Julien Marcus

Emergence of surfactant-free micelles from ternary solutions

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.

How to explain microemulsions formed by solvent mixtures without conventional surfactants

Significance Beginning over 40 y ago, a curious type of microemulsions—as transparent dispersion of two immiscible liquids separated by an interfacial film—at thermodynamic equilibrium has been described as “pre-Ouzo,” “detergentless,” or surfactant-free microemulsions. The experiments in ternary systems containing one hydrotropic cosolvent were ambiguous, and therefore, there was no need to come up with a general theory. Recent evidence obtained by specific deuteration in neutron scattering established the need for the extension of self-assembly theories based on Derjaguin-Landau-Verwey-Overbeck theory (DLVO), bending, or phase transfer energy. Here, we introduce a general free energy expression for weak self-assembly, where solvation effects and entropy compete without the influence of film bending, that explains all experimental results for this class of microemulsions. Ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids, both being soluble in the hydrotrope at any proportion, show unexpected solubilization power and allow strange but yet unexplained membrane enzyme activity. We study the system ethanol-water-octanol as a simple model of such kinds of ternary solutions. The stability of “detergentless” micelles or microemulsions in such mixtures was proposed in the pioneering works of Barden and coworkers [Smith GD, Donelan CE, Barden RE (1977) J Colloid Interface Sci 60(3):488–496 and Keiser BA, Varie D, Barden RE, Holt SL (1979) J Phys Chem 83(10):1276–1281] in the 1970s and then, neglected, because no general explanation for the observations was available. Recent direct microstructural evidence by light, X-ray, and neutron scattering using contrast variation reopened the debate. We propose here a general principle for solubilization without conventional surfactants: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.

Influence of high intensity sweeteners and sugar alcohols on a beverage microemulsion

The present paper shows the effects of added sugars and sweeteners on the clearing temperature of a highly water dilutable fatty acid salt microemulsion used as a model of a beverage concentrate. There is a twofold interest in this work. The first one is practical and relates to the fact that many fatty acid salt surfactants can be used in food without major regulatory restrictions. As is shown here, they allow making highly stable microemulsions even at neutral and acidic pH. The second one is more of scientific interest. The model system can be used to study the effect of sugars and sweeteners on the formulation stability depending on their charges, amphiphilic properties, and localization in the microemulsion interfacial film. An important practical result is the discovery of the possibility to formulate highly dilutable microemulsions at neutral or slightly acid pH with a good taste in presence of sucralose. Further, a significant decrease of the pKA of the fatty acid is observed in presence of stevia, thus allowing transparent, fairly stable systems at neutral pH.

Formulation and stability of a soap microemulsion and the apparent pKA herein

The influence of composition, and added salts and polyols on the stability of an oil-in-water microemulsion formulation and on the apparent pKA (apKA) of the used oleate surfactant is investigated. High temperature favours a decrease of the apKA and leads to the formation of more hydrated micelles. The apKA decreases also when the percentage of ethanol increases. Citronellol molecules do not significantly influence the apKA at concentrations between 0% and 2% w/w. By contrast, with increasing limonene concentration, the apKA increases. It was observed that anions of sodium salts destabilize the microemulsion and high temperatures are needed to recover it. By increasing the concentration of NaCl, a slight increase of the apKA is observed, which can be associated with a non-specific, electrostatic (Debye-Hückel) effect. Cations of chloride salts have different effects depending on their ability to exchange with Na(+) near the carboxylate group. Li(+), Na(+) and K(+) have apparently a salting-out effect. Tetramethylammonium chloride and choline chloride have salting-in effects until respectively 0.6 and 0.4 mol kg(-1). The associations of sorbitol or glycerol with ethanol lead to a salting-in effect and to a decrease of the apKA of Na-Oleate.