Synthesis and Aggregation of Benzyl(2-acylaminoethyl)dimethylammonium Chloride Surfactants

Abstract

The following scheme has been employed to synthesize the title cationic surfactants: RCO2H (+NH2CH2CH2N(CH3)2, toluene) → RCONHCH2CH2N(CH3)2 (1a−1d) (+PhCH2Cl, CH3CN) → RCONHCH2CH2N+(CH3)2CH2C6H5Cl- (2a−2d). RCO2H refers to decanoic, dodecanoic, tetradecanoic, and hexadecanoic acid, respectively. Aggregation of these surfactants in water has been studied at 25 °C by measuring solution conductivity, surface tension, and electromotive force and by using Fourier transform infrared spectroscopy (FTIR). Increasing the length of R resulted in an increase of the aggregation number and a decrease in minimum area/surfactant at the solution/air interface, critical micelle concentration, and degree of counterion dissociation. Gibbs free energies of adsorption at the solution/air interface and of micelle formation were calculated and compared to those of other cationic surfactants. Contributions to these free energies from methylene groups of the hydrophobic tail and surfactant headgroup were calculated. The former are similar to those of other cationic surfactants, whereas the latter are smaller, i.e., more negative. That is, transfer of the headgroup from bulk water to the interface and/or to the micelle is more favorable. This is attributed to intermolecular H-bonding of monomers at the solution/air interface and/or in the aggregate, via the amide group, in agreement with our FTIR data.