2-(Acylaminoethyl)trimethylammonium chloride surfactants: synthesis and properties of aqueous solutions

The title cationic surfactants have been synthesized by reaction of carboxylic acids with N, N-dimethylethylenediamine to give an intermediate amidoamine. The latter was quaternized with methyl iodide; the product was transformed into the corresponding chloride surfactant by ion-exchange on a macroporous resin. Adsorption and aggregation of these surfactants in H ₂O have been studied by surface tension measurement. Additionally, solution conductivity, electromotive force (H ₂O), and Fourier transform IR spectroscopy (D ₂O) have been employed to investigate micelle formation. Increasing the length of R resulted in the following changes: an increase in the micelle aggregation number; a decrease in the minimum area per surfactant at the solution/air interface, the critical micelle concentration, and the degree of counterion dissociation. Gibbs free energies of adsorption at the solution/air interface and micelle formation in water were calculated and compared to those of alkyltrimethylammonium chlorides. The contribution to these free energies from surfactant methylene groups (in the hydrophobic tail) and the head group was calculated. The former are similar to those of other cationic surfactants. The corresponding free-energy contributions of head groups are smaller (i.e., more negative), indicating that the transfer of this group from bulk water to the interface (for adsorption) and/or to the micelle (aggregate formation) is more favorable. This is attributed to intermolecular hydrogen bonding of monomers at the interface, and/or in the aggregate, via the amide group, in agreement with our Fourier transform IR data. Our results are compatible with a micellar interface closer to the amide nitrogen than to the quaternary ammonium ion.