Sven Holger Behrens

The charge of glass and silica surfaces

We present a method of calculating the electric charge density of glass and silica surfaces in contact with aqueous electrolytes for two cases of practical relevance that are not amenable to standard techniques: surfaces of low specific area at low ionic strength and surfaces interacting strongly with a second anionic surface.

Charge Regulation in the Electrical Double Layer: Ion Adsorption and Surface Interactions.

Charge regulation in the electrical double layer has important implications for ion adsorption, interparticle forces, colloidal stability, and deposition phenomena. Although charge regulation generally receives little attention, its consequences can be major, especially when considering interactions between unequally charged surfaces. The present article discusses common approaches to quantify such phenomena, especially within classical Poisson-Boltzmann theory, and pinpoints numerous situations where a consideration of charge regulation is essential. For the interpretation of interaction energy profiles, we advocate the use of the constant regulation approximation, which summarizes the surface properties in terms of two quantities, namely, the diffuse layer potential and the regulation parameter. This description also captures some pronounced regulation effects observed in the presence of multivalent ions.

Electric charging in nonpolar liquids because of nonionizable surfactants.

Nonpolar liquids do not easily accommodate electric charges, but it is known that surfactant additives can raise the conductivity and lead to electric charging of immersed solid surfaces. Here, we study the rarely considered conductivity effects induced by surfactant molecules without ionizable groups. Precision conductometry, light scattering, and Karl Fischer titration of sorbitan oleate solutions in hexane reveal a distinctly electrostatic action of the nonionic surfactant. The conductivity in dilute hexane solutions of sorbitan trioleate (Span 85) exhibits two regimes of linear scaling with surfactant concentration and a transition around the critical micelle concentration (cmc). Both regimes can be described with a statistical model of equilibrium charge fluctuations. The behavior observed above the cmc has a direct analogy in systems of ionic surfactants and can be explained by charge disproportionation of reverse micelles. The observed conductivity of Span 85 solutions below the cmc, however, represents a qualitative departure from the behavior reported for ionic surfactants. In the studied surfactant systems, the availability of ionic species may stem from a complexation of the surfactant with ionizable impurities; nonetheless, the ionization process appears to be limited entirely by the surfactant and not by the level of impurity. We therefore propose that nonionizable surfactants can offer a new and robust way of controlling the conductivity in nonpolar liquids.

Novel emulsions stabilized by pH and temperature sensitive microgels

Surfactant-free oil-in-water emulsions prepared with temperature and pH sensitive poly(N-isopropylacrylamide)(PNIPAM) microgel particles offer unprecedented control of emulsion stability.

Permeability control in stimulus-responsive colloidosomes

Self-assembly of colloidal particles in the liquid interface of double emulsion droplets can be used to fabricate “colloidosome” microcapsules, which have great potential as vehicles for the controlled delivery of drugs or other cargoes. Here we present a novel class of aqueous core colloidosomes that combine the benefit of low capsule permeability (good cargo retention) with the option of a stimulus-triggered fast release in a target environment. Complete or partial dissolution of the capsule walls in response to a mild pH change is achieved in each case through the use of responsive particles made from polymers with pH-switchable solubility. We demonstrate three methods of controlling the capsule permeability prior to release while maintaining the intended response to the release trigger.

Electric double layer interaction of ionizable surfaces: Charge regulation for arbitrary potentials

Electrostatic interactions between charged surfaces across an electrolyte solution are commonly described by boundary conditions of constant charge or constant surface potential. These two extremes are in general not appropriate for an equilibrium description of materials with ionizable surface groups, but do provide an upper and lower bound for the interaction energy. We propose a quantitative criterion which permits to evaluate the degree of charge regulation for surfaces with arbitrary electrostatic potential, as considered in the Poisson–Boltzmann theory. Our approach represents a generalization of the linearized regulation model proposed earlier for the framework of Debye–Huckel theory only. Like in the case of low surface potentials, the regulation behavior is generally determined by the competition between the capacities of the diffuse and the compact part of the electric double layer. Our results suggest a new way of using the limiting conditions of constant charge and constant potential to approx...

Electrostatic Double Layer Forces in the Case of Extreme Charge Regulation

We analyze the interaction forces between charged surfaces across aqueous solutions under the conditions of extreme charge regulation. Under such conditions, interactions may be weaker than those given by the constant potential (CP) boundary conditions. Thermodynamically, even vanishing electrostatic interactions are conceivable. Within the constant regulation approximation, the known results can be extended to this sub-CP regime by adopting regulation parameters outside of the common range. A mean-field lattice model of an adsorbed layer shows that such conditions are most likely found near critical points within the adsorbed layer.

Interaction and structure of surfaces coated by poly(vinyl amines) of different line charge densities

Interactions between preadsorbed films of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed films were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a flat configuration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous films. As revealed by reflectivity measurements, such preadsorbed PVA films are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA films have very similar diffuse layer charge densities of about 1.5 mC/m2. Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reflectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the films is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the films behave differently. While the highly charged PVA shows unspecific adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances.

Surfactant mediated charging of polymer particles in a nonpolar liquid.

We study the charging behavior of polystyrene and polymethyl methacrylate particles with different functional surface groups in water and in decane containing either ionic (AOT) or nonionic surfactant (Span 85). Electrophoretic mobilities in the nonpolar media are measured as a function of surfactant concentration and the applied electric field strength by phase analysis light scattering (PALS); acid-base characteristics of the particles and the surfactant are investigated via contact angle measurement and interfacial tensiometry, and the residual water content of the non-aqueous dispersions is assessed by Karl Fischer titration. The results suggest a competition of several mechanisms for particle charging in nonpolar media. At high concentrations of the nonionic surfactant, particle charging becomes insensitive to the functional surface groups responsible for charging in aqueous dispersions, but consistent with a charge transfer between the polymer surface and the surfactant due to acid-base interactions, which can be rationalized in terms of measurable acid-base parameters. By contrast, particle charging in nonpolar solutions of the ionic surfactant (with significantly larger amounts of residual water) suggests a strong influence of surface headgroup ionization, and of dissociated surfactants adsorbed to the particle surface.

Salt-Induced Aggregation of a Monoclonal Human Immunoglobulin G1

Physical stability is critical for any therapeutic proteins efficacy and economic viability. No reliable theory exists to predict stability de novo, and modeling aggregation is challenging as this phenomenon can involve orientation effects, unfolding, and the rearrangement of noncovalent bonds inter- and intramolecularly in a complex sequence of poorly understood events. Despite this complexity, the simple observation of protein concentration-dependent diffusivity in stable, low ionic-strength solutions can provide valuable information about a proteins propensity to aggregate at higher salt concentrations and over longer times. We recently verified this notion using two model proteins, and others have shown that this strategy may be applicable to antibodies as well. Here, we expand our previous study to a monoclonal human immunoglobulin G1 antibody and discuss both merits and limitations of stability assessments based on the diffusional virial coefficient k(D). We find this parameter to be a good predictor of relative protein stability in solutions of different chaotropic salts, and a telling heuristic for the effect of kosmotropes. Both temperature and glycosylation are seen to have a strong influence on k(D), and we examine how these factors affect stability assessments. Protein unfolding is monitored with a fluorescence assay to assist in interpreting the observed aggregation rates.