(583e) Volumetric Behavior of Water-Alcohol Mixtures at 673.15 K under High Pressures. I. Methanol and Ethanol Solutions

For a better understanding of chemical and physical processes in fluids, it is important to study the physical properties of fluids. Density is one of the most fundamental thermodynamic properties, and the nature of molecular transport in fluids depends on the density. It is generally agreed that water is an environmentally benign green solvent, and its high temperature and pressure region has recently attracted much attention in various industrial processes due to the wide tunability for its various solvent properties. A water molecule has four hydrogen-bonding site, two of which are donors and others acceptors. Water molecules form the three-dimensional tetrahedral network structure by the effect of intermolecular hydrogen bonding which plays a dominant role for determining the solvent characteristic properties of aqueous solutions in sub- and supercritical region when a solute was introduced in aqueous solutions. Alcohols are also typical hydrogen-bonded fluids whose molecules have not only the hydrogen-bonding sites like water but also hydrophobic alkyl groups. For its amphiphilicity, the interactions between water and hydrophilic or hydrophobic group of alcohol have interested in the study of aqueous solutions. Alcohol molecules strongly affect the three-dimensional water structure, and the water-alcohol mixtures whose structure varies from bulk water are expected to very unique solvent depending on their composition for industrial green chemical applications. The structures of water-alcohol mixtures have been discussed exclusively below the boiling point of water in the literatures. At high temperature and pressure including sub- and supercritical region, however, there have been few studies on the temperature and pressure dependences for a series of water-alcohol mixtures. The discussion of the volumetric properties and the solvent structure for a series of water-alcohol mixtures at high temperatures and pressures will provide useful information about new reaction field and the design of new aqueous solvents in industrial processes. In this study, the densities of water-methanol and water-ethanol mixtures were measured with the previously developed laser-doppler type vibrating tube densimeter at 673.15 K in the pressure range of 25-40 MPa. This is the first step of our volumetric study of water-alcohol mixtures at higher temperatures and pressures. The concentration dependences of the excess molar volumes for water-alcohol mixtures were evaluated from the experimental values and fitted with the Redlich-Kister equation. For all the mixtures studied, the densities at 25 MPa slightly increased with increasing the alcohol composition in water. In the pressure range of 30-40 MPa, on the other hand, the densities decreased in the water-rich region and slightly increased in the alcohol-rich region. The excess molar volumes of all water-alcohol systems studied are positive in the whole composition range and the composition dependences of the excess molar volumes show maxima around 30-40 mol% of alcohol in water. This indicates that the effect of the apparent ?repulsive' interaction between water and a hydrophobic group of alcohol molecule plays a dominant role of the positive deviation from the ideal mixture. In the region of water-rich compositions, the excess molar volumes of ethanol solutions are larger than those of methanol ones. In the region of alcohol-rich compositions, on the other hand, the difference of the excess molar volumes between two solutions was not significant. These results indicate that the effect of the structural change for mixtures in the region of water-rich compositions is larger than that in the region of alcohol-rich compositions. In the region of water-rich compositions, the magnitude of the apparent ?repulsive' interaction depends on the length of alkyl group of alcohol molecules. At high pressures, hydrogen bonds among molecules are strengthened and then broken with increasing pressure and molecules in mixtures begin to condense. These concerted effects of the apparent ?repulsive' interaction and pressure may give the maximum around 30 MPa for the pressure dependences of the excess molar volumes.