Effect of alkyl chain length of imidazolium cations on foam properties of anionic surface active ionic liquids: Experimental and DFT studies

Abstract

Two anionic surface active ionic liquids (SAILs), 1-hexyl-3-methylimidazolium dodecyl sulfate ([Hmim][C12H25SO4]) and 1-octyl-3-methylimidazolium dodecyl sulfate ([Omim][C12H25SO4]) were synthesized using imidazolium ionic liquids (ILs) and sodium dodecyl sulfate (SDS). The critical micelle concentration (CMC) values, thermodynamic properties, and aggregation parameters were determined using surface tension measurements, steady state fluorescence tests, and electrical conductivity measurements. CMC values (from surface tension) of [Hmim][C12H25SO4] and [Omim][C12H25SO4] were 1.075 mmol/L and 0.278 mmol/L, respectively. The micellization process of [Omim][C12H25SO4] and [Hmim][C12H25SO4] was spontaneous, endothermic, and entropy-driven. Foaming performances (foaming ability, foam stability, and foam composite index) of three SAILs ([Bmim][C12H25SO4], [Hmim][C12H25SO4], and [Omim][C12H25SO4]) were determined using Ross-Miles methods. The foaming abilities of SAILs followed the order: [Hmim][C12H25SO4] > [Bmim][C12H25SO4] > [Omim][C12H25SO4]. While the foam stabilities and foam composite index (FCI) of SAILs followed the order: [Omim][C12H25SO4] > [Hmim][C12H25SO4] > [Bmim][C12H25SO4]. The structure effect of three SAILs ([Bmim][C12H25SO4], [Hmim][C12H25SO4], and [Omim][C12H25SO4]) on foam performances were analyzed by density functional theory (DFT) calculations. Binding energy of three SAIL hydrates followed the trend: [Omim][C12H25SO4](H2O)n > [Hmim][C12H25SO4](H2O)n > [Bmim][C12H25SO4](H2O)n. DFT results showed the increase of alkyl chain length of imidazolium cation was beneficial to the stability of SAIL hydrates, which resulted in higher foam performances. The information will shed light on rapid screening of novel surface active ionic liquids for surfactant foam enhanced remediation (SFER).