Interaction between ionic liquid and zwitterionic surfactant: a comparative study of two ionic liquids with different anions

Room temperature ionic liquids (ILs) may have enormous potential as far as modifying the properties of aqueous surfactant solutions is concerned. A comparative study on the changes in the physicochemical properties of aqueous solution of a commonly used zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB-12) in the presence of up to 2 wt% of two ILs with same cation but different anions is undertaken using conductance, fluorescence spectroscopy, and dynamic light scattering. The two ILs used are 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]). It is demonstrated that the addition of IL alters the important properties of aqueous SB-12. Critical micelle concentration, aggregation number, micellar size, and microfluidity of micellar pseudo-phase change when the IL is added to aqueous SB-12. The extent to which these properties are altered is observed to be significantly more for [bmim][PF(6)] as compared to that for [bmim][BF(4)]. The results demonstrate the presence of electrostatic attractive interactions between the ions of ILs and the zwitterion of SB-12 to be the major reason for these observations. While the interaction between the IL cation and the anionic sulfonate termini of SB-12 is same for the two ILs, that between IL anion and cationic quaternary ammonium of SB-12 is proposed to be more efficient for [bmim][PF(6)] due to the bigger size of PF(6)(-). Important role of IL anion in modifying properties of aqueous SB-12 is demonstrated. It is noted that the changes in the properties of aqueous surfactant systems upon IL addition depend strongly on the nature of the surfactant head group.