Benzyl (3-Acylaminopropyl) Dimethylammonium Chloride Surfactants: Structure and Some Properties of the Micellar Aggregates

Abstract

The title cationic surfactants were synthesized by the scheme in Fig. 1, where RCO2H 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 1H 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, αmic. 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.