Molecular Modeling at Interfaces: Phase Equilibria of Complex Fluids, and Mechanical Properties of Nanostructures

Molecular simulation is a very powerful technique that allows us to predict thermodynamic and transport properties of bulk and confined phases, as well as phase equilibria and interfacial properties. These properties are often crucial to the design of chemical and related industrial processes. Molecular simulation can predict these properties over a wide range of conditions, in contrast with experiments, which at extreme conditions (e.g., high temperature and/or high pressure) are often very difficult and in some cases dangerous. Furthermore, semi-empirical and empirical engineering models can frequently only be used for the specific systems to which they are fitted – that is, they are interpolative rather than predictive. Therefore molecular modeling methods, including simulation, can play a very useful role in the design of new processes, as well as the prediction of new phenomena.

In this thesis, we applied molecular simulation methods to four separate problems: vapor-liquid equilibrium for a polarizable model of water, liquid-liquid interfacial properties, phase equilibrium in confined systems, and mechanical properties of nano scale systems.

The first three problems imply the study of phases in equilibrium under different conditions. The most simple is the vapor-liquid equilibrium of a single component.

Thermophysical properties such as coexistence densities, vapor pressure, surface tension, and interfacial thickness were obtained for a polarizable model of water and compared with other simpler potential models and experimental results. Using the same methodology, the interfacial properties of binary and ternary mixtures with polar and non-polar fluids exhibiting liquid-liquid equilibrium were studied. The dependence of the interfacial properties with increasing molecular size of one compound was studied. For ternary mixtures, the presence of a surfactant molecule was studied at different concentrations of the surfactant. Phase equilibria inside single carbon nanotubes were studied for single and binary aqueous systems, the coexistence liquid densities were calculated and compared with results of water in hydrophobic nanopores, and in the bulk. The phase equilibria behavior was studied indirectly in terms of the pressure inside the nanotube.

Molecular simulation is a very suitable tool to study mechanical properties of systems at the nanoscale. The interlayer friction forces in double-wall carbon nanotubes were studied for systems with axial length up to 100 nm. The oscillatory behavior resulting when the inner tube is pulled out and released was studied as a function of nanotube length, temperature, and internal conformation. The latter enabled the study of systems with different degree of commensurability.