MICROEMULSIONS AS MACROELECTROLYTES

Water-in-oil microemulsions, composed of discrete aqueous droplets dis persed in a continuous oil medium, constitute a special class of elect rolyte solutions. Such macroelectrolytes are analogous to conventional electrolyte solutions in most respects, with the notable difference t hat, in a microemulsion, the ionic (droplet) charge is not fixed but d epends on the droplet interactions. Describing the microemulsion as a primitive-model electrolyte mixture with ions of variable charge and e valuating the statistical mechanics within the mean-spherical approxim ation (MSA), we construct a self-consistent theory of charge fluctuati ons and droplet interactions in ionic microemulsions. The droplet char ge distribution is calculated as a function of the size, shape, polydi spersity, and volume fraction of the droplets. We argue that the net d roplet charges can have a decisive influence on microemulsion structur e, especially at the higher volume fractions where clustering and spin odal decomposition are observed. At lower volume fractions, where the MSA treatment should be quantitatively accurate, the Coulomb interacti on between charged droplets has no effect on the structure factor dedu ced from scattering data. (C) 1995 American Institute of Physics.