Modulations in the aggregation behavior of ionic liquid 1-butyl-3-methylimidazolium octylsulfate in aqueous alcohol solutions

Abstract

The aggregation behavior of a surface active ionic liquid (SAIL) 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8OSO3], has been investigated with the addition of different alcohol additives 1,2-propanediol, 1-propanol, and 2-propanol at lower compositions (10%, 15% and 20%) in aqueous medium. The aggregation properties have been studied using surface tension and fluorescence techniques at 298.15K and using conductance at temperatures 288.15, 298.15, and 308.15K. The critical aggregation concentration (cac) of [C4mim][C8OSO3] is found to increase with the addition of alcohols. Various thermodynamic parameters of aggregation process viz. Gibbs free energy of aggregation (ΔGagg.o), standard enthalpy of aggregation (ΔHagg.o), and standard entropy of aggregation (ΔSagg.o) are calculated from the temperature dependent cac values. The negative values of (ΔGagg.o) suggest the spontaneous nature of aggregation process of [C4mim][C8OSO3] in presence of alcohols in aqueous medium. The large positive entropy values indicate that the process of aggregation is entropy driven in all investigated systems at studied temperature range. The cac was also measured from the variation in fluorescent vibronic bands of pyrene as probe (I376/I387) which is sensitive to the polarity of environment. A series of various surface active parameters such as surface tension at cac (γcac), surface pressure at the interface (Πcac), maximum surface excess concentration (Γmax), and minimum area occupied by the single SAIL molecule at air-solvent interface (Amin) are derived from surface tension measurements.