Interfacial and aggregation behavior of ionic liquid-bile salt conjugates

Bile salt derived nontoxic surface-active ionic liquids (SAILs), 1-butyl-3-methylimidazolium cholate ([bmim]C) and deoxycholate ([bmim]DC) were synthesized by reacting [bmim] tetrafluoroborate separately with sodium cholate (NaC) and sodium deoxycholate (NaDC) which were subsequently characterized by FTIR, ¹H-NMR, ¹³C-NMR, HRMS, PXRD, and TGA-DTA studies. Interfacial and aggregation behavior of the SAILs were assessed by surface tension, conductance, pyrene fluorescence, dynamic light scattering, and isothermal titration calorimetric studies. Critical micelle concentration values were substantially lower than their precursor sodium salts. SAILs were less surface active than their corresponding sodium salts due to enhanced aqueous solubility induced by the ionic liquid cation. Both the aggregation number and size of the SAIL micelles were higher/larger where the SAIL micelles were more compact than NaC/NaDC. Exothermic changes in the enthalpy of micellization were higher for SAILs, where the micellization processes were found to be entropy driven. Such comprehensive studies can shed further light on the microstructure, besides the fundamental understanding on SAIL aggregation. The combined advantages of surfactant and ionic liquid in the SAILs can result in superior amphiphilic characteristics. Such nontoxic aggregates are capable to enhance the limited solubility of organic compounds in water besides their possible uses in catalysis, drug delivery, and energy storage, etc.