A model for molecular emulsions: Water and weak water mixtures

The SPC/E water model is mixed with three << weaker >> versions of it, obtained by reducing the initial partial charges by multiplicative factors of 1/3, 2/3 and 4/5, respectively, while keeping the same diameter as water, and adjusting only the energy parameters such as to keep each neat substance in a dense liquid phase under ambient conditions. These models cover the observed behaviour of many realistic aqueous mixtures, ranging from demixing (the 1/3 model) to fully mixed hydrophobic-like (2/3) or hydrophilic-like (4/5) situations, the latter both showing strong and weak micro-heterogeneity, respectively. The simulations show that micro-segregation arises even when all constituents have the same length scale, under the sole influence of hydrogen bonding interactions. However, this micro-heterogeneity itself introduces a second length scale by producing domain oscillations in the distribution functions in the nanometer range, that can be captured by making a formal analogy with micro-emulsions. This approach explains the origin of the anomalously large Kirkwood-Buff integrals, often obtained in simulations of realistic aqueous mixtures, as a transient behaviour in the domain range. The analogy can be used to calculate the correct integrals by accounting for domain statistics, without the need to perform expensive large scale simulations. (C) 2012 Elsevier B.V. All rights reserved.