Molecular dynamics study of the thermodynamic and structural properties of methanol and polarizable/non-polarizable carbon tetrachloride mixtures

Abstract

Molecular dynamics simulations have been performed for methanol/carbon tetrachloride mixtures over the whole composition range at 323 K and zero pressure. The OPLS (optimized potentials for liquid simulation) potential energy parameters by Jorgensen were used to model the methanol potential. Both a non-polarizable carbon tetrachloride model taken from McDonald, Bounds, and Klein [Mol. Phys. 45, 521 (1982)] as well as a polarizable model were used. The latter model was devised by combining the model of McDonald, Bounds, and Klein with the atomic polarizabilities proposed by Applequist, Carl, and Fung [J. Am. Chem. Soc. 94, 2952 (1972)]. We show that the role of the methanol–carbon tetrachloride interactions are very important in discussing the thermodynamic mixing properties. In order to reproduce the asymmetric behavior of the excess enthalpies with respect to composition it is necessary to include the non-additive polarization interaction. The structure and especially hydrogen bonding properties are discussed. Radial distribution functions show a strong tendency of methanol to preserve the local order similar to the one in the pure fluid. The deviations from random mixing are more pronounced at the lower mole fractions. This is explained by a frustration model. At low methanol concentrations the molecules get more freedom to align themselves in energetically favorable (hydrogen bonded) configurations. Throughout the composition range, the majority of the methanol molecules is found to be engaged in two hydrogen bond, As in the pure fluid, this leads to the pattern of hydrogen bonded winding chains. Upon dilution the degree of cross-linking between the chains diminishes whereas the free monomer fraction rises. Furthermore a significant number of the remaining chains close to form cyclic polymers.