Propiedades interfaciales y equilibrio de fase de mezclas fluidas mediante simulación Monte Carlo

Laburpena

The understanding, from a molecular perspective, of the phase equilibria and interfacial properties of condensed systems, has increased in the last years. However, the determination of the thermodynamic and structural properties of inhomogeneous systems of complex mixtures (molecular chains, associating systems...), such as the interfacial thickness, adsorption, and surface tension, among others, is limited compared with that of homogenous systems. The precise knowledge of the interfacial properties is essential from a theoretical point of view and for the design of process of industrial interest. Nowadays, it is possible to determine the global phase diagram and the interfacial properties of a given model using molecular simulation. This is probably due to the enormous increasing of the available computational resources and also due to the development of new and more effective computational methods during the last years. In this Thesis, we combine different Monte Carlo simulation techniques to understand, from the Statistical Mechanics point of view, how the microscopic parameters of a given molecular model determine the interfacial properties of systems that exhibit vapour-liquid and liquid-liquid equilibria. The analysis on how the long-range corrections affect the behaviour of the system, particularly due to the discontinuities existing in continuous potential that are truncated at a given distance, is an important and delicate issue. In this Thesis, we extend and improve an existing methodology in the literature to account for these long-range corrections. We put special emphasis on how these corrections affect to the interfacial tension and extend and improve the standard method for determining the pressure tensor in inhomogeneous systems (virial or mechanical route). This methodology is easy to apply and implement in systems form from spherically symmetric molecules, but this is not the case of complex molecular systems. In this Thesis, we use alternative methods, based on the thermodynamic route, such as the volume perturbation technique, the Test-Area (TA) methodology, and the Wandering Interface Method (WIM). We have determined the vapour-liquid and liquid-liquid interfacial tension of binary mixtures of spherical systems, and the vapour-liquid surface tension of flexible chains form from monomers that interact through the Lennard- Jones and Square-Well intermolecular potentials, among many other interfacial properties. The Soft-SAFT approach, based on Wertheim’s theory, is used to calculate the global phase diagram of the mixtures studied in order to compare the simulation results obtained in this Thesis. In the case of flexible Lennard-Jones chains, we have studied the effect of chain length and flexibility on the phase equilibria and interfacial properties. We have also calculated the vapour pressure of Square-Well chains along the vapour-liquid coexistence combining Monte Carlo simulations of the interface and the thermodynamic integration technique, as well as the associated error from the Sinthetic Method. Finally, we have also analysed some universal scaling relationships for Lennard-Jones and Square-Well molecular chains with different degrees of flexibility.