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

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.