Surfactant-free microemulsions with cleavable constituents

Abstract

Surfactant-free microemulsions (SFME) comprising a cleavable hydrotrope/hydrophobic component were investigated regarding potential applications for green, microemulsion-mediated extraction processes. To this purpose, dynamic light scattering (DLS) experiments were performed for ternary systems consisting of water/(2,2-dimethyl-1,3-dioxolan-4-yl)methanol (Solketal)/oleic acid and water/ethanol/triethyl citrate (TEC) in order to detect mesoscopic structured compositions. Further, kinetic measurements were performed for the acid-catalyzed hydrolysis of Solketal (cleavable hydrotrope) and base-catalyzed hydrolysis of TEC (cleavable hydrophobic component) at different temperatures/different pH of the aqueous pseudo-phase. As proof of concept, α-tocopherol and vanillic acid were dissolved in the ternary mixtures as extractable surrogates in order to simulate an extraction and separation process. It was found that well-structured SFME can be obtained for both systems, which basically allows microemulsion-mediated extraction. Moderate changes of temperature and/or pH, lead to rapid hydrolysis of Solketal and TEC within a few hours. Finally, it could be shown that upon hydrolysis of Solketal/TEC, separation of the used surrogate solutes from the hydrolysed products is facilitated and quantitative separation could be achieved.