Cationic Hofmeister Series of Wettability Alteration in Mica-Water-Alkane Systems.

The specific interaction of ions with macromolecules and solid-liquid interfaces is of crucial importance to many processes in biochemistry, colloid science, and engineering, as first pointed out by Hofmeister in the context of (de)stabilization of protein solutions. Here, we use contact angle goniometry to demonstrate that the macroscopic contact angle of aqueous chloride salt solutions on mica immersed in ambient alkane increases from near-zero to values exceeding 10°, depending on the type and concentration of cations and pH. Our observations result in a series of increasing ability of cations to induce partial wetting in the order Na+, K+ < Li+ < Rb+ < Cs+ < Ca2+ < Mg2+ < Ba2+. Complementary atomic force microscopy measurements show that the transition to partial wetting is accompanied by cation adsorption to the mica-electrolyte interface, which leads to charge reversal in the case of divalent cations. In addition to electrostatics, hydration forces seem to play an important role, in particular for the monovalent cations.

Templated co-assembly into nanorods of polyanions and artificial virus capsid proteins.

Recombinant triblock polypeptides C-Sn-B, where C is a 400 amino acid long hydrophilic random coil block, Sn is a multimer of the silk-like octapeptide S = (GAGAGAGQ), and B = K12 is an oligolysine, have previously been shown to encapsulate double stranded DNA into rod-shaped, virus-like particles. In order to gain insight of the co-assembly process, and in order to be able to use these proteins for templating other types of nanorods, we here explore their co-assembly with a range of polyanionic templates: poly(acrylic acids) (PAA) of a wide range of lengths, poly(styrene sulphonate) (PSS) and the stiff anionic polysaccharide xanthan. The formation of the complexes was characterized using Dynamic Light Scattering (DLS), cryogenic Transmission Electronic Microscopy (Cryo-TEM) and Atomic Force Microscopy (AFM). Except at very high molar masses, we find that flexible anionic PAA and PSS lead to co-assembly of proteins with single polyanion chains into nanorods, with a packing factor as expected on the basis of charge stochiometry. Only for very long PAA templates (8 × 105 Da) we find evidence for heterogeneous complexes with thin and thick sections. For the very stiff xanthan chains, we find that its stiffness precludes co-assembly with the artificial viral capsid proteins into condensed and regular nanorods. Given the simple and robust formation of rod-like structures with a range of polyanionic templates, we anticipate that the artificial virus proteins will be useful for preparing high-aspect ratio nanoparticles and scaffolds of precise size and find applications in nanotechnology and materials science for which currently natural rod-like viruses are being explored.

Harnessing the advantages of hard and soft colloids by the use of core-shell particles as interfacial stabilizers.

The ability of colloidal particles to penetrate fluid interfaces is a crucial factor in the preparation of particle stabilized disperse systems such as foams and emulsions. For hard micron-sized particles the insertion into fluid interfaces requires substantial energy input, but soft particles are known to adsorb spontaneously. Particle hardness, however, may also affect foam and emulsion stability. The high compliance of soft particles may compromise their ability to withstand the lateral compression associated with disproportionation. Hence, particles which can spontaneously adsorb onto fluid interfaces, and yet depict low compliance may be ideal as interfacial stabilizers. In the present work, we prepared core-shell particles comprising a hard, polystyrene core and a soft poly(N-isopropylacrylamide) based shell. We found that such core-shell particles adsorb spontaneously onto various fluid interfaces. The absence of a pronounced energy barrier for interfacial adsorption allowed the facile preparation of particle-stabilized bubbles as well as emulsion droplets. For bubbles, the stability was better than that of bubbles stabilized by entirely soft particles, but disproportionation was not stopped completely. Emulsion droplets, in contrast, showed excellent stability against both coalescence and disproportionation. Lateral compression of core-shell particles due to disproportionation was clearly limited by the presence of the polystyrene core, leading to long-lasting stability. For emulsions, we even observed non-spherical droplets, indicating a negligible Laplace pressure. Our results indicate that core-shell particles comprising a hard core and a soft shell combine the advantageous properties of hard and soft particles, namely spontaneous adsorption and limited compliance, and can therefore be superior materials for the preparation of particle-stabilized dispersions.

Surfactant induced autophobing.

Surfactant adsorption in a three-phase system and its influence on wetting properties are relevant in various applications. Here, we report a hitherto not observed phenomenon, namely the retraction of an aqueous drop on hydrophilic solid substrates (which we refer to as 'autophobing') in ambient oil containing water-insoluble fatty acids, caused by the deposition of these fatty acids from the ambient oil onto the solid substrate. AFM measurements confirm that the surfactant is deposited on the solid by the moving contact line. This leads to a more hydrophobic substrate, the retraction of the contact line and a concomitant increase in the contact angle. The deposition process is enabled by the formation of a reaction product between deprotonated fatty acids and Ca(2+) ions at the oil/water interface. We investigate how the transition to a new equilibrium depends on the concentrations of the fatty acids, the aqueous solute, the chain lengths of the fatty acid, and the types of alkane solvent and silica or mica substrates. This phenomenon is observed on both substrates and for all explored combinations of fatty acids and solvents and thus appears to be generic. In order to capture the evolution of the contact angle, we develop a theoretical model in which the rate of adsorption at the oil-water interface governs the overall kinetics of autophobing, and transfer to the solid is determined by a mass flux balance (similar to a Langmuir Blodgett transfer).

Assembly of jammed colloidal shells onto micron-sized bubbles by ultrasound.

Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compression due to bubble shrinkage. The combination of both effects stops coarsening once the interface is jammed with particles. As a result, stable bubbles are formed. Individual particles can desorb from the interface upon surfactant addition, though. The latter fact confirms that the particle shell is not covalently linked due to thermal sintering, but is solely held together by capillary interaction. In summary, we show that our ultrasound approach allows for the straightforward creation of micron-sized particle stabilized bubbles with high stability towards coarsening.

Colloids in Flatland: a perspective on 2D phase-separated systems, characterisation methods, and lineactant design.

In 1861 Thomas Graham gave birth to a new field of science, today known as colloid science. Nowadays, the notion "colloid" is often used referring to systems consisting of two immiscible phases, one of which is finely dispersed into the other. Research on colloids deals mostly with sols (solids dispersed in a liquid), emulsions (liquids dispersed in liquid), and foams (gas dispersed in a liquid). Because the dispersed particles are small, there is a lot of interface per unit mass. Not surprisingly, therefore, the properties of the interface have often a decisive effect on the behaviour of colloids. Water-air interfaces have a special relevance in this field: many water-insoluble molecules can be spread on water and, given the right spreading conditions and enough available surface area, their spreading proceeds until a monolayer (a one-molecule thick layer) eventually remains. Several 2D phases have been identified for such monolayers, like "gas", "liquid expanded", "liquid condensed", and "solid". The central question of this review is whether these 2D phases can also exist as colloidal systems, and what stabilizes the dispersed state in such systems. We shall present several systems capable of yielding 2D phase separation, from those based on either natural or fluorinated amphiphiles, to polymer-based ones. We shall seek for analogies in 3D and we shall try to clarify if the lines between these 2D objects play a similar role as the interfaces between 3D colloidal systems. In particular, we shall consider the special role of molecules that tend to accumulate at the phase boundaries, that is, at the contact lines, which will therefore be denoted "line-actants" (molecules that adsorb at a 1D interface, separating two 2D colloidal entities), by analogy to the term "surfactant" (which indicates a molecule that adsorbs at a 2D interface separating two 3D colloidal entities).

On the stability of the polymer brushes formed by adsorption of ionomer complexes on hydrophilic and hydrophobic surfaces.

We have studied the effect of normal forces and shear forces on the stability and functionality of a polymer brush layer formed upon adsorption of polymeric micelles on hydrophilic and hydrophobic surfaces. The micelles consist of oppositely charged polyelectrolyte blocks (poly(acrylic acid) and poly(N-methyl 2-vinyl pyridinium iodide), and a neutral block (poly(vinyl alcohol)) or neutral grafts (poly(ethylene oxide)). The strength of the attachment of the micellar layers to various substrates was evaluated with Atomic Force Microscopy. Flow cell experiments allowed for the evaluation of long-term stability of coatings in lateral flow. Fixed angle optical reflectometry was used to quantify protein (BSA) adsorption on the micellar layers after their exposure to flow. The results show that adsorbed micellar layers are relatively weakly attached to hydrophobic surfaces and much stronger to hydrophilic surfaces, which has a significant impact on their stability. Adsorbed layers maintain their ability to suppress protein adsorption on hydrophilic surfaces but not on hydrophobic surfaces. Due to the relatively weak attachment to hydrophobic surfaces the structure of adsorbed layers may easily be disrupted by lateral forces, such that the complex coacervate-brush structure no longer exists.

Towards a structural characterization of charge-driven polymer micelles.

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.

Reduction of protein adsorption to a solid surface by a coating composed of polymeric micelles with a glass-like core.

Adsorption studies by optical reflectometry show that complex coacervate core micelles (C3Ms) composed of poly([4-(2-amino-ethylthio)-butylene] hydrochloride)(49)-block-poly(ethylene oxide)(212) and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)(47)-block-poly(ethylene oxide)(212) adsorb in equal amounts to both silica and cross-linked 1,2-polybutadiene (PB). The C3Ms have an almost glass-like core and atomic force microscopy of a dried layer of adsorbed C3Ms shows densely packed flattened spheres on silica, which very probably are adsorbed C3Ms. Experiments were performed with different types of surfaces, solvents, and proteins; bare silica and cross-linked 1,2-PB, NaNO(3) and phosphate buffer, and lysozyme, bovine serum albumin, beta-lactoglobulin, and fibrinogen. On the hydrophilic surface the coating reduces protein adsorption >90% in 0.1 M phosphate buffer, whereas the reduction on the coated hydrophobic surface is much lower. Reduction is better in phosphate buffer than in NaNO(3), except for the positively charged lysozyme, where the effect is reversed.

Phase behavior of flowerlike micelles in a SCF cell model.

We study the interactions between flowerlike micelles, self-assembled from telechelic associative polymers, using a molecular self-consistent field (SCF) theory and discuss the corresponding phase behavior. In these calculations we do not impose properties such as aggregation number, micellar structure and number of bridging chains. Adopting a SCF cell model, we calculate the free energy of interaction between a central micelle surrounded by others. Based on these results, we predict the binodal for coexistence of dilute and dense liquid phases, as a function of the length of the hydrophobic and hydrophilic blocks. In the same cell model we compute the number of bridges between micelles, allowing us to predict the network transition. Several quantitative trends obtained from the numerical results can be rationalized in terms of transparent scaling arguments.

Complex coacervate core micro-emulsions.

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-µEs) 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 ζ-potential is nearly zero. For shorter neutral blocks the ζ-potential is around -10 mV. This shows that the C3-µEs have excess charge, which can be almost completely screened by long enough neutral blocks.

Shear banding and rheochaos in associative polymer networks.

We present experimental evidence of an instability in the shear flow of transient networks formed by telechelic associative polymers. Velocimetry experiments show the formation of shear bands, following a complex pattern upon increasing the overall shear rate. The chaotic nature of the stress response in transient flow is indicative of spatiotemporal fluctuations of the banded structure. This is supported by time-resolved velocimetry measurements.

Equilibrium capillary forces with atomic force microscopy.

We present measurements of equilibrium forces resulting from capillary condensation. The results give access to the ultralow interfacial tensions between the capillary bridge and the coexisting bulk phase. We demonstrate this with solutions of associative polymers and an aqueous mixture of gelatin and dextran, with interfacial tensions around 10 microN/m. The equilibrium nature of the capillary forces is attributed to the combination of a low interfacial tension and a microscopic confinement geometry, based on nucleation and growth arguments.

Bending rigidity of mixed phospholipid bilayers and the equilibrium radius of corresponding vesicles.

In spite of the large mean bending moduli observed for phospholipid bilayers, stable vesicle phases were recently observed for dilute solutions of charged phospholipids. A correspondingly large negative Gaussian bending modulus associated with charged membranes results in an overall curvature energy that is so low that entropic stabilization is possible. The mean bending modulus determines the membrane persistence length and therefore it is reasonable that there is a correlation between the membrane rigidity and the size of the lipid vesicles. Here we show that in mixtures of the anionic phospholipid dioleoylphosphatidylglycerol and the zwitterionic phospholipid dioleoylphosphatidylcholine the radius of vesicles produced by repetitive freeze-thaw cycles is considerably smaller than expected from the rigidities of the corresponding pure lipid bilayers. Self-consistent field calculations indicate that the changes in the equilibrium radius of mixed bilayers can be attributed to the dependences of the mean bending modulus k(c) on lipid mixing and the average surface charge density.

Entropic stabilization and equilibrium size of lipid vesicles.

We have studied the phase behavior of zwitterionic phospholipid dioleoylphosphatidylcholine (DOPC) vesicles (membranes) and interpreted our results using scaling arguments in combination with molecular realistic self-consistent field (SCF) calculations. DOPC membranes acquire a partial negative charge per lipid molecule at intermediate NaBr concentrations. As a result of this, dilute DOPC solutions form stable unilamellar vesicles. Both at low and high salt concentrations phase separation into a lamellar and a vesicular phase is observed. The vesicle radius decreases as a power law with decreasing lipid concentration. This power-law concentration dependence indicates that the vesicle phase is entropically stabilized; the size of the DOPC vesicles result from a competition between the bending energy and translation and undulation entropy. This scaling behavior breaks down for very small vesicles. This appears to be consistent with SCF predictions that point to the fact that in this regime the mean bending modulus kc increases with curvature. The SCF theory predicts that, at low ionic strength, the membrane stability improves when there is more charge on the lipids. Upon a decrease of the ionic strength, lipids with a full negative charge form vesicles that grow exponentially in size because the mean bending modulus increases with decreasing ionic strength. At the same time the Gaussian bending modulus becomes increasingly negative such that the overall bending energy tends to zero. This indicates that small micelles become the dominant species. The SCF theory thus predicts a catastrophic break down of giant vesicles in favor of small micelles at sufficiently low ionic strength and high charge density on the lipids.

Rheology of a reversible supramolecular polymer studied by comparison of the effects of temperature and chain stoppers.

The rheology of a reversible supramolecular polymer is studied by comparing the effects of an increase in temperature and the addition of chain stoppers. The dependence of the zero-shear viscosity and the terminal relaxation time on temperature is exponential, and the activation energy for viscous flow can be calculated. Above a critical stopper fraction, power laws describe the stopper dependence of the viscosity and relaxation time. A simple model for the effect of the addition of chain stoppers on the average degree of polymerization adequately describes the results. A comparison of flow curves at several temperatures and stopper fractions reveals considerable differences between solutions with the same zero-shear viscosity. These are mainly associated with differences in the terminal relaxation time. A mechanism of shear-induced alignment and subsequent elongation of chains is proposed, with which the experimental results are consistent.

On the binding of calcium by micelles composed of carboxy-modified pluronics measured by means of differential potentiometric titration and modeled with a self-consistent-field theory.

We perform differential potentiometric titration measurements for the binding of Ca2+ ions to micelles composed of the carboxylic acid end-standing Pluronic P85 block copolymer (i.e., CAE-85 (COOH-(EO)26-(PO)39-(EO)26-COOH)). Two different ion-selective electrodes (ISEs) are used to detect the free calcium concentration; the first ISE is an indicator electrode, and the second is a reference electrode. The titration is done by adding the block copolymers to a known solution of Ca2+ at neutral pH and high enough temperature (above the critical micellization temperature CMT) and various amount of added monovalent salt. By measuring the difference in the electromotive force between the two ISEs, the amount of Ca2+ that is bound by the micelles is calculated. This is then used to determine the binding constant of Ca2+ with the micelles, which is a missing parameter needed to perform molecular realistic self-consistent-field (SCF) calculations. It turns out that the micelles from block copolymer CAE-85 bind Ca2+ ions both electrostatically and specifically. The specific binding between Ca2+ and carboxylic groups in the corona of the micelles is modeled through the reaction equilibrium -COOCa+ <==> -COO- + Ca2+ with pKCa = 1.7 +/- 0.06.

Long-range depletion forces induced by associating small molecules.

This is the first report of experimental observations of depletion interactions in solutions of a (hydrogen-bonded) reversible supramolecular polymer. Depletion forces were measured directly by colloidal probe atomic force microscopy. The range of the depletion force is consistent with existing independent experimental data. The interaction can be tuned by adding monofunctional chain stoppers to the solution, a possibility which is unique to supramolecular polymers. The depletion force is shown to be strong enough to induce phase separation in a colloidal suspension.

Thinning of wetting films formed from aqueous solutions of non-ionic surfactant.

We investigated the thinning of wetting films formed from aqueous solution of non-ionic triblock copolymer Pluronic F127 on the surface of silica using a home-made thin film balance and time-resolved ellipsometry. Imaging ellipsometry was used to visualize the film structures at subsequent stages of their development. The results unambiguously show that the time required for the formation of steady films strongly depends on the electrolyte concentration. When increasing the latter from 10(-4) to 0.1 M, this time typically increases with several orders of magnitude, from a few minutes to several hours. Moreover, for sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified. After initial quick thinning, further thinning slows down enormously. These typical kinetic regimes are thought to result from the coupled dependencies of the bulk and interfacial properties of F127 on salt concentration. Possible explanations of the phenomenon are discussed.

Why surfaces modified by flexible polymers often have a finite contact angle for good solvents.

Polymers adsorbing from a dilute solution onto the solvent-vapor interface generate a nonzero surface pressure. When the same polymers are end-grafted onto a surface such that a so-called polymer brush is formed, one will find that the solvent wets this compound interface partially. The partial wetting and the finite surface pressure are intimately linked properties of the polymer-solvent-vapor combination. It is shown that the spreading parameter in the wetting problem is proportional to the surface pressure in the adsorption case. Complete wetting is only possible when this surface pressure is nonpositive. The wetting characteristics are hardly influenced by the grafting density and chain length characterizing the brush. We argue that the grafted polymer chains can bridge to the solvent-vapor interface, thereby preventing the wetting film to become macroscopically thick. We present experimental data underpinning our self-consistent field analysis. Indeed, finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films.