(290e) Determining the Free Energy of Nano-Confined Fluids

It is well know that when fluids are confined to the nanoscale, between molecularly smooth surfaces, their properties differ dramatically to the bulk fluid at the same thermodynamic conditions. However, the exact nature of these differences and, most importantly, the underlying structural changes causing them, remain somewhat unclear. Clarifying this issue purely experimentally is incredibly challenging since direct observation of the structure of the nano-confined fluid is currently not possible. By contrast molecular simulation, with its inherent atomic resolution, is able to provide precise details of any structural change. However there are reasonable doubts as to whether the changes observed via simulation represent stable equilibrium structures. In the determination of stable structures, the calculation of the free energy of a system is fundamental. However, whilst the calculation of fluid phase free energies is relatively straight forward and, thus, widely utilized, the calculation for solid structures remains non-trivial. The most commonly used method for calculating the absolute free energy of a crystalline solid is the Einstein Crystal method of Frenkel and Ladd and here we adapt and apply it to calculated the free energy of a Lennard-Jones solids nano-confined between molecularly smooth, but structured, surfaces.