Charge fluctuation in reverse micelles

Abstract

Electrostatic interactions in and among reverse micelles in the microemulsion are studied by the grand canonical ensemble Monte Carlo simulation (GCEMC) and by the linear response/Poisson–Boltzmann approximation (LPB). The latter theory is found to reproduce the ion-correlation effects among the micelles seen in the earlier canonical Monte Carlo approach. The open ensemble simulation and the Poisson–Boltzmann (PB) equation are used to study solubilization of simple electrolyte in the microemulsion. In systems with monovalent ions, the PB approximation agrees well with the simulation. Since individual ions are allowed to enter or leave the micelle in our GCEMC simulation, the deviations from the electroneutrality of the droplets are also studied. A good agreement with the LPB theory and with the simple charge fluctuation models considering only the Born solvation energy of charged droplets is observed. For large reverse micelles, the LPB theory and the continuum approach of Eicke and co-workers [J. Phys. Chem. 93, 314 (1989)] proved satisfactory while only the discrete model of Hall [J. Phys. Chem. 94, 429 (1990)] agrees with the simulation for small droplets. The GCEMC results lend support to recent interpretations of electric conductivity of microemulsions. The intermicellar forces due to the correlated charge fluctuation on adjacent droplets are discussed. An approximate expression for the charge fluctuation contribution to the long-ranged pair potential among large droplets with the radius Rm of the form w(r)/kT≂−(1/2)(Rm/r)2 is derived.