Simulations of nonuniform fluids with long-ranged and short-ranged interactions

Abstract

The study of nonuniform fluids is of great importance in many areas of chemistry, biophysics,and materials science. Computer simulations of model systems have provided great insight into fundamental physical issues. We have studied two model systems: amphiphiles on a hydrophilic silica surface and charge overcompensation at the boundary of a colloid by multivalent ions. In the first model system, the organization of propionitrile and methanol near the surface has been studied via simulations. Both molecules can form highly organized bilayer-like structure near the surface. For propionitrile molecules, inside the bilayer-like structure hydrocarbon molecules intertwine with each other and form a closely packed structure. For methanol, molecules are strongly bonded to the silica surface with hydrogen bonds, resulting in much stronger hydrogen bonding than in the bulk and extremely slow dynamics.

In our second model system, we use the local molecular field (LMF) theory to calculate the structure and solvation free energy of the system with a highly charged colloid immersed in trivalent salt. A mimic system with only short-ranged interactions was constructed using the LMF theory. By solving the LMF equation self-consistently, we have obtained the correct structure that indicates overcharging, where the charge of the colloid is overcompensated by the charge of trivalent ions. Then by taking a series of steps in a thermodynamic cycle, we have also calculated the solvation free energy of the colloid, using only results from the mimic system, and found very good agreement with more costly calculations carried out in the full long-ranged system.