Monte Carlo simulation for solubility and spatial structure of fatty acid and higher alcohol in supercritical carbon dioxide with octane

Monte Carlo simulation has been applied to calculate the static properties such as solubilities and spatial structures of the fatty acids palmitic acid (C₁₅H₃₁COOH) and stearic acid (C₁₇H₃₅COOH), and of the higher alcohol stearyl alcohol (C₁₈H₃₇OH) in supercritical carbon dioxide with octane at 308.2 K. Carbon dioxide and octane were treated as single-site molecules for simplification, while the chain molecules (fatty acids and higher alcohol) were approximated as many-site molecules. The residual chemical potentials of the chain molecules in supercritical carbon dioxide with octane were calculated by the isothermal-isobaric Kirkwood method. It was shown that the solubilities (solid-gas equilibria) of fatty acids and higher alcohol in supercritical carbon dioxide with octane as a cosolvent can be calculated quantitatively by introducing an inter-site interaction parameter between unlike pair sites. Further, the mean-square end-to-end separations and the radial distribution functions of carbon dioxide and octane for chain molecules are reported as fundamental knowledge of the microstructure of chain molecules in the supercritical fluid phase.