Effect of size and polarity of solutes on partial molar volumes and intermolecular interaction in supercritical fluids

The densities of supercritical (SC) n-pentane and n-pentane–methane, n-pentane–propane, n-pentane–n-heptane, n-pentane–n-decane, n-pentane–carbon dioxide, n-pentane–ethanol, and n-pentane–acetonitrile binary SC mixtures were measured at 476.6 K in the pressure range from 2 to 5 MPa, and n-pentane was SC solvent. The molalities of the solutes were 1.3×10−4 mol g−1. The partial molar volumes (PMVs) of the solutes were calculated on the basis of determined densities. It was observed that the PMVs of methane, propane, n-heptane and n-decane in SC n-pentane change from positive to negative as their size increases, but the peaks of the PMVs always corresponded to highest isothermal compressibility of the fluids. Ethanol, acetonitrile, carbon dioxide, and propane have different structure and polarity, but PMVs of the four solutes exhibit similar behavior, suggesting that the polarity and structure of the solutes are not dominant factors to influence the PMVs for the systems studied in this work under the experimental conditions. Monte Carlo simulation was carried out to study the microstructure of the fluids under some typical conditions where the density measurements were conducted.