Molecular dynamics simulations of aqueous solutions of short chain alcohols. Excess properties and the temperature of maximum density

Abstract

The excess enthalpy h and excess volume v of the binary systems {methanol, ethanol, 1-propanol, 2-propanol, and tert-butanol + water} over the whole composition range at 298.15 K and the temperature of maximum density, TMD, in the alcohol diluted region were studied by MD simulations at atmospheric pressure. Our model systems were defined using the flexible version of the TIP4P/2005 model of water, the OPLS-AA for alcohols, and the geometric combining rule for all the Lennard-Jones cross interactions, with the exception of those between oxygen sites of water and alcohol molecules. These latter values were fitted to reproduce the experimental excess properties of the mixtures. This new parameterization allows for an appropriate description of the excess thermodynamics of these short-chain alcohol/water systems over the whole composition range. The changes in the temperature of maximum density of water induced by the addition of small amounts of alcohol concentration are, however, not adequately reproduced by our models. In all instances we observe a pronounced decrease of the simulated TMDs when alcohol concentration increases. This is in sharp contrast with the experimental behavior, which at very high dilution displays slight increases in the TMD right before the decreasing regime sets in for higher alcohol concentrations. For similar alcohol mole fractions, the simulated decrease of the TMDs grows in parallel with the size of the alkyl group of the alcohol. A similar situation is found experimentally for concentrations higher than those that increase the TMD. These differences disappear when the TMD change is expressed in terms of the mass concentration of alcohol. This leads to very similar Despretz constants for all solutions studied in this work.