Vapor–liquid equilibria and heat effects of hydrogen fluoride from molecular simulation

Abstract

The vapor–liquid coexistence densities, vapor pressure, and heat of vaporization of hydrogen fluoride (HF) is calculated via Monte Carlo simulation from three intermolecular potential models that are found in the literature. The first is a pure pair potential based solely on ab initio data, the second is a semi-empirical pair potential which uses an ab initio derived surface fitted with dimer spectroscopic data, and the third is an effective pair potential that was fit to experimental data for the condensed phase. As expected, the effective potential reproduces the saturated liquid densities more accurately than the others do, while all the potential models predict the wrong slope and curvature in the vapor pressure curve. The inability to reproduce the vapor pressure dependence on temperature is connected to the models’ poor prediction of the heat of vaporization at temperatures below 400 K. A biasing algorithm is introduced to study the superheated-vapor heat capacity, density, association number, and oligomer distribution along three low-pressure isobars using both the semi-empirical and effective pair potentials. It is found that both these potential models do predict a peak in the heat capacity, however, they are at cooler temperatures and only about half the magnitude relative to the experiment. When comparing the potential models to each other, it is found that the semi-empirical pair potential predicts the onset of near-ideal gas conditions at about 30 K cooler than the effective pair potential. Additionally, the percentage of ring oligomers predicted by both models is considerable at all but the highest temperatures. Both models also agree that the monomer and cyclic tetramer are the two most important species at the nonideal states.