Aqueous tert-butanol mixtures: a model for molecular-emulsions

By analogy with micro-emulsion, we introduce the molecular-emulsion picture to describe particular aqueous mixtures. The analogy is set by introducing the equivalent of the Teubner-Strey structure factor, the latter which is traditionally used to describe the structure of micro-emulsions. The main difference resides in the fact that the size of the oil and water domains are not in the micrometer, but in the nanometer scale. This implies that the molecular size and the molecular geometry cannot be neglected anymore. The introduction of this analogy is used to settle the problem of properly describing with computer simulations highly micro-heterogeneous aqueous mixtures. In particular, the issue of whether or not the Kirkwood-Buff integrals represent solely concentration fluctuations is settled by showing the contribution of the micro-heterogeneity to these integrals through the presence of an associated pre-peak in the structure factors. Both the Optimized Potentials for Liquid State (OPLS) and Transferable Potential for Phase Equilibria-United Atoms (TraPPE-UA) force fields for tert-butanol turn out to be remarkably good in describing the structure of the corresponding aqueous mixtures, when the above-mentioned analogy with micro-emulsion is introduced to correct for the computational artifacts in the Kirkwood-Buff integrals.