The phase equilibrium behavior of icy bodies' surface mixtures:The simulation of the methane + acetonitrile system on Titan

Abstract

Titan's surface has a hydrological system based on methane. The interaction of solar ultraviolet radiation and energetic particles from Saturn's magnetosphere results in the creation of nitrogen-containing molecules. When these molecules dissolve in Titan's seas, it could represent the initial step of complex organic chemistry processes. Of particular interest is acetonitrile, which has been detected in the atmosphere as a gas and potentially as ice on the surface. Despite the importance of knowing the phase equilibrium behavior of methane and acetonitrile for understanding the physical and chemical phenomena occurring on Titan's surface, no data exist for this binary mixture. This article presents the first experimental study of phase equilibrium behavior for the binary mixture methane + acetonitrile. In particular, vapor-liquid equilibrium (T, P, x, y) data have been obtained from 223.26 to 293.09 K and for pressures up to about 10 MPa. These data have allowed regressing the parameters of an equation of state for the representation of the fluid phases of the mixture. The equation of state has been used in the classical approach framework for predicting the phase equilibrium behavior of the methane + acetonitrile mixture at temperature and pressure conditions of interest for studying the phenomena occurring on Titan's surface. Some results have also been obtained with molecular dynamics techniques for the vapor-liquid equilibrium of the binary mixture, at low pressure.