Molecular dynamics study of the local structure and diffusivity of partially miscible water/n-alcohols binary mixtures

Abstract

In this work, we look at partially miscible water/n-alcohol mixtures that have been studied extensively by experiment and, in some cases, by classical simulation methods. As a reference point, we chose Su et al.'s [J. Chem. Phys. 132 (2010), 044506] experimental study of the diffusion of water in its binary mixtures with n-alcohols and n-alkanes. Their measurements showed that diffusivity of water in n-alcohols is substantially lower than in n-alkanes of similar size whereas its decrease with increasing n-alcohol viscosity violates Stokes–Einstein relation, a result attributed by them to the hydrogen bonded water molecules spending most of their time near the n-alcohol hydroxyls. To explore their hypothesis that that is the main determinant of water's dynamic behaviour in those mixtures, we employed the molecular dynamics method to simulate the organic phase of binary mixtures of water and n-alcohols (butan-1-ol, pentan-1-ol, and hexan-1-ol) at ambient conditions. The chosen force fields (SPC/E and TraPPE) were shown to predict accurately the systems' volumetric properties. Also, self-diffusion coefficients of water compared very well with experimental measurements and previous simulations. As the local structure and dynamics of the mixtures are not fully understood so far, special attention was paid to the interaction sites taking part in hydrogen bonding, their pair distribution functions and the local structure in their vicinity. Our results, expressed in terms of specific quantitative descriptors, provide a clear picture of hydrogen-bonding connectivity between n-alcohol molecules, water molecules and each other. In particular, it is shown that water and n-alcohol molecules take part in complex extended structures with a rather stable topology, thus confirming, and at the same time, enriching Su et al.'s initial hypothesis. Both species occupy places in these structures in proportions depending on the mixtures' composition — and that is the main cause of their particular diffusive behaviour.