Confined fluid and the fluid-solid transition: Evidence from absolute free energy calculations

The debate on whether an organic fluid nanoconfined by mica sheets will undergo a fluid-to-solid transition as the fluid film thickness is reduced below a critical value has lasted over two decades. Extensive experimental and simulation investigations have thus far left this question only partially addressed. In this work, we adapt and apply absolute free energy calculations to analyze the phase behavior of a simple model for nanoconfined fluids, consisting of spherical Lennard-Jones (LJ) molecules confined between LJ solid walls, which we use in combination with grand-canonical molecular dynamics simulations. Absolute Helmholtz free energy calculations of the simulated nanoconfined systems directly support the existence of order-disorder phase transition as a function of decreasing wall separation, providing results in close agreement with previous experiments and detailed atomistic simulations.