranen - membranes - transport - bioenergetics - electrical properties - artificial membranes

    The aim of this study was to make accurate predictions on the interaction of biologically relevant molecules with lipid bilayer membranes. We emphasised on the partitioning of these molecules between the membrane phase, and the aqueous phase quantified by the partition coefficient. To make detailed predictions a theory had to be set up along the lines of the self-consistent-field theory developed by Scheutjens and Fleer and extended by Evers, Leermakers, Van Lent, Böhmer, Barneveld, Israëls, Wijmans, Van der Linden, and others for (chain) molecules in inhomogeneous systems.

    As a first step towards this goal we have investigated bare membranes in chapter 1. A membrane of dimyristoylphosphatidylcholine (DMPC) has been used as a model for biological membranes, existing in plant and animals, in which this molecule is a major component. For comparison, membranes consisting of the anionic dimyristoylphosphatidylserine (DMPS) were also modelled. It was found that the zwitterionic phosphatidylcholine (PC) head group was laying on average flat on the membrane surface. This result is in line with experiments and in accordance with other theoretical calculations. At high salt concentrations however, two preferred conformations had to be distinguished, both about equally populated, one with the choline moiety closer to the water phase than the phosphate moiety and one the other way around. Due to the out of plane tilting of the head group, an electrostatic potential profile develops. The electrostatic potential is positive in the membrane centre and on the membrane surface, but negative in the middle of the head group region at the average position of the phosphate groups.

    The ionic head groups of DMPS are found tilted towards the aqueous solution. Counterions interpenetrate the head group region and compensate the charge to a large extent even at low ionic strength. At high salt concentrations ions are depleted from the head group space but, due to asymmetric depletion of anions and cations, charge compensation is still achieved.

    The variability of the PC head group orientation was investigated theoretically by attaching a hydrophilic chain to the choline moiety of DMPC. Varying the chain length had two effects: First, due to the interchain steric repulsion the head group area increased and therefore the dimension of the hydrophobic core decreased which eventually destabilised the membranes. Second, the head group orientation changed non-monotonously. Short chains attached to the choline moiety 'drag' it towards the water phase, while longer chains attached to it do not affect the average orientation of the dipole, which is parallel to the membrane surface. This is caused by the fact that, for longer hydrophilic chains, the most bulky part of the head group is located further from the hydrophobic core in the centre of the polymeric coil. This relaxes the packing constraints at the position of the choline. Hence the phosphate moieties and the electrostatics forces, that favour a flat conformation, meet less opposition.

    In the second chapter we concentrate on the interplay of the electrostatic potential profile across the membrane and the valence of the ions present in solution. From the calculations it can be concluded that the electrostatic interactions can explain the accumulation of charges in the head group area without introduction of specific chemical interactions between e.g. a divalent ion like calcium and the phosphate group.

    An important issue in the modelling of non-interacting, free-standing membranes is the proof that the modelled bilayers are thermodynamically the most stable structures. From thermodynamic arguments, it can be shown that the surface tension of these layers should vanish. This is a necessary, but not sufficient condition. For isolated, free-standing bilayers to be stable, the membranes should be mutually repulsive. The interaction between bilayers is the topic of chapter 3. In this chapter a thermodynamic derivation is presented of the various ways the interaction curve can be calculated from the self-consistent-anisotropic-field (SCAF) theory. The results show that three force-distance regimes can be distinguished for a DMPC bilayer in a moderate salt concentration: two repulsive regimes, one of electrostatic and one of steric origin flank an attractive one that was shown to be of entropic origin. The entropic attraction is caused by an increase in the number of head group conformations. At large separation the head groups are oriented mostly parallel to the membrane surface. Upon closer approach of two bilayers the head group conformation is allowed to change. The head group can now cross the gap between the bilayers without an electrostatic penalty. As a function of the screening of the electrostatic interactions we observe various changes in the interaction curves. At high salt concentration both the electrostatic repulsion and the entropic attraction become negligible. At low salt concentration the entropic attraction increases whereas, at the same time, the electrostatic repulsion vanishes. This last effect is caused by the perfect alignment of the head group parallel to the membrane surface so that virtually no charge separation occurs.

    Stretching the membranes increases the entropic attraction but decreases the electrostatic repulsion. We did not incorporate undulations in our theory so the decrease of undulations was not the phenomenon that caused the stress-induced tendency to adhere. In our model stretched membranes have a larger head group area which relaxes the head group packing constraints. This allows these groups to assume a position more parallel to the bilayer surface, leading to a reduced electrostatic repulsion and a stronger entropic attraction.

    Addition of non-ionic surfactants (dodecanol) to DMPC bilayers caused the membranes to grow thicker without changing the interaction curve. Ionic surfactants (e.g. dodecylammonium and dodecylsulphide) did not change the overall membrane thickness but made, due to a modification of the electrostatic interactions, the interaction profile completely repulsive. Cationic surfactants had a more pronounced effect than anionics. Cationics push the choline moieties outwards while the anionics pull these more inwards towards the membrane centre.

    Finally in chapter 4 the SCAF theory for molecules containing rigid structures is given. The coupling of the segment potential {u(z)} to the volume fraction profile {(p(z)} is in principle accomplished according to the following simple, basic method. First, all conformations of the molecules are generated and then the statistical weights of these conformations in the potential field are added and normalised. For flexible molecules, that can assume very many conformations, an efficient technique exists that is just doing this in one single operation, it generate the conformations, calculate the statistical weight, and add the results to obtain the densities (the propagator method). For rigid structures, that can assume relatively few conformations, the basic methods is already effective. A hybrid scheme was developed for partly rigid molecules, where the basic method for the rigid parts was combined with the propagator method for the flexible parts.

    Partition coefficients calculated for a number of linear and branched alcohols as well as for phenols were compared with measurements. Good quantitative comparison was found for these molecules. Trends known from literature, like the exponential dependence of the partition coefficient on the chain length in homologue series, were reproduced.

    To illustrate the possibilities of the theory some results were presented of calculations on three groups of molecules having the same zwitterionic isomer (C 22N+S -, containing a benzene-like structure). The calculations showed that in DMPC membranes the partition coefficient can change by a factor of ten depending on the molecular architecture. The positional and orientational data revealed that negatively charged units partition much more readily into the membrane core than positively charged segments do. This can be rationalised by the electrostatic potential profile which, as told above, is positive both in the centre and on the outskirts of the membrane while being negative at the average position of the phosphate segments.

    Calculations on a number of substituted tetrahydroxynaphthalenes showed that, with only small changes in the partition coefficient, large orientational and positional variations can be realised, changing from spanning the membrane for 2,3,6,7- tetrahydroxy naphthalene to parallel to the membrane surface positioned at the head group-hydrophobic core interface for 1,3,5,7-tetrahydroxy naphthalene. This kind of large orientational changes, while keeping the partition coefficient virtually constant, can be of great importance in the development and the improvement of new drugs, or in elucidating the working mechanism of existing ones.

    Our model has provided a detailed insight into the nature of model lipid membranes and will hopefully advance the development of products and contribute to the optimisation and interpretation of experiments in which lipid bilayers play a role. At present reasonable (semi) quantitative agreement with many experimental results have already been achieved. There are however cases where the present theory does not yet give good enough predictions. For this it is good to know that the theory can be readily extended to incorporate more details in the calculations.

    Theory of planar polyelectrolyte brush immersed in solution of asymmetric salt.
    Zhulina, E.B. ; Israëls, R. ; Fleer, G.J. - \ 1994
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 86 (1994). - ISSN 0927-7757 - p. 11 - 24.
    Theory of planar polyelectrolyte brush immersed in solution of asymmetric salt.
    Zhulina, E.B. ; Israëls, R. ; Fleer, G.J. - \ 1993
    In: Abstract Polymers at Interfaces; Bristol, UK (1993) Vol. 1
    Adsorption of ionic copolymers.
    Israëls, R. - \ 1993
    In: Abstract 4th Eur. Student Conf.; Loch Lomond, Scotland (1993)
    Adsorption of ionic block copolymers: self-consistent-field analysis and scaling predictions.
    Israëls, R. ; Scheutjens, J.M.H.M. ; Fleer, G.J. - \ 1993
    Macromolecules 26 (1993). - ISSN 0024-9297 - p. 5405 - 5413.
    Adsorption of charged block copolymers: scaling predictions and SCF calculations.
    Israëls, R. ; Fleer, G.J. - \ 1993
    In: Abstract 67th Colloid and Surface Symp : Am. Chem. Soc., Toronto, Canada - p. 140 - 140.
    Self avoiding random walks.
    Israëls, R. - \ 1991
    In: Abstract 3rd Int. Student Conf. Polymers, colloids and interfaces., Haamstede, The Netherlands - p. 44 - 44.
    Interaction between hairy surfaces and the effect of free polymer.
    Lent, B. van; Israels, R. ; Scheutjens, J.M.H.M. ; Fleer, G.J. - \ 1990
    Journal of Colloid and Interface Science 137 (1990). - ISSN 0021-9797 - p. 380 - 394.
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