Combined molecular dynamics (MD) and small angle scattering (SAS) analysis of organization on a nanometer-scale in ternary solvent solutions containing a hydrotrope

Abstract

Mixtures of three solvents, with two immiscible liquids and a third one miscible to both – the solvotrope, may exhibit structuration. We explore the phase diagram of n-octanol/ethanol/water, where ethanol is the hydrotrope, varying composition from the water-rich side to the n-octanol-rich side at constant ethanol fraction. We resolve nanometer-sized structures experimentally by mean of four contrasts: three from Small Angle Neutron Scattering (SANS) and one from Small Angle X-ray Scattering (SAXS). On the water-rich side, we confirm the existence of droplets associated to a critical point stabilized by an excess adsorption of the hydrotrope: the ultra-flexible microemulsion (UFME) domain. The n-octanol-rich side is better described as a dynamic random network of chain-like associations of hydroxy groups. The continuous evolution from oil clusters to a dynamic network of hydroxy groups is demonstrated by the features of scattering patterns, successfully compared for all contrasts to Molecular Dynamics (MD) simulations, allowing to illustrate with snapshots the structuration of solvents. The free energy of transfer of the hydrotrope obtained from MD is low (∼1 kBT/molecule). This study suggests that Ouzo spontaneous emulsions may be in dynamic equilibrium with a pre-Ouzo, similarly to nanoemulsions kinetically stabilized by the co-existence of a microemulsion.