Phase behavior and mesoscale solubilization in aqueous solutions of hydrotropes

Abstract

Hydrotropes are amphiphilic molecules that are too small to spontaneously form equilibrium structures in aqueous solutions, but form dynamic, non-covalent assemblies, referred to as clusters. In the presence of a hydrophobic compound, these clusters seem to get stabilized leading to the formation of long-lived, highly stable mesoscopic droplets, a phenomenon that we call "mesoscale solubilization". In this work, we focus on the unusual mesoscopic properties of aqueous solutions of a nonionic hydrotrope, namely tertiary butyl alcohol (TBA), on addition of various hydrophobic compounds. Aqueous TBA solutions, in about 3 mol % to 8 mol % TBA concentration range and about 0 °C to 25 °C temperature range, show the presence of short-ranged (~ 0.5 nm), short-lived (tens of picoseconds) molecular clusters which result in anomalies of the thermodynamic properties. These clusters are transient but do not relax by diffusion, thus distinctly different from conventional concentration fluctuations. In this concentration and temperature range, upon the addition of a third (more hydrophobic) component to TBA-water solutions, long-lived mesoscopic droplets of about 100 nm size are observed. In this work, we clarify the ambiguity behind the definition of solubility and elucidate the phenomenon of mesoscale solubilization. A systematic study of the macroscopic and mesoscopic phase behavior of three ternary systems TBA-water-propylene oxide, TBA-water-isobutyl alcohol, and TBA-water-cyclohexane has been carried out. We differentiate between molecular solubility, mesoscale solubilization, and macroscopic phase separation. We have confirmed that practically stable aqueous colloids can be created from small molecules, without addition of surfactants or polymers. Such kind of novel materials may find applications in the design of various processes and products, ranging from pharmaceuticals to cosmetics and agrochemicals.

Mesoscale solubilization and critical phenomena in binary and quasi-binary solutions of hydrotropes

“…[28,29,31,32]. In Moscow, the measurements were performed at the scattering angles of 20, 30 and 90 .…”

“…It has been shown that such inhomogeneities occur in aqueous solutions of nonionic hydrotropes when the solution contains a third, more hydrophobic, component [12,13,[28][29][30][31][32]. Remarkably, these inhomogeneities (emerging in water-rich ternary systems) are only pronounced in the hydrotrope concentration range where molecular clustering and thermodynamic anomalies are observed in the original binary hydrotrope-water solutions.…”

“…The hypothesized structure of these mesoscopic droplets is such that they have a hydrophobe-rich core, surrounded by a H-bonded shell of water and hydrotrope molecules [12,13,22,31,32]. These droplets can be extremely long-lived, being occasionally stable for over a year [32]. The phenomenon of the formation of mesoscopic inhomogeneities in aqueous solutions of nonionic hydrotropes induced by hydrophobic "impurities" is referred to as "mesoscale solubilization" [12,13,31,32].…”

“…These droplets can be extremely long-lived, being occasionally stable for over a year [32]. The phenomenon of the formation of mesoscopic inhomogeneities in aqueous solutions of nonionic hydrotropes induced by hydrophobic "impurities" is referred to as "mesoscale solubilization" [12,13,31,32]. Mesoscale solubilization may represent a ubiquitous feature of certain nonionic hydrotropes that exhibit molecular clustering in water and may have important practical applications in areas, such as drug delivery, if the replacement of traditional surfactants is necessary.…”

Liquid-Liquid Equilibrium and Interfacial Tension in Solutions of Hydrotropes

“…Положение указанной границы качественно совпало с ранее опубликованными данными [10]. Однако проведенное нами определение составов фаз расслоенной системы позволило установить, что положение критической точки расслоения существенно отли-чается от ранее опубликованного: на треугольной фазовой диаграмме системы критиче-ская точка смещена к вершине воды (см.…”