Micellization of conventional and gemini surfactants in aquoline: A case of exclusively water based deep eutectic solvent

Abstract

Deep eutectic solvents (DES) based on water as the only hydrogen bond donor (HBD) have been introduced recently (aquoline). There is not a single report on surfactant association behaviour in aquoline. This study deals with the determination of physical properties (rheology, specific conductance, pH, micro polarity and apparent dielectric constant) of pure aquoline (with different molar ratios of choline chloride (ChCl): water, DES I - DES IV) and association behaviour of ionic surfactants in such aquoline systems. DESs were found Newtonian and highly polar / conducting fluids with nearly neutral pH (6.9–7.1). Micellization behaviour of different ionic surfactants (sodium dodecyl sulphate (SDS), sodium dodecane-1-sulphonate (SDSo), sodium dodecyl benzene sulphonates (SDBS), P, P'-1,4-butanedieyl, P, P'-didodecylester, disodium salt (12-4-12A), cetyltrimethylammonium ammonium bromide (CTAB) or dodecyl trimethyl ammonium bromide (DTAB)) has been studied fluorometrically in various aquolines and compare data with an aqueous medium. In most of the cases, critical micelle concentration (CMC) values have been found lower than water which may be due to the presence of choline chloride (ChCl, one of the components of aquoline), which can interact micelle electrostatically and hydrophobically. For different head groups in anionic surfactant, CMC follows the order 12-4-12A < SDBS < SDS < SDSo, which fits in the Hoffmeister like series of head groups. For cationic surfactants (CTAB and DTAB), CMC shows a similar chain length effect as observed in water. Overall, CMC data allow to propose that CMC decreases with an increase in molar content of water in a typical aquoline under eutectic limit. Micellar aggregation number (Nm), Stern-Volmer constant, micellar polarity, and apparent dielectric constant were also computed. It has been observed that micelles of lower Nm, with high polarity, are formed in a typical aquoline system (DES III). The study may find applications where organized assemblies are required in highly polar solvents.