MD simulation of the self-diffusion coefficient and dielectric properties of expanded liquids - I. Methanol and carbon dioxide mixtures

Methanol can dissolve considerable amounts of CO₂ under pressure. When this occurs, viscosity decreases and volume increases, which is a typical feature of expanded liquids. In this study, molecular dynamics (MD) simulations were conducted for methanol and CO₂ mixtures at 50°C and high pressures to explore features of the expanded liquid state in terms of solution structure. Radial distribution function for methanol-methanol molecules showed that methanol molecules formed hydrogen bonds and nearest hydrogen bonds distance was not changed. The self-diffusion coefficients of both methanol and CO₂ were found to decrease monotonically from the pure CO₂ side and then not to change appreciably at methanol mole fractions higher than about 0.5. It should also be noted that the simulation results could qualitatively present the dielectric spectroscopy results reported in the literature. These results showed that methanol molecules make hydrogen bond networks and hydrogen bond networks surrounded CO₂ molecules at mole fractions higher than about 0.5. Further addition of CO₂ into methanol caused the hydrogen bond networks to break up and to form smaller hydrogen bond aggregates.