Amphiphile-water systems and electrostatic interactions

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Abstract
Electrostatic interactions in an aqueous medium are analysed with amphiphile-water systems as models. It is concluded that the mean field Poisson-Boltzmann approximation is often adequate to describe electrostatic effects in ionic systems particularly with monovalent counterions. Two approximations inherent in the Poisson-Boltzmann equation are discussed in detail: (i) the neglect of ion-ion correlations, which can be serious when considering the force between two particles and (ii) the continuum ideal dielectric treatment of the solvent, which can be questioned both with respect to the local character of the dielectric response, dielectric saturation and dielectric inhomogeneities. Saturation effects are least problematic, while the situation is unclear with respect to the local character of the dielectric response. As regards dielectric inhomogeneities there are clearly unsolved problems, particularly for the interfacial region. The force between zwitterionic surfaces is analysed in terms of a deviation from the asymptotic behaviour obtained by the Lifshitz theory. One should expect a substantial k-dependence of the dielectric response for a zwitterionic layer and this effect may be essential for understanding the interaction between two such surfaces at intermediate and short separations