Thermodynamics of micellization of cationic surfactants in aqueous solutions: consequences of the presence of the 2-acylaminoethyl moiety in the surfactant head group

Abstract

The enthalpies of micellization of the following surfactant series have been determined by calorimetry: benzyl (2-acylaminoethyl)dimethylammonium chlorides, RABzMe2Cl, and alkyldimethylbenzylammonium chlorides, R′BzMe2Cl, where A, Bz and Me refer to amide, benzyl, and methyl groups, respectively and the acyl (for RABzMe2Cl) and/or the alkyl (for R′BzMe2Cl) groups C10, C12, C14, and C16, respectively. For both series, the shapes of the calorimetric titration curves (enthalpograms) depend on the following micellar parameters: critical micelle concentration, aggregation number, and degree of counterion binding. The calorimetric-based critical micelle concentrations are in excellent agreement with those determined by conductivity. The Gibbs free energy, the enthalpy and the entropy of micellization were calculated, and divided into contributions from the CH2 groups of the hydrophobic tail, and the terminal CH3 plus head group of the surfactant. For both surfactant series, all thermodynamic parameters per CH2 group were found to be similar, since their transfer (from bulk solution to the micelle) is independent of the surfactant head-group structure. The Gibbs free energy, the enthalpy, and the entropy of transfer of the head group of RABzMe2Cl are more favorable than their counterparts for R′BzMe2Cl, because of direct and/or water mediated hygrogen bonding of the amide groups in the micelle.