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

Abstract

Aggregation of the following surfactant series in D2O has been studied by 1H and 13C NMR spectroscopy: RCONH(CH2)2N+(CH3)3Cl-, where RCO is an acyl group containing 10−16 carbon atoms. Micelle formation has been followed by measuring observed chemical shifts, δobs, and apparent transverse relaxation times, 1/T2*, of the surfactant discrete groups as a function of surfactant concentration, below and above its critical micelle concentration, cmc. Plots of δobs and/or 1/T2* 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, Nagg; and the chemical shifts of the monomer, δmon, and the micelle, δmic, 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 Nagg values are in excellent agreement with those previously determined by independent techniques. K and Nagg increase as a function of increasing the length of the surfactant hydrophobic tail. Gibbs free energies of micellization, ΔG°mic, were calculated and divided into contributions from the CH2 groups in the hydrophobic chain and from the (terminal CH3 + headgroup). Both quantities agree with those previously calculated from conductivity data. The contribution of the (terminal CH3 + headgroup) to ΔG°mic shows the importance to micellization of direct and/or water-mediated H-bonding of the surfactant amide group. Comparison of (δmic − δmon) with data in bulk solvents (CDCl3 and CD3OD/D2O, respectively) shows that the monomers are probably not fully exposed to D2O below the cmc, in agreement with previous NMR investigations of cationic surfactants.