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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    Interaction between lysozyme and humic acid in layer-by-layer assemblies: Effects of pH and ionic strength
    Tan, W.F. ; Norde, W. ; Koopal, L.K. - \ 2014
    Journal of Colloid and Interface Science 430 (2014). - ISSN 0021-9797 - p. 40 - 46.
    donnan model parameters - 2d nmr-spectroscopy - interpolyelectrolyte complexes - electrostatic interactions - optical reflectometry - adsorption-kinetics - proton binding - solid-surfaces - protein - polyelectrolyte
    The interaction between protein and soluble organic matter is studied through layer-by-layer assembly of lysozyme (LSZ) and purified Aldrich humic acid (PAHA) at a solid surface (2-D) and in solution (3-D). By bringing a silica surface in alternating contact with solutions of LSZ and PAHA a layer-by-layer LSZ-PAHA assembly is formed. At pH 5 the negative charge density of PAHA is about 3 times that of the positive LSZ; the layers of LSZ and PAHA are stable and the adsorbed amounts decrease with increasing ionic strength. The mass ratios PAHA/LSZ in the layers depend on the ionic strength; K+ incorporation is relatively large (similar to 25%) when PAHA is the outer layer of the assembly. At pH 6 and 8, and moderate ionic strength (0-100 mmol L-1 KCl) the assembly is accompanied by partial solubilization of positive LSZ by the much more negative PAHA followed by desorption of the complex. The solubilization increases with increasing pH, and decreases with increasing KCl concentration. At 400 mmol L-1 KCl the electrostatic interactions are so well screened that the assembly is no longer accompanied by layer erosion. Assembly of PAHA and LSZ in solution is also investigated at pH 5 and 5 mmol L-1 KCl. The PAHA/LSZ mass ratio at the iso-electric point of the assembly depends on the order of the addition. When LSZ is added to the negative assembly K+ is incorporated in the complex, but when PAHA is added to the positive assembly PAHA and LSZ neutralize each other. (C) 2014 Elsevier Inc. All rights reserved.
    Complex coacervate core micelles as diffusional nanoprobes
    Bourouina, N. ; Cohen Stuart, M.A. ; Kleijn, J.M. - \ 2014
    Soft Matter 10 (2014). - ISSN 1744-683X - p. 320 - 331.
    polymer-solutions - fluorescence recovery - diblock copolymer - probe diffusion - gels - polyelectrolyte - nanoparticles - microscopy - stability - glutaraldehyde
    Because of their ease of preparation and versatile modification opportunities, complex coacervate core micelles (C3Ms) may be a good alternative for expensive diffusional probes, such as dendrimers. However, C3Ms are unstable at high salt concentrations and may fall apart in contact with other polymers or (solid) materials. Therefore, we designed and characterized small (15 nm radius), stable fluorescent C3Ms. These were formed by electrostatic interactions between poly(ethylene oxide-methacrylic acid) (PEO–PMAA) and fluorescently labelled poly(allylamine hydrochloride) (PAH) and irreversible cross-linking of the core through amide bonds. We compared the properties of the cross-linked and non-cross-linked micelles. The radii of the two types of micelles were quite similar and independent of the ionic strength. Surprisingly, both were found to be stable at salt concentrations as high as 1.5 M. However, unlike the non-cross-linked C3Ms, the stability of the cross-linked C3Ms is independent of the pH. As a first example of their application as diffusional nanoprobes, we present results on the diffusion of the fluorescent micelles measured in xanthan solutions using fluorescence recovery after photobleaching (FRAP).
    A physical cross-linking process of cellulose nanofibril gels with shear-controlled fibril orientation
    Fall, A.B. ; Lindstrom, S.B. ; Sprakel, J.H.B. ; Wagberg, L. - \ 2013
    Soft Matter 9 (2013)6. - ISSN 1744-683X - p. 1852 - 1863.
    dynamic light-scattering - microfibrillated cellulose - nanocomposites - polyelectrolyte - mechanism - networks - modulus
    Cellulose nanofibrils constitute the smallest fibrous components of wood, with a width of approximately 4 nm and a length in the micrometer range. They consist of aligned linear cellulose chains with crystallinity exceeding 60%, rendering stiff, high-aspect-ratio rods. These properties are advantageous in the reinforcement components of composites. Cross-linked networks of fibrils can be used as templates into which a polymer enters. In the semi-concentrated regime (i.e. slightly above the overlap concentration), carboxy methylated fibrils dispersed in water have been physically cross-linked to form a volume-spanning network (a gel) by reducing the pH or adding salt, which diminishes the electrostatic repulsion between fibrils. By applying shear during or after this gelation process, we can orient the fibrils in a preferred direction within the gel, for the purpose of fully utilizing the high stiffness and strength of the fibrils as reinforcement components. Using these gels as templates enables precise control of the spatial distribution and orientation of the dispersed phase of the composites, optimizing the potentially very large reinforcement capacity of the nanofibrils.
    A case of adaptive self-assembly
    Ding, Y. ; Yang, Y. ; Yang, L. ; Yan, Yun ; Huang, J. ; Cohen Stuart, M.A. - \ 2012
    ACS Nano 6 (2012)2. - ISSN 1936-0851 - p. 1004 - 1010.
    metallo-supramolecular polymers - phase-transition - spin-crossover - polyelectrolyte - complex - ligand
    We report in this paper direct observation of redox-induced uptake of a charged species in micelles with a complex coacervate core, using a system consisting of negatively charged iron-coordination polymers and positively charged-b-neutral block co-polyelectrolytes. Neutral, charge-balanced micelles are first prepared by stoichiometric mixing of the oppositely charged components. Upon a redox stimulus, the micelles develop excess charges, which (as proposed in our previous work) most likely lead to sequestration of oppositely charged species, as the charge balance has to be restored. In this work we verify this prediction by using a rigid, rod-like iron coordination polymer, namely, the positively charged MEPE, as the species to be taken up. After uptake of this rigid cargo, the morphology of the micelles was found to transform from spheres to banana-shaped bundles and fibers, which clearly indicate the uptake of MEPE in the micellar core. Our result proves that the redox stimulus indeed induces excess charges in the core, which forces the self-assembled particles to change both composition and shape. As an interesting example of “adaptive self-assembly”, our findings also pave the way to novel redox-triggered uptake and release systems.
    Complex coacervate core micelles from iron-based coordination polymers
    Wang, J.Y. ; Keizer, A. de; Fokkink, R.G. ; Yan, Y. ; Cohen Stuart, M.A. ; Gucht, J. van der - \ 2010
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 114 (2010)25. - ISSN 1520-6106 - p. 8313 - 8319.
    diblock copolymer - block-copolymers - polyelectrolyte - nanoparticles - stability - lysozyme - ionomers - delivery - design - length
    Complex coacervate core micelles (C3Ms) from cationic poly(N-methyl-2-vinyl-pyridinium iodide)-b-poly(ethylene oxide) (P2MVP41-b-PEO205) and anionic iron coordination polymers are investigated in the present work. Micelle formation is studied by light scattering for both Fe(II)- and Fe(III)-containing C3Ms. At the stoichiometric charge ratio, both Fe(II)-C3Ms and Fe(III)-C3Ms are stable for at least 1 week at room temperature. Excess of iron coordination polymers has almost no effect on the formed Fe(II)-C3Ms and Fe(III)-C3Ms, whereas excess of P2MVP41-b-PEO205 copolymers in the solution can dissociate the formed micelles. Upon increasing salt concentration, the scattering intensity decreases. This decrease is due to both a decrease in the number of micelles (or an increase in CMC) and a decrease in aggregation number. The salt dependence of the CMC and the aggregation number is explained using a scaling argument for C3M formation. Compared with Fe(II)-C3Ms, Fe(III)-C3Ms have a lower CMC and a higher stability against dissociation by added salt.
    Multi-responsive reversible gels based on charge-driven assembly
    Lemmers, M. ; Sprakel, J.H.B. ; Voets, I.K. ; Gucht, J. van der; Cohen Stuart, M.A. - \ 2010
    Angewandte Chemie-International Edition 49 (2010)4. - ISSN 1433-7851 - p. 708 - 711.
    transfer radical polymerization - indirect fourier transformation - small-angle scattering - rheological properties - block-copolymers - methyl-methacrylate - chain-length - polyelectrolyte - micelles - polymers
    Linked in? Coassembly of an ABA triblock copolymer with charged end blocks and an oppositely charged polyelectrolyte yields gels that respond to changes in concentration, temperature, ionic strength, pH value, and charge composition. Above the critical gel concentration, the triblock copolymers bridge micelles, forming a sample-spanning transient network of interconnected micelles
    Towards a structural characterization of charge driven polymer micelles
    Voets, I.K. ; Vries, R.J. de; Fokkink, R.G. ; Sprakel, J.H.B. ; May, R.P. ; Keizer, A. de; Cohen Stuart, M.A. - \ 2009
    European Physical Journal E. Soft Matter and Biological Physics 30 (2009)4. - ISSN 1292-8941 - p. 351 - 359.
    diblock copolymer micelles - block-copolymers - core micelles - polyelectrolyte - complexes - solvent
    Light scattering and small-angle neutron scattering experiments were performed on comicelles of several combinations of oppositely charged (block co)polymers in aqueous solutions. Fundamental differences between the internal structure of this novel type of micelle --termed complex coacervate core micelle (C3Ms), polyion complex (PIC) micelle, block ionomer complex (BIC), or interpolyelectrolyte complex (IPEC)-- and its traditional counterpart, i.e., a micelle formed via self-assembly of polymeric amphiphiles, give rise to differences in scaling behaviour. Indeed, the observed dependencies of micellar size and aggregation number on corona block length, N corona , are inconsistent with scaling predictions developed for polymeric micelles in the star-like and crew-cut regime. Generic C3M characteristics, such as the relatively high core solvent fraction, the low core-corona interfacial tension, and the high solubility of the coronal chains, are causing the deviations. A recently proposed scaling theory for the cross-over regime, as well as a primitive first-order self-consistent field (SCF) theory for obligatory co-assembly, follow our data more closely
    Spontaneous symmetry breaking: formation of Janus micelles
    Voets, I.K. ; Fokkink, R.G. ; Hellweg, T. ; King, S.M. ; Waard, P. de; Keizer, A. de; Cohen Stuart, M.A. - \ 2009
    Soft Matter 5 (2009). - ISSN 1744-683X - p. 999 - 1005.
    mixed polymeric micelles - charged block-copolymers - coacervation core micelles - ellipsoidal molecules - diblock copolymer - aqueous-solution - polyelectrolyte - particles - complexes - separation
    We describe the preparation and solution properties of Janus micelles, i.e., non-centrosymmetric nanoparticles with compartmentalized shells, via co-assembly of two fully water-soluble block copolymers. They consist of a mixed core of poly(N-methyl-2-vinyl pyridinium iodide) (P2MVP) and poly(acrylic acid) (PAA), and a shell segregated into two sides, consisting of poly(ethylene oxide) (PEO) or poly(acryl amide) PAAm. These Janus particles form spontaneously and reversibly, i.e., association, dissocation, and reassociation can be carefully controlled via parameters, such as polymer mixing fraction, solution pH, and ionic strength. Dynamic (polarized and depolarized) and static light scattering, cryogenic transmission electron microscopy, small angle neutron and X-ray scattering, and two-dimensional nuclear magnetic resonance spectroscopy are used to monitor the micelle formation and to characterize the micellar structure. The Janus particles were found to be ellipsoidal, with a cigar-like overall shape and a disc-like core. This peculiar morphology is driven by the delicate interplay between two opposing forces: an attraction between the oppositely charged core blocks and a subtle repulsion between the water-soluble, neutral corona blocks.
    Spherocylindrical coacervate core micelles formed by a supramolecular coordination polymer and a diblock copolymer
    Yan, Y. ; Harnau, L. ; Besseling, N.A.M. ; Keizer, A. de; Ballauff, M. ; Rosenfeldt, S. ; Cohen Stuart, M.A. - \ 2008
    Soft Matter 4 (2008)11. - ISSN 1744-683X - p. 2207 - 2212.
    polyion complex micelles - block ionomer complexes - polyelectrolyte - scattering - nanoparticles - acid) - ph
    We investigated the hierarchical structure of complex coacervate core micelles formed by mixing a supramolecular coordination polymer and a diblock copolymer. Cryogenic transmission electron microscopy (cryo-TEM) on those systems was only possible for very dilute samples and suggested the existence of wormlike micelles or strings of spherical micelles (Y. Yan, N. A. M. Besseling, A. de Keizer, R. Fokkink, M. Drechsler and M. A. Cohen Stuart, J. Phys. Chem. B 2007, 111, 11662). Using small-angle X-ray scattering (SAXS) we investigated those mixtures with a concentration of up to 0.6 vol%. Depending on the charge ratio and the concentration the above mixtures was proven to build spherocylindrical objects of different sizes. In the case of charge neutrality spherical micelles are found. Thus, we confirmed by SAXS experiments that the structures seen by cryo-TEM indeed exist in solution
    Self-consistent field theory for obligatory coassembly
    Voets, I.K. ; Leermakers, F.A.M. - \ 2008
    Physical Review. E, Statistical nonlinear, and soft matter physics 78 (2008)6. - ISSN 1539-3755 - 15 p.
    coacervate core micelles - statistical thermodynamics - association colloids - diblock copolymer - aqueous-solutions - block-copolymers - polyelectrolyte - complexes
    We present a first-order model for obligatory coassembly of block copolymers via an associative driving force in a nonselective solvent, making use of the classical self-consistent field (SCF) theory. The key idea is to use a generic associative driving force to bring two polymer blocks together into the core of the micelle and to employ one block of the copolymer(s) to provide a classical stopping mechanism for micelle formation. The driving force is generated by assuming a negative value for the relevant short-range Flory-Huggins interaction parameter. Hence, the model may be adopted to study micellization via H bonding, acceptor-donor interactions, and electrostatic interactions. Here, we limit ourselves to systems that resemble experimental ones where the mechanism of coassembly is electrostatic attraction leading to charge compensation. The resulting micelles are termed complex coacervate core micelles (CCCMs). We show that the predictions are qualitatively consistent with a wide variety of experimentally observed phenomena, even though the model does not yet account for the charges explicitly. For example, it successfully mimics the effect of salt on CCCMs. In the absence of salt CCCMs are far more stable than in excess salt, where the driving force for self-assembly is screened. The main limitations of the SCF model are related to the occurrence of soluble complexes, i.e., soluble, charged particles that coexist with the CCCMs
    Temperature responsive complex coacervate core micelles with a PEO and PNIPAAm corona
    Voets, I.K. ; Moll, P.M. ; Aqil, A. ; Jerome, C. ; Detrembleur, C. ; Waard, P. de; Keizer, A. de; Cohen Stuart, M.A. - \ 2008
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 112 (2008)35. - ISSN 1520-6106 - p. 10833 - 10840.
    laser-light scattering - 2 diblock copolymers - block-copolymers - raft polymerization - aqueous-solutions - poly(n-isopropylacrylamide-co-acrylic acid) - n-isopropylacrylamide - polyelectrolyte - water - aggregation
    We report on the stability of complex coacervate core micelles, i.e., C3Ms (or PIC, BIC micelles), containing metal coordination polymers. In aqueous solutions these micelles are formed between charged-neutral diblock copolymers and oppositely charged coordination polymers formed from metal ions and bisligand molecules. The influence of added salt, polymer concentration, and charge composition was investigated by using light scattering and cryo-TEM techniques. The scattering intensity decreases strongly with increasing salt concentration until a critical salt concentration beyond which no micelles exist. The critical micelle concentration increases almost exponentially with the salt concentration. From the scattering results it follows that the aggregation number decreases with the square root of the salt concentration, but the hydrodynamic radius remains constant or increases slightly. It was concluded that the density of the core decreases with increasing ionic strength. This is in agreement with theoretical predictions and is also confirmed by cryo-TEM measurements. A complete composition diagram was constructed based on the composition boundaries obtained from light scattering titrations.
    Stability of complex coacervate core micelles containing metal coordination polymer
    Yan, Y. ; Keizer, A. de; Cohen Stuart, M.A. ; Drechsler, M. ; Besseling, N.A.M. - \ 2008
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 112 (2008)35. - ISSN 1520-6106 - p. 10908 - 10914.
    block ionomer complexes - entrapping enzyme molecules - diblock copolymer - physicochemical properties - polyelectrolyte - nanoparticles - lysozyme - delivery - surface - length
    We report on the stability of complex coacervate core micelles, i.e., C3Ms (or PIC, BIC micelles), containing metal coordination polymers. In aqueous solutions these micelles are formed between charged-neutral diblock copolymers and oppositely charged coordination polymers formed from metal ions and bisligand molecules. The influence of added salt, polymer concentration, and charge composition was investigated by using light scattering and cryo-TEM techniques. The scattering intensity decreases strongly with increasing salt concentration until a critical salt concentration beyond which no micelles exist. The critical micelle concentration increases almost exponentially with the salt concentration. From the scattering results it follows that the aggregation number decreases with the square root of the salt concentration, but the hydrodynamic radius remains constant or increases slightly. It was concluded that the density of the core decreases with increasing ionic strength. This is in agreement with theoretical predictions and is also confirmed by cryo-TEM measurements. A complete composition diagram was constructed based on the composition boundaries obtained from light scattering titrations
    Complex coacervate core micro-emulsions
    Hofs, P.S. ; Keizer, A. de; Burgh, S. van der; Leermakers, F.A.M. ; Cohen Stuart, M.A. ; Millard, P.E. ; Muller, A.H.E. - \ 2008
    Soft Matter 4 (2008)7. - ISSN 1744-683X - p. 1473 - 1482.
    block-copolymer micelles - interacting chain molecules - diblock copolymers - statistical-theory - aqueous-media - polyelectrolyte - kinetics - mechanism - polymerization - solubilization
    Complex coacervate core micelles form in aqueous solutions from poly(acrylic acid)-block-poly(acrylamide) (PAAxPAAmy, x and y denote degree of polymerization) and poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA150) around the stoichiometric charge ratio of the two components. The hydrodynamic radius, Rh, can be increased by adding oppositely charged homopolyelectrolytes, PAA140 and PDMAEMA150, at the stoichiometric charge ratio. Mixing the components in NaNO3 gives particles in highly aggregated metastable states, whose Rh remain unchanged (less than 5% deviation) for at least 1 month. The Rh increases more strongly with increasing addition of oppositely charged homopolyelectrolytes than is predicted by a geometrical packing model, which relates surface and volume of the particles. Preparation in a phosphate buffer ¿ known to weaken the electrostatic interactions between PAA and PDMAEMA ¿ yields swollen particles called complex coacervate core micro-emulsions (C3-Es) whose Rh increase is close to that predicted by the model. These are believed to be in the stable state (lowest free energy). A two-regime increase in Rh is observed, which is attributed to a transition from more star-like to crew-cut-like, as shown by self-consistent field calculations. Varying the length of the neutral and polyelectrolyte block in electrophoretic mobility measurements shows that for long neutral blocks (PAA26PAAm405 and PAA39PAAm381) the -potential is nearly zero. For shorter neutral blocks the -potential is around -10 mV. This shows that the C3-Es have excess charge, which can be almost completely screened by long enough neutral blocks.
    Characteristic differences in the formation of complex coacervate core micelles from neodymium and zinc-based coordination polymers
    Yan, Y. ; Besseling, N.A.M. ; Keizer, A. de; Cohen Stuart, M.A. - \ 2007
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 111 (2007)21. - ISSN 1520-6106 - p. 5811 - 5818.
    diblock copolymer - block-copolymers - metal-ions - polyelectrolyte - recognition - stability - lysozyme
    In this paper we compare the formation of complex coacervate core micelles (C3Ms) from two different tricompontent mixtures, namely neodymium, the bisligand L2EO4 and the poly(cation)-block-poly(neutral) diblock copolymer P2MVP41-b-PEO205, and zinc, L2EO4 and P2MVP41-b-PEO205 mixed systems. Three sets of titration experiments were carried out for each system: (i) titration of diblock copolymer P2MVP41-b-PEO205 with the stoichiometric mixture of metal ions and bisligands, (ii) titration of a mixture of diblock copolymer and bisligand with metal ions, and (iii) titration of a mixture of diblock copolymer and metal ions with bisligands. In all the above three cases, micelles are found to form either in a broad range of charge ratios or in a broad range of metal/bisligand ratios. Upon addition of Nd2-(L2EO4)3 coordination polymer to P2MVP41-b-PEO205 solution, and upon addition of Nd3+ to a mixture of L2EO4 and P2MVP41-b-PEO205, micelles are found to form immediately after the first addition, whereas micelles show up in the similar zinc system only after a certain threshold Zn-(L2EO4) or Zn2+ concentration. This difference can be traced to the different structures of the Nd2-(L2EO4)3 and Zn-(L2EO4) coordination compounds. At very low concentrations, Zn-(L2EO4) are ring-like oligomers, but Nd2-(L2EO4)3 are larger networks. The network structure favors the formation of coacervate micellar core with P2MVP41-b-PEO205. Moreover, excess of Nd3+ ions will break up the C3Ms, while the same amount of Zn2+ has hardly any effect on the C3Ms. The breakdown of C3Ms by Nd3+ is due to the charge inversion of the coordination complex with increasing [Nd3+]/[L2EO4] ratio, which results in repulsive interaction between the coordination complex and the diblock copolymer, whereas no such interaction can occur in the zinc system.
    Complex coacervate core micelles with a lysozyme-modified corona
    Danial, M. ; Klok, H.A. ; Norde, W. ; Cohen Stuart, M.A. - \ 2007
    Langmuir 23 (2007)15. - ISSN 0743-7463 - p. 8003 - 8009.
    block-copolymers - diblock copolymers - ionic-strength - drug-delivery - polyelectrolyte - adsorption - behavior - brushes - cells - ph
    This paper describes the preparation, characterization, and enzymatic activity of complex coacervate core micelles (C3Ms) composed of poly(acrylic acid) (PAA) and poly(N-methyl-2-vinyl pyridinium iodide)-b-poly(ethylene oxide) (PQ2VP-PEO) to which the antibacterial enzyme lysozyme is end-attached. C3Ms were prepared by polyelectrolyte complex formation between PAA and mixtures containing different ratios of aldehyde and hydroxyl end-functionalized PQ2VP-PEO. This resulted in the formation of C3Ms containing 0-40% (w/w) of the aldehyde end-functionalized PQ2VP-PEO block copolymer (PQ2VP-PEO-CHO). Chemical conjugation of lysozyme was achieved via reductive amination of the aldehyde groups, which are exposed at the surface of the C3M, with the amine groups present in the side chains of the lysine residues of the protein. Dynamic and static light scattering indicated that the conjugation of lysozyme to C3Ms prepared using 10 and 20% (w/w) PQ2VP-PEO-CHO resulted in the formation of unimicellar particles. Multimicellar aggregates, in contrast, were obtained when lysozyme was conjugated to C3Ms prepared using 30 or 40% (w/w) PQ2VP-PEO-CHO. The enzymatic activity of the unimicellar lysozyme-C3M conjugates toward the hydrolysis of the bacterial substrate Micrococcus lysodeikticus was comparable to that of free lysozyme. For the multimicellar particles, in contrast, significantly reduced enzymatic rates of hydrolysis, altered circular dichroism, and red-shifted tryptophan fluorescence spectra were measured. These results are attributed to the occlusion of lysozyme in the interior of the multimicellar conjugates.
    Hierarchical Self-Assembly in Solutions Containing Metal Ions, Ligand, and Diblock Copolymer
    Yan Yun, ; Besseling, N.A.M. ; Keizer, A. de; Marcelis, A.T.M. ; Drechsler, M. ; Cohen Stuart, M.A. - \ 2007
    Angewandte Chemie-International Edition 46 (2007)11. - ISSN 1433-7851 - p. 1807 - 1809.
    coordination polymers - block-copolymer - micelles - core - polyelectrolyte - recognition - complexes - lysozyme
    Bending rigidity and induced persistence length of molecular bottle brushes : a self-consistent-field theory
    Feuz, L. ; Leermakers, F.A.M. ; Textor, M. ; Borisov, O.V. - \ 2005
    Macromolecules 38 (2005)21. - ISSN 0024-9297 - p. 8891 - 8901.
    transfer radical polymerization - monte-carlo simulation - comb-branched polymers - cylindrical brushes - lyotropic behavior - side-chains - conformation - polyelectrolyte - architecture - elasticity
    The two-gradient version of the Scheutjens-Fleer self-consistent-field (SF-SCF) approach is employed for the analysis of the average conformations of side chains and corresponding contribution to the bending rigidity, or equivalently the induced persistence length, of molecular bottle brushes both under good and theta solvent conditions. This study is targeted to unravel conformational properties of poly(L-lysine)-graft-poly(ethylene glycol) copolymers in dilute aqueous solutions, where variation of temperature changes the solvent strength for poly(ethylene glycol) in a wide range. We focus on molecular brushes with moderate and high grafting density and large degree of polymerization of grafted chains. In this limit the predictions of an analytical mean-field theory for the dependences of the structural properties of the bottle brush on the architectural parameters are well confirmed. Both the induced persistence length and the ratio between the induced persistence length and the cross-sectional thickness of the bottle brush increase with increasing grafting density and/or increasing degree of polymerization of the grafted chains. However, in the range of moderate chain lengths and grafting densities this ratio remains small, which explains why the effects of the induced bending rigidity on the apparent persistence length have not been observed in earlier numerical experiments. We argue that only molecular bottle brushes with densely grafted long chains possess the potential for lyotropic ordering, both in solutions and at interfaces, due to the expected high effective segment asymmetry.
    Electrokinetics of diffuse soft interfaces. III. Interpretation of data on the polyacrylamide/water interface
    Yezek, L.P. ; Duval, J.F.L. ; Leeuwen, H.P. van - \ 2005
    Langmuir 21 (2005)14. - ISSN 0743-7463 - p. 6220 - 6227.
    acrylamide-based hydrogels - aqueous salt-solutions - swelling behavior - electrophoretic mobility - microgel particles - donnan potentials - charge-density - gel layers - polyelectrolyte - temperature
    Streaming potential measurements were carried out on a family of polyacrylamide-co-sodium acrylate gels cross-linked with N,N¿- methylenebisacrylamide in a homemade electrokinetic cell. Measurements of the ionic conductivity within thin films of these gels allowed the equilibrium Donnan potential difference between the bulk gel and the bulk electrolyte environments to be estimated at various ionic strengths. The resulting Donnan potential data were combined with the directly measurable streaming potential data and used to evaluate the diffuse soft interface model of electrokinetics (Langmuir 2004, 20, 10324). The model introduces the concept of a gradual decay of polymer density and fixed charge density within a narrow inter-phase at the gel/solution interface. The nature of the decay at the interface has a dramatic effect on the magnitude of the streaming potential as predicted by the diffuse soft interface model. In this investigation, the gradual decay of polymer density within the inter-phase is described with a hyperbolic tangent function. For the gels mentioned, the characteristic length scale of the decay, ¿, as calculated from the fit to the model, increases significantly with decreasing ionic strength, suggesting an osmotically driven swelling of the loosely cross-linked polymer chains at the interface. The experimental data and the results of the fitting are discussed in terms of the physical picture of the interface and compared to fitting results for a model which assumes a simple step function at the gel-solution interface.
    Electrokinetics of diffuse soft interfaces. I. Limit of low Donnan potentials
    Duval, J.F.L. ; Leeuwen, H.P. van - \ 2004
    Langmuir 20 (2004)23. - ISSN 0743-7463 - p. 10324 - 10336.
    segment density distribution - sensitive hydrogel layers - cell-surface properties - electrophoretic mobility - microgel particles - charge-density - gel layers - polyelectrolyte - temperature - isotherms
    The current theoretical approaches to electrokinetics of gels or polyelectrolyte layers are based on the assumption that the position of the very interface between the aqueous medium and the gel phase is well defined. Within this assumption, spatial profiles for the volume fraction of polymer segments (), the density of fixed charges in the porous layer (fix), and the coefficient modeling the friction to hydrodynamic flow (k) follow a step-function. In reality, the "fuzzy" nature of the charged soft layer is intrinsically incompatible with the concept of a sharp interface and therefore necessarily calls for more detailed spatial representations for , fix, and k. In this paper, the notion of diffuse interface is introduced. For the sake of illustration, linear spatial distributions for and fix are considered in the interfacial zone between the bulk of the porous charged layer and the bulk electrolyte solution. The corresponding distribution for k is inferred from the Brinkman equation, which for low reduces to Stokes' equation. Linear electrostatics, hydrodynamics, and electroosmosis issues are analytically solved within the context of streaming current and streaming potential of charged surface layers in a thin-layer cell. The hydrodynamic analysis clearly demonstrates the physical incorrectness of the concept of a discrete slip plane for diffuse interfaces. For moderate to low electrolyte concentrations and nanoscale spatial transition of from zero (bulk electrolyte) to o (bulk gel), the electrokinetic properties of the soft layer as predicted by the theory considerably deviate from those calculated on the basis of the discontinuous approximation by Ohshima.
    Complex coacervation of whey proteins and gum arabic
    Weinbreck, F. ; Vries, R.J. de; Schrooyen, P. ; Kruif, C.G. de - \ 2003
    Biomacromolecules 4 (2003). - ISSN 1525-7797 - p. 293 - 303.
    bovine serum-albumin - beta-lactoglobulin - polysaccharide interactions - acacia gum - polyelectrolyte - adsorption - phase - separation - surfaces - behavior
    Mixtures of gum arabic and whey protein (whey protein isolate, WP) form an electrostatic complex in a specific PH range. Three phase boundaries (PHc, pHphi(l), pHphi(2)) have been determined using an original titration method, newly applied to complex coacervation. It consists of monitoring the turbidity and light scattering intensity under slow acidification in situ with glucono-delta-lactone. Furthermore, the particle size could also be measured in parallel by dynamic light scattering. When the PH is lowered, whey proteins and gum arabic first form soluble complexes. This boundary is designated as PHc.When the interaction is stronger (at lower PH), phase separation takes place (at pHphi(1)). Finally, at pHphi(2) complexation was suppressed by the charge reduction of the gum arabic. The major constituent of the whey protein preparation used was beta-lactoglobulin (beta-1g), and it was shown that beta-1g was indeed the main complex-forming protein. Moreover, an increase of the ionic strength shifted the PH boundaries to lower PH values, which was summarized in a state diagram. The experimental PH., values were compared to a newly developed theory for polyelectrolyte adsorption on heterogeneous surfaces. Finally, the influence of the total biopolymer concentration (0-20% w/w) was represented in a phase diagram. For concentrations below 12%, the results are consistent with the theory on complex coacervation developed by Overbeek and Voorn. However, for concentrations above 12%, phase diagrams surprisingly revealed a "metastable" region delimited by a percolation line. Overall, a strong similarity is seen between the behavior of this system and a colloidal gas-liquid phase separation.
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