Effect of temperature and pressure on a water–benzene mixture as studied by infrared spectroscopy

A water–benzene mixture has been studied by infrared spectroscopy at temperatures and pressures in the 473–648 K and 100–350 bar ranges. The OH stretching band intensity has been obtained as a measure of water concentration in the mixture. In the temperature range of 473–523 K, the intensity is almost independent of pressure. On the contrary, at higher temperatures, the absorption intensities exhibit remarkable pressure dependence. Moreover, the intensities increase with increasing temperature towards a certain temperature and then steeply decrease at higher temperatures. It has been found that the turning points of the intensities are located on an extended line of the liquid–liquid–gas three-phase equilibrium curve of the water–benzene mixture and near the critical curve between the two-phase and one-phase regions. This fact suggests that density of the mixture changes steeply across those lines. We have also observed coalescence of two bands, which are assigned to hydrogen-bonded and hydrogen-bond-free OH, respectively, at the higher temperatures, and discussed rotational motion of a water molecule in a hydrogen-bonded cluster.