Concentration-dependent dual behavior of hydrophilic ionic liquid in changing properties of aqueous sodium dodecyl sulfate.

Modifying physicochemical properties of aqueous surfactant solutions in favorable fashion by addition of environmentally benign room-temperature ionic liquids (ILs) has enormous future potential. Due to its unusual properties, an IL may demonstrate a unique role in altering the properties of aqueous surfactant solutions. Changes in the properties of aqueous sodium dodecyl sulfate (SDS), an anionic surfactant, upon addition of a common and popular "hydrophilic" ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] are presented. Addition of low concentrations of [bmim][BF4] (i.e., <or=2 wt %) to aqueous SDS results in a decrease in critical micelle concentration (cmc) as well as an increase in aggregation number (N(agg)) and average micellar size. An increase in dipolarity and a decrease in microfluidity of the micellar phase is reported by solvatochromic fluorescence probes pyrene and 1,3-bis-(1-pyrenyl)propane as up to 2 wt % [bmim][BF4] is added to aqueous SDS. The IL is observed to behave similarly to common electrolytes in the concentration range 0 < [bmim][BF4] <or= 2 wt %. A further increase in [bmim][BF4] concentration to 30 wt % results in an increase in cmc which is accompanied by a decrease in N(agg) and average micellar size. Change in the microfluidity of the micellar phase is observed to become insignificant in this concentration range. Evidence of the presence of micellar aggregates at such high concentration of [bmim][BF4] is furnished by dynamic light scattering. The role of an IL in altering the properties of aqueous SDS in the range 2 wt % < [bmim][BF4] <or=30 wt % is similar to those of polar cosolvents. The concentration-dependent dual role of the IL [bmim][BF4] in altering physicochemical properties of aqueous SDS is demonstrated.