Phase diagrams and formulation mechanism of microemulsions with hydroxyl group functional anionic surface active ionic liquids

Abstract

One functional hydroxyl group-containing anionic surface active ionic liquids (SAILs), 1-(3-hydroxypropyl)-3-methylimidazolium dodecyl sulfate ([MPmim][DS]) and 1-(3-hydroxypropyl)-3-butylimidazolium dodecyl sulfate ([MPbim][DS]) were synthesized and characterized with 1HNMR, 13CNMR, FT-IR spectroscopy, thermogravimetric analysis (TGA), and elemental analysis. The critical micellar concentration (CMC) values of [MPmim][DS] and [MPbim][DS] were obtained through measurements of surface tension and electrical conductivity. The capability of [MPmim][DS] and [MPbim][DS] to form microemulsions was assessed. The influence of the hydroxyl group of imidazolium cation on microemulsion formulation was elucidated using Density functional theory (DFT) simulations. Critical micellar concentration (CMC) values for [MPbim][DS] and [MPmim][DS] are 0.51 mmol/L and 0.99 mmol/L, respectively. The micellization processes of [MPbim][DS] and [MPmim][DS] are spontaneous, endothermic, and entropy-driven. [MPmim][DS] or [MPbim][DS] as surfactant, ethyl laurate microemulsions are formulated. The single-phase region area (SME) percent of ethyl laurate microemulsions are 60% and 66%. The single-phase region area percent of different microemulsions follows the trend: [MPbim][DS] > [MPmim][DS]. Introducing the functional hydroxyl group into the imidazolium cation significantly expands the SME area of microemulsions. DFT simulations are employed to analyze hydrogen bonding and electronic energy interactions among cations, DS anion, [MPmim][DS], [MPbim][DS], and ethyl laurate. Two types of hydrogen bonds are formed in [MPmim][DS] or [MPbim][DS], as C-H…O, and O-H…O. In [MPmim][DS], the hydroxyl group of imidazolium cation participates in the formation of a strong hydrogen bond with DS anion, exemplified by strong hydrogen bond O18-H19…O64 (1.972 Å). These advancements offer critical insights into the design of surface active ionic liquids, enhancing their functional properties and broadening the potential for innovative microemulsion applications in various industries.