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.

Carboxyl latex particles of two different sizes were used to study the early stages of aggregation in dilute colloidal suspensions. The charging behavior as a function of solution pH was characterized in acid-base titration and electrophoresis experiments at fixed ionic strength; absolute aggregation rate constants were measured by combined static and dynamic light scattering as a function of pH and ionic strength. Up to an ionic strength of 10 mM in a KCl solution, the classical DLVO theory of colloidal stability is seen to work quantitatively. At higher ionic strength, however, well-known discrepancies between theory and experiment are observed. An analysis of the theoretical pair interaction energy suggests that quantitative agreement can be achieved when the energy barrier for reaction-limited aggregation lies at surface separations of at least 1-2 nm. This result is consistent with recent measurements of colloidal forces and interaction energies, as well as with earlier aggregation and deposition studies typically carried out in the unfavorable situation of barriers at subnanometer distances. The theoretical discussion further considers the appropriate choice of a Hamaker constant, the effect of nonlinearity in the Poisson-Boltzmann equation on stability predictions, as well as the role of charge regulation and the error introduced by the Derjaguin approximation.

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

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.

No abstract

When two surfaces with ionizable groups interact across an electrolyte solution, both their equilibrium charge density and the corresponding electrostatic surface potential will depend on the surface separation (charge regulation). The corresponding nonlinear boundary conditions are often replaced, for simplicity, by the limiting conditions of constant charge or constant surface potential. A strategy to linearize the boundary conditions, initially devised for the case of low potentials only, has recently been adapted to situations of arbitrary potential. Within a 1-pK Basic Stern Model suitable for a large class of surface materials, we now address the implications of charge regulation on the level of Poisson−Boltzmann theory. The regulation behavior can be characterized in terms of a single parameter taking values between 0 for constant potential and 1 for constant charge conditions. This parameter depends on the capacities associated with the diffuse part and the compact part of the electrical double layer and can be inferred from acid−base titrations. We discuss the effect of regulation on a variety of measurable quantities for exemplary surfaces of carboxyl latex, silica, and iron hydroxide.

Using stimulus-sensitive microgel particles as an emulsifier, we have prepared a new type of emulsion responsive to pH, ionic strength, and temperature changes. Each of these environmental changes can trigger a volume phase transition in poly(N-isopropylacrylamide) (PNIPAM) microgel particles containing some carboxylic groups. Depending on their hydrophobicity and charging state, such PNIPAM microgel particles can adsorb to the droplets of an octanol-in-water emulsion and provide excellent stability against coalescence and ripening. We have studied in detail the correlation between the particles' response to changes in the solution conditions and the corresponding response of particle-decorated emulsion droplets. In their swollen, hydrophilic state, the microgel particles consistently stabilize the octanol droplets, but inducing a microgel collapse usually results in a destabilization of the emulsion and eventually in phase separation. A notable exception was found at high pH where particles are highly charged: in this regime emulsions remain stable even upon a temperatureinduced collapse of the microgel particles and prove sensitive only to high levels of screening ions. Microscopy studies of toluene-in-water emulsions stabilized by compact polystyrene particles of variable surface charge further suggest an intimate connection between the charge and packing density of interfacial particles and hint at a charge-induced interparticle attraction.

Although equally charged colloidal particles dispersed in clean water are expected to repel each other, an unexplained long-range attraction has consistently been reported for charged colloidal spheres confined by charged macroscopic surfaces. We present an alternative equilibrium measurement of the pair interaction energy for charged spheres near a single charged wall. Analyzing their radial distribution functions for different concentrations reveals a purely repulsive sphere-sphere interaction that is well described by a screened Coulomb potential.