Computer experiments on aqueous solution. I. Monte Carlo calculation on the hydration of methanol in an infinitely dilute aqueous solution with a new water–methanol pair potential

Abstract

Monte Carlo calculations have been carried out both for pure water and an infinitely dilute aqueous solution of methanol at 298.15 K and ordinary density by the Metropolis scheme in NVT ensemble. The total number of molecules is 216, one of which is methanol in the case of aqueous solution. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–methanol interaction from ab initio LCAO SCF MO calculations for more than 450 different dimeric configurations with a STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. For the aqueous methanol solution, 4 500 000 configurations have been generated and the final 1 500 000 are used to obtain average quantities for energy and various distribution functions. It is found that, with the introduction of one methanol molecule, the potential energy and structure of water tend to stabilize as a whole. Such stabilizations in energy and structure are found to be angular dependent, owing to anisotropic nature of methanol–water interactions. Calculations for several regions classified according to the direction from the methanol molecule have been attempted to examine the contribution of hydrophilic and hydrophobic regions to the total hydration structure. The hydrophilic region as a whole does not show a large energetic stabilization, though pair interaction distribution function clearly indicate a larger hydrogen bonding interaction for some water molecules in this region. In contrast to this, the hydrophobic region is characterized by larger overall energy stabilization as a result of the sum of a large number of smaller interactions. Strong hydrogen bondings in the hydrophilic region seem to act cooperatively with the structural stabilization in the hydrophobic region to form a fairly stable hydration structure around methanol. This is clearly shown by density distribution diagrams for water around methanol which visualize an iceberglike structure of water, remarkably in comparison with the structure in pure water.