Steric asymmetry vs charge asymmetry in dilute solution containing large weakly charged ions

Abstract

From Molecular Dynamic (MD) simulations we obtain the osmotic equation of state (EOS) and the energy for solution of large, weakly and asymmetrically charged rodlike ions in a neutralizing background that consists of continuous solvent and small spherical counterions. We elucidate the relative roles of the electrostatic and the steric interactions in the thermodynamic behavior of a fluid, estimate contributions of these interactions to the EOS, and describe the effect of electrostatic screening in solution of asymmetric ions. Electrostatic contribution to the EOS is derived from the variational-field theory in the weak coupling approximation; expressions for the screening parameter are given in a closed form. We show that charge asymmetry has strong impact on the electrostatic parts of both the pressure and the energy. Nevertheless for fluids containing weakly charged bulky ions, repulsion forces tend to overwhelm the electrostatic contribution at very moderate concentrations. Soft repulsion gives an important contribution and diminishes the effect of steric asymmetry. For fluids of electrically neutral particles, our simulations are in good agreement with predictions from the scaled particle theory. A simple empirical EOS is proposed for isotropic fluids containing large, weakly charged cylindrical particles of a moderate aspect ratio.