Curious effects ranging from enzyme activity to anomalies in evaporation rates that have been known for over fifty years suggest the existence and thermodynamic stability of surfactant-free micelles. Only recently, joint X-ray, light and neutron scattering experiments have demonstrated that aggregates and bulk pseudo-phases coexist in presumably normal solutions, in which a water insoluble component is solubilized in a certain domain of concentration of a hydrotrope component like ethanol. Nevertheless, nothing is known about the molecular-level shape and structure of such aggregates. In this work we characterize mixtures of octanol, ethanol, and water by molecular dynamics simulations. For compositions in the “pre-ouzo” region (close to the single phase stability limit) we observe micelle-like aggregates that are clearly distinct from simple critical density fluctuations. We define an ethanol partition in the pseudo-phase from an integral of the van der Waals dispersion energy term. From this partition, octanol-rich aggregates swollen with ethanol appear with an emerging interface. Ethanol is present in the water pseudo-phase with an exponential decay similar to the one predicted by Marcelja and Radic forty years ago.

The behavior of halide salts at the vapor/water interface has been the focus of a tremendous amount of work in the past ten years. A molecular view of the interface has been introduced with the observation that large anions have some affinity for the interface, but a quantitative description of the driving forces that determine ion adsorption or repulsion at the interface is still missing. This review discusses recent developments that are based on classical and quantum-chemical molecular simulations as well as developments that are based on simple potential models.

We show that three different morphologies exist near the two-phase boundary of ternary systems containing a hydrotropic cosolvent. Based on synchrotron small- and wide-angle X-ray scattering combined with molecular dynamics, we rationalize the specific scattering signature of direct, bicontinuous, and reverse mesoscale solubilization. Surprisingly, these mesostructures are resilient toward strong acids, which are required in industrial applications. However, on a macroscopic scale, the phase boundary shifts in salting-in and salting-out in the direct and respectively reverse regime, leading to a crossing of the binodals.

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.