Simulations of Nonuniform Fluids with Long-ranged and Short-ranged Interactions

Title of dissertation: SIMULATIONS OF NONUNIFORM FLUIDS WITH LONG-RANGED AND SHORT-RANGED INTERACTIONS Shule Liu, Doctor of Philosophy, 2012 Dissertation directed by: Professor John D. Weeks Department of Chemistry and Biochemistry Institute for Physical Science and Technology 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. SIMULATIONS OF NONUNIFORM FLUIDS WITH LONG-RANGED AND SHORT-RANGED INTERACTIONS by Shule Liu Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2012 Advisory Committee: Professor John D. Weeks, Chair/Advisor Professor Millard Alexander Professor Christopher Jarzynski Professor Devarajan Thirumalai Professor Sergei Sukharev, Dean’s Representative c © Copyright by Shule Liu 2012