Accurate solution of a highly asymmetric electrolyte: Molecular dynamics simulation and integral equation

Abstract

An asymmetric electrolyte has been investigated by means of molecular dynamics simulations and integral equation techniques. The charge asymmetry of the electrolyte is 1:20 which corresponds to small micelles formed by ionic amphiphiles. In the simulations, the long ranged Coulombic force was replaced by a spherical Ewald truncated (SET) potential. The effect of the long ranged part was taken into account by applying the reference hypernetted chain (RHNC) perturbation method on the simulation data. The result of this procedure converges to the result of the Coulombic potential as the force gets more long ranged in the simulation. An alternative way to treat the long ranged interaction in simulations, the minimum image convention, breaks down for the present system. The almost exact thermodynamic and structural results for the Coulombic system are used to examine the validity of the hypernetted chain (HNC) approximation. Although the HNC equation gives accurate energy and pressure, it overestimates the micelle–ion and ion–ion correlation which results in a too weak effective micelle–micelle repulsion. Moreover, the HNC approximation results in a too high critical temperature and the present results suggest that mixed integral equations may be useful to obtain more reliable phase boundaries.