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

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 1 HNMR, 13 CNMR, 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 (S ME ) 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 S ME 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.