Volumetric behavior of water–methanol mixtures in the vicinity of the critical region

Densities of water–methanol mixtures at 573 and 588 K and at pressures in the 100–200 bar range have been measured with a vibrating-tube densimeter. Temperature and pressure dependence of the excess molar volumes together with the previous results was discussed. A large negative-to-positive sigmoidal change of the excess molar volumes as a function of methanol mole fraction was interpreted on the basis of an estimated critical locus of the mixtures. The volumetric behavior of the mixtures was compared with that of the previously reported water–benzene mixtures by estimating the relative volume change on mixing. A large negative volume change at the lower methanol concentrations is in sharp contrast to the large positive change for the water–benzene mixtures. This contrast may be attributable to characteristic features of aqueous solutions of hydrophilic and hydrophobic substances in the vicinity of the critical region. The behavior of the water–methanol mixtures at the lower methanol mole fractions was discussed in terms of the local solute–solvent structure by estimating radial distribution functions and self-diffusion coefficients from molecular dynamics calculations.