Linking excess order to co-solvent aggregation in solvent mixtures

Abstract

Small amphiphilic molecules dissolved in alcohol-water mixtures have been seen to have a tendency to aggregate. Here we present a thorough neutron diffraction experiment, aided by H/D isotopic substitution and empirical potential structure refinement simulations of four mixtures of tert-butyl alcohol (x t =0.03 molar) in methanol and water (x m =0.27, 0.54, 0.73, 1.0 molar). The tert-butyl alcohol is found to segregate in the methanol rich regions at all concentrations; at x m =0.27 the mixture is found to be a bi-percolating mixture, with water forming one interconnected cluster, and methanol and tert-butyl alcohol (apolar-apolar) forming another. At the same x m =0.27 concentration, tert-butyl alcohol molecules become closer each other at this concentration, forming more dimers than at the other concentrations, and even trimers. Results are discussed in terms of the ability of water molecules to form flexible tetrahedral networks.