Phase transitions of fluids confined in nanopores

Abstract

Confined nanoscale fluids occur in many technologically important phenomenon such as boundary lubrication, adsorption, adhesion and catalysis. Fluids confined at these length scales are spatially inhomogeneous and fluid properties can be significantly different from the corresponding bulk fluid. The focus of this thesis is to study phase transitions of fluids confined in slit-shaped nanopores using both molecular simulation techniques and density functional theory. In the first part of this thesis we present our results on free energy computations for methane confined in a slit-shaped graphite pore. Using a combination of GCMC simulations with umbrella sampling we calculate the Landau firee energy landscape for the system. All interaction potentials were assumed to have a 12-6 Lennard-Jones (LJ) form. Simulations are carried out for a bilayer system at a pore width of 0.95 nm where the square lattice is stable at low temperatures. Structural quantities, such as the pair correlation function, bond order correlation function and bond order parameters are used to classify the different phases. Our results indicate that the melting of a square lattice to a liquid occurs by a two step process with an intermediate tetratic phase. In the first transition, the solid melts to a tetratic phase which is characterized by positional disorder and quasi-long-range orientational order. In the second transition the tetratic phase melts to a liquid. From the Landau free energy landscapes the liquid-tetratic coexistence is found to occur at 224 K and the tetratic-crystal coexistence occurs at 208 K. The value of the exponent which corresponds to the slope of the algebraically decaying envelope of the bond order correlation function ranges from 0.21 to 0.039 across the transition. Density functional theories have been used to study the adsorption in confined inhomogeneous fluids. Although density functional theories have been used to study freezing in the bulk, extensions to study freezing transitions between different lattices that have been observed for confined fluids have not been attempted. In the last part of the thesis we present our preliminary results on the application ofdensity functional theories to confined inhomogeneous fluids. Towards this end, we have studied a one dimensional weighted density free energy functional formulation to capture the density oscillations that are present for a hard sphere fluid confined between hard walls. Some preliminary work on the application of the functionals based on the fundamental measure theories to hard discs is also presented.