Molecular dynamics simulations of 1,2-dimethoxypropane and 1,2-dimethoxyethane in aqueous solution

We report on the thermodynamic properties, conformations, and local structure of aqueous solutions of 1,2-dimethoxypropane (DMP) as a function of composition and temperature as obtained from molecular dynamics simulations and compare results with simulations of aqueous solutions of 1,2-dimethoxyethane (DME). The density of DMP/water solutions was found to be systematically lower than that for DME solutions of the same composition, reflecting the lower density of neat DMP compared to DME. The excess volume of DMP/water solutions closely resembles that of DME/water solutions, indicating that the nature of DMP solvation is similar to that of DME. The magnitude and composition dependence of the self-diffusion coefficient of water and ether in the DMP/water solutions were also found to closely resemble those of DME/water solutions. As in DME/water solutions, the populations of DMP conformers with favorable dipole-dipole interactions between the ether and solvating water (i.e., hydrophilic conformers) were found to increase with increasing dilution (water content). The free energy of stabilization (relative to the respective ttt conformers) for hydrophilic conformers in dilute solution was found to be around 1.5 kcal/mol for both DME and DMP solutions. The local structure, e.g., pair distribution functions and extent of hydrogen bonding, is similar for the more dilute DMP and DME solutions. However, the intrinsically higher conformational energy of the hydrophilic DMP conformers results in a much lower fraction of these conformers for all solution compositions compared to DME solutions. At higher ether content, this leads to significantly greater clustering of water molecules in DMP solutions and is probably the principal reason for the disparate phase behavior seen for poly(ethylene oxide) and poly(propylene oxide).