Electrostatic Interactions in Complex Liquids : A Thermodynamic Analysis Based on the Poisson-Boltzmann Equation and the Flexible Surface Model

Sammanfattning: The aim of this study was to analyse physico-chemical properties of some complex liquids using the Poisson-Boltzmann (PB) cell model; to extend the theory to cover bilayer systems with complex global geometry; finally to apply this new extended model to such bilayer systems. The PB cell model provides a practical tool to examine counterion association and phase behaviour for structures of simple geometries; cylindrical and spherical. The model applies to self-assembled aggregates of ionic surfactants as well as to a polyelectrolyte system. The framework of the PB cell model provides a consistent route to derive the electrostatic contribution to Helfrich's bending energy at finite salt- and surfactant concentrations. The bending energy concept, with the extension of a variable electrostatic contribution to the bending rigidity, the saddle splay modulus, and the product (bending rigidity)'Ho (where Ho is the spontaneous mean curvature) was adopted for the description of the fluid mono- and bilayer phases, bicontinous microemulsion and L3 (sponge), respectively. This formalism successfully describes many features of a ionic microemulsion system and the inclusion of both salt and surfactant concentration dependence was necessary for this purpose. The same formalism provided the basis for a model description of the ternary system AerosolOT/NaCl/Water. A theoretical phase diagram, containing the L3 phase in competition with the lamellar phase and dilute solution, was calculated. The calculations captured the features typical for L3: narrowness in the one phase region and the characteristic sequence of phase transitions.