Peripheral proteins can trigger the formation of domains in mixed fluid-like lipid membranes. We analyze the mechanism underlying this process for proteins that bind electrostatically onto a flat two-component membrane, composed of charged and neutral lipid species. Of particular interest are membranes in which the hydrocarbon lipid tails tend to segregate owing to nonideal chain mixing, but the (protein-free) lipid membrane is nevertheless stable due to the electrostatic repulsion between the charged lipid headgroups. The adsorption of charged, say basic, proteins onto a membrane containing anionic lipids induces local lipid demixing, whereby charged lipids migrate toward (or away from) the adsorption site, so as to minimize the electrostatic binding free energy. Apart from reducing lipid headgroup repulsion, this process creates a gradient in lipid composition around the adsorption zone, and hence a line energy whose magnitude depends on the protein's size and charge and the extent of lipid chain nonideality. Above a certain critical lipid nonideality, the line energy is large enough to induce domain formation, i.e., protein aggregation and, concomitantly, macroscopic lipid phase separation. We quantitatively analyze the thermodynamic stability of the dressed membrane based on nonlinear Poisson-Boltzmann theory, accounting for both the microscopic characteristics of the proteins and lipid composition modulations at and around the adsorption zone. Spinodal surfaces and critical points of the dressed membranes are calculated for several different model proteins of spherical and disk-like shapes. Among the models studied we find the most substantial protein-induced membrane destabilization for disk-like proteins whose charges are concentrated in the membrane-facing surface. If additional charges reside on the side faces of the proteins, direct protein-protein repulsion diminishes considerably the propensity for domain formation. Generally, a highly charged flat face of a macroion appears most efficient in inducing large compositional gradients, hence a large and unfavorable line energy and consequently lateral macroion aggregation and, concomitantly, macroscopic lipid phase separation.

The current interest in mixed cationic-zwitterionic lipid membranes derives from their potential use as transfer vectors in nonviral gene therapy. Mixed cationic-zwitterionic lipid membranes have a number of structural properties that are distinct from the corresponding anionic-zwitterionic lipid membranes. As known from experiment and reproduced by computer simulations, the average cross-sectional area per lipid changes nonmonotonically with the mole fraction of the charged lipid, passing through a minimum at a roughly equimolar mixture. At the same time, the average orientation of the zwitterionic headgroup dipoles changes from more parallel to the membrane plane to more perpendicular. We suggest a simple mean-field model that reveals the physical mechanisms underlying the observed structural properties. To backup the mean-field calculations, we have also performed Monte Carlo simulations. Our model extends Poisson-Boltzmann theory to include (besides the cationic headgroup charges) the individual charges of the zwitterionic lipid headgroups. We model these charges to be arranged as dipoles of fixed length with rotational degrees of freedom. Our model includes, in a phenomenological manner, the changes in steric headgroup interactions upon reorientation of the zwitterionic headgroups. Our numerical results suggest that two different mechanisms contribute to the observed structural properties: one involves the lateral electrostatic pressure and the other the zwitterionic headgroup orientation, the latter modifying steric headgroup interactions. The two mechanisms operate in parallel as they both originate in the electrostatic properties of the involved lipids. We have also applied our model to a mixed anionic-zwitterionic lipid membrane for which neither a significant headgroup reorientation nor a nonmonotonic change in the average lateral cross-sectional area is predicted.

Within the framework of linear and non-linear Poisson-Boltzmann theory, we study the effective interaction of a single charged colloidal sphere in an aqueous electrolytic solution with an air-water interface. The effects of varying the salt concentration and the colloidal surface charge density on the effective interaction are being investigated, with a view to understanding some physical phenomena, which include electrostatic adsorption and trapping at the air-water interface. Results show an electrostatic double-layer barrier to the colloid's approach to the interface which can be lowered considerably by increasing the salt concentration. For enough added salt, the charged colloid should be able to suddenly pop up at the air-water surface, an effect which has actually been observed in recent experiments. We discuss the relevance of our results to other experimental observations, and emphasize the close analogy between the problem considered here and the classical problem of the interaction of two colloids in a bulk suspension.

This article deals with the electric double-layer force between a charged colloidal sphere and a charged dielectric planar wall. To introduce the problem and to uncover the basic physics involved, we start by first reviewing the effective wall-colloid potentials that one obtains in linearized Poisson-Boltzmann theory. The important key concepts in this context are: charge renormalization, confinement effects, salty interfaces, and image-charge effects due to the dielectric discontinuity at the wall. Starting from the potentials derived in linear theory, we then come to approximate wall-colloid potentials that are valid also in the parameter regime where the non-linearity of the Poisson-Boltzmann equation becomes important. The range of validity of these potentials is systematically investigated by comparing them with potentials based on the exact numerical solution to the Poisson-Boltzmann equation. The important parameters of the calculation are the salt content of the electrolytic solution, the colloidal sphere radius, and the surface charge densities on both the wall and the colloid. We then briefly discuss what additional effect a concentrated suspension of such colloidal spheres has on the interfacial colloid, and close with a short report of an optical experiment that has recently been performed to measure the approximate wall-colloid potentials investigated here.