Proton and carbon-13 NMR study of the aggregation of benzyl(2-acylaminoethyl)dimethylammonium chloride surfactants in D2O

Abstract

¹H and ¹³C NMR spectroscopy have been employed to study aggregation of the following surfactant series in D₂O: RNH(CH₂)₂N⁺(CH₃)₂CH₂C₆H₅ Cl⁻, where R is an acyl group containing 10 to 16 carbon atoms. Observed chemical shifts, δ_obs, and apparent transverse relaxation times, 1/T₂* of the surfactant discrete groups were measured at concentrations below and above the critical micelle concentration, c.m.c. Plots of δ_obs and/or 1/T₂* versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. The parameters determined by this fit were: the equilibrium constant for micelle formation, K; c.m.c. and chemical shifts of the monomer, δ_mon and the micelle, δ_mic. NMR-determined c.m.c.s are in excellent agreement with those previously determined by conductance and surface tension measurements. K increases as a function of increasing 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 (terminal CH₃ + head-group). Both quantities agree with those previously determined from conductivity data. Contribution of (terminal CH₃ + head-group) to ΔG°_mic shows the importance to micellization of direct and/or water-mediated H-bonding of the surfactant amide group. Peak splitting as a function of increasing [surfactant], (δ_mic − δ_mon) and (δ_{organic solvent} − δ_mon) supports a micellar structure where the benzyl group folds back toward the aggregate interior.