Emergence of surfactant-free micelles from ternary solutions

Abstract

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.

Nanostructures in clear and homogeneous mixtures of rapeseed oil and ethanol in the presence of green additives

“…It can also be noted that the phase diagrams have exactly the same topology than those reported in our last studies [19][20][21][22] concerning the pre-ouzo effect as they result from the mixture of two immiscible molecules, being cosolubilized by a third one, which is miscible with both immiscible liquids.…”

“…The wide composition extension of the well-defined correlation functions is a strong hint in favor of nanostructure fluctuations and not of simple molecular fluctuations close to a demixing line or a critical point. It can also be noted that the extent of these well-established correlation functions covers a larger area in the realms of existence of clear and homogeneous solutions than in our previous studies on water-based systems [19][20][21][22]. Remarkably, correlation functions are even measured for the mixtures 1-heptanol/rapeseed oil, which suggests the formation of pseudo-phases even in these binary systems.…”

“…In previous works [19][20][21][22], we have investigated the presence of nanostructures in different water/ethanol/hydrophobic molecule systems. In the present study, the formation of liquid nanostructures is investigated in media with ethanol and rapeseed oil using dynamic and static light scattering, small-angle neutron scattering and conductivity measurements.…”

How to explain microemulsions formed by solvent mixtures without conventional surfactants

“…The SANS experiments show the presence of polydisperse octanol-rich globular domains surrounded by water-rich domains, which can be considered as pseudophases (20). The octanol-rich domains are in thermodynamic equilibrium and separated by a surface film, which contains a slight accumulation of ethanol molecules at the interface (21) as also suggested by molecular dynamics (MD) simulations (22). This microstructure corresponds to the IUPAC definition of microemulsions, because two immiscible fluids are separated by an interface, which in the case described here, is a nanometerthick diffuse film composed mainly of ethanol (23).…”

“…With increasing ethanol concentration, the microdomains progressively vanish, and the mixture becomes mainly a structureless molecular solution (22). Hydration vs. van der Waals balance may also contribute to the stability of emulsion droplets in the Ouzo regime.…”