Dynamic properties of water/alcohol mixtures studied by computer simulation

We have studied mixtures of alcohol and water in an extensive series of 465 molecular-dynamics simulations with an aggregate length of 713 ns, in order to study excess properties of mixing, in particular the relation between mobility and viscosity. Methanol/water, ethanol/water, and 1-propanol/water mixtures were simulated using an alcohol content of 0–100 mass % in steps of 10%, using the OPLS (optimized potential for liquid simulations) force field for the alcohol molecules and the TIP4P (transferable intermolecular potential with four particles) water model. Computed densities and energies show very good agreement with experimental data for bulk simulations and the mixtures are satisfactory as well. The shear viscosity was computed using nonequilibrium molecular-dynamics simulations. Other properties studied include diffusion constants and rotational correlation times. We find the mobility to correlate well with the viscosity data, i.e., at intermediate alcohol concentrations the viscosity is maximal and the mobility is minimal. Furthermore, we have combined the viscosity and diffusion calculations in order to compute an effective hydrodynamic radius of the particles in the mixtures, using the Stokes–Einstein relation. This analysis indicates that there is no collective diffusion of molecular clusters in these mixtures. For all properties we find that the excess values are underestimated in the simulations, which, given that the pure liquids are described rather well, raises the question whether the potential function is too simplistic to describe mixtures quantitatively. The set of simulations presented here can hence be regarded as a force-field benchmark.