Molecular dynamics simulations of 1,2-dimethoxyethane/water solutions. 1. Conformational and structural properties

Molecular dynamics simulations of solutions of 1,2-dimethoxyethane (DME) and water have been performed in order to investigate the conformations of DME and the local solution structure as a function of solution composition and temperature. The simulations employ a previously developed force field based upon ab initio quantum chemistry calculations of DME/water interactions. The DME conformer populations show a strong dependence on temperature and solution composition and are quite different in aqueous solutions from those found in the gas phase or neat liquid. An analysis of the dependence of the conformer population distribution on temperature allows us to estimate the relative free energies of solvation for different conformers in water solution, which are found to be primarily energetic in origin for dilute solutions. The conformation dependence of the solvation free energy, resulting in "hydrophilic" and "hydrophobic" conformers, is explained in terms of differences in polar interactions between DME and neighboring water molecules. Analysis of the local structure of water around different DME conformers reveals that the degree of DME/water hydrogen bonding is nearly independent of the DME conformer for dilute solutions. The extent of water/water hydrogen bonding is also found to be nearly independent of composition over a wide composition range. To maintain a nearly constant level of water-water hydrogen bonding with increasing DME concentration, water is not randomly distributed in the system on a nearest-neighbor length scale but rather tends to form clusters of 4-5 molecules, The entropic penalty associated with this water structure increases with increasing DME concentration and temperature, thereby increasing the free energy of hydrophilic conformers relative to hydrophobic conformers.