The title cationic surfactants were synthesized by the scheme in Fig. 1, where RCO
H refers to decanoic, dodecanoic, tetradecanoic and hexadecanoic acid, respectively. In aqueous solution, the micelle/water interface may be located at the quaternary ammonium ion or at the amide group. The following pieces of evidence indicate that the interface lies at the latter site: theoretically calculated aggregation numbers and those determined by static light scattering; dependence on surfactant concentration, below and above the critical micelle concentration, cmc, of both the IR frequency of amide I band and
H NMR chemical shifts of the discrete surfactant protons. Solution conductance and calorimetric titration have been employed to study the aggregation of these surfactants in water at 25 °C. Increasing the length of R resulted in a decrease of the cmc and the degree of counter-ion dissociation, α
. Gibbs free energies of micelle formation were calculated and divided into contributions from the methylene groups of the hydrophobic tail, and the terminal methyl plus head-group. The former are similar to those of other surfactants, whereas the latter are more negative, i.e., the transfer of the head-group from bulk water to the micelle is more favorable. This is attributed to direct or water-mediated H-bonding of the micellized surfactant molecules, via the amide group, in agreement with the IR data presented.