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.