1H and 13C NMR Study on the Aggregation of (2-Acylaminoethyl)trimethylammonium Chloride Surfactants in D2O

Aggregation of the following surfactant series in D 2 O has been studied by 1 H and 13 C NMR spectroscopy: RCONH(CH 2 ) 2 N + (CH 3 ) 3 Cl -, where RCO is an acyl group containing 10−16 carbon atoms. Micelle formation has been followed by measuring observed chemical shifts, δ obs, and apparent transverse relaxation times, 1/ T 2 *, of the surfactant discrete groups as a function of surfactant concentration, below and above its critical micelle concentration, cmc. Plots of δ obs and/or 1/ T 2 * versus [surfactant] are sigmoidal and were fitted to a model based on the mass-action law. A modified computation procedure was introduced in order to calculate the following: cmc; the equilibrium constant of micelle formation, K; the micelle aggregation number, N agg; and the chemical shifts of the monomer, δ mon, and the micelle, δ mic, respectively. The modification introduced permits simple and accurate calculation of the above-mentioned micellar parameters from the same set of experimental data. NMR-based cmc and N agg values are in excellent agreement with those previously determined by independent techniques. K and N agg increase as a function of increasing the length of the surfactant hydrophobic tail. Gibbs free energies of micellization, Δ G ° mic, were calculated and divided into contributions from the CH 2 groups in the hydrophobic chain and from the (terminal CH 3 + headgroup). Both quantities agree with those previously calculated from conductivity data. The contribution of the (terminal CH 3 + headgroup) to Δ G ° mic shows the importance to micellization of direct and/or water-mediated H-bonding of the surfactant amide group. Comparison of (δ mic − δ mon ) with data in bulk solvents (CDCl 3 and CD 3 OD/D 2 O, respectively) shows that the monomers are probably not fully exposed to D 2 O below the cmc, in agreement with previous NMR investigations of cationic surfactants.