Revisiting aqueous-acetone mixtures through the concept of molecular emulsions

Aqueous acetone mixtures represent a good example of perfectly miscible liquids in reality, and that hard to mix in silico. This is related to a key problem in molecular simulations, which is to distinguish between strongly micro-segregated mixtures from phase-separated ones. The Kirkwood-Buff integrals of in silico aqueous mixtures are often found to be dramatically higher than the experimental ones, hinting at a possible underlying phase separation. This is the case for many combinations of the force field models chosen for water or for acetone. Herein, we demonstrate, through a proper handling of the tail of the correlation functions, that these high values of the Kirkwood-Buff integrals represent in fact a transient regime within the segregated spatial domains, and that they asymptotically settle down to values in much better agreement with the experimental ones. The concept central to this new approach is that of molecular emulsions, where the long range part of the correlations is modulated by the micro-segregated domains, and that it is necessary to take into this modulation in order to recover the correct thermodynamical properties.