(123e) Solvent Properties of Water+Methanol Binary Mixture at High Temperatures Via MD Simulation

Alcohol and water are expected to become very effective solvents at higher temperatures because of their thermodynamically and chemically stability. The properties of these components are very similar at room temperature and completely miscible but become fairly different at supercritical state. This is one reason why supercritical water and supercritical methanol have been applied separately to various polymer-decomposition processes with the aim of inducing different reaction mechanism and product selectivity.

For wider applicability of these solvents at higher temperature, we have tried to use the mixture of alcoholand water at higher temperature. We focused on density and have measured the density of methanol+water mixture with a newly developed vibration tube densimeter in the temperature ranging from 573 to 673 K under pressures. The experimental results indicated that the excess molar volumes VE for methanol+water mixture was positive at 673 K and 25 MPa in the whole composition range and changed from negative to positive with increasing the methanol composition (XM) at 623 K and 20 MPa. This suggested that methanol+water solution may have attractive and repulsive force dependent on the composition at high temperature, which can be attributable to the balance of potential energy and kinetic energy.

We disclose the above behavior by using MD simulation[1][2]. The calculated radial distribution function and potential mean force indicated the possible change in intermolecular force from attractive to repulsive with increasing XM. The calculated diffusion coefficients of water(DW) and methanol(DM) were also evaluated from mean square displacement. The values of DW and DM exhibited similar behavior and increased drastically at XM = 0.2 ~ 0.4, indicated that the molecular mobility was dominated by water at lower XM and by methanol at higher XM and that the transition was induced by the hydrogen bonding network structure change in the presence of the methanol molecules in this region.

[1] T. Honma et al., Fluid Phase Equilibria, 194-197 271. (2002).

[2] T. Honma et al., J. Phys. Chem. A, 107 3960. (2003).