Thermodynamics and micellization behavior of ethanolammonium carboxylate surface active ionic liquids in aqueous media

A systematic experimental study of the micellization of ethanolammonium carboxylate surface active ionic liquids (SAILs) in aqueous solutions was performed. The effect of two separate anions having different alkyl chain length, laurate and palmitate on the micellization behavior was studied. The effect of different cationic head groups, n-butyl, tertiary butyl as well as phenyl group on the micellization parameters was also investigated. The critical micelle concentration (cmc) values and the surface active parameters were evaluated from tensiometric methods. The values of cmc are observed to be lower for palmitate anion SAILs in comparison to the laurate anions. The values of surface tension reduction (πcmc) and surface excess (Γmax) show considerable variation as the anion chain length and the cationic head groups changed. The thermodynamics of the micellization parameters, standard Gibbs free energy of micellization (∆Gm°), enthalpy of micellization (∆Hm°) and entropy of micellization (∆Sm°) were evaluated by conductometric measurements. Using pyrene as a probe molecule, the cmc values were evaluated by steady state fluorescence. The values of critical micelle concentration obtained from three different methods for all the studied SAILs are found to be in good match with each other. The order of cmc for the synthesized SAILs were as follows: [NPEA][PAL] < [TBEA][PAL] < [NBEA][PAL] < [NPEA][LAU] < [TBEA][LAU] < [NBEA][LAU].