Near-Infrared Study of Water and Water-Benzene Mixtures at High Temperatures and Pressures

Near-infrared absorption of water and water-benzene mixtures has been measured at temperatures and pressures in the range of subcritical and supercritical condition of water. The molar absorption intensity of the OH stretching overtone transition of neat water was obtained. It decreases linearly with increasing pressure in the region of gas and gaslike density. Enthalpy for dimerization has been estimated to be 15 3 kJ/mol from temperature dependence of the slopes. Molar absorption intensity in the liquidlike-density region exhibits a good correlation with the first band moment. Using this correlation, we have estimated concentrations of water in both the water-rich phase and the benzene-rich phase of the mixtures from the integrated absorption intensities of the OH stretching overtone band. Benzene concentrations were obtained from the CH stretching overtone band of benzene. From these concentrations, the densities of the both phases were estimated and compared with the average densities before mixing, which were calculated from literature densities of neat water and neat benzene. It is found that anomalously large volume expansion on the mixing occurs in the vicinity of the critical region of the mixtures. Furthermore, magnitude of the relative volume change increases with decreasing molar fraction of benzene in the present experimental range. It is suggested that dissolving a small amount of benzene in water induces a change in the fluid density from a liquid-like condition to a gas-like condition in the vicinity of the critical region.