Aggregation of cationic surfactants in D2O:: A proton NMR study on effects of the structure of the headgroup

Aggregation of the following surfactants in D2O has been studied by H-1 NMR spectroscopy at 500.13 MHz, 298 K: series I, dodecyltrialkylammonium bromides, alkyl group = methyl, ethyl, n-propyl, and n-butyl; series II, dodecyldimethyl (2-hydroxyethyl)ammonium bromide, dodecylmethyl bis(2-hydroxyethyl) ammonium bromide, and dodecyl tris(2-hydroxyethyl)ammonium bromide. Chemical shifts of the surfactant discrete groups were measured at concentrations below and above the surfactant critical micelle concentration, c.m.c., whose value was determined by conductance measurements in H2O. Plots of observed chemical shifts versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. Excellent fits were obtained for the discrete groups of all surfactants. The parameters determined by these fits were: the equilibrium constant for micelle formation, K, c.m.c., and the chemical shifts of the monomer, delta(mon) and the micelle, delta(mic). H-1 NMR-based c.m.c. values are in excellent agreement with those which we determined by conductance measurements. Values of K increase as a function of increasing the size of the headgroup of series I surfactants, but the incremental contribution to the Gibbs free energy per CH2 group at the interface is small, its value increases from 0.13 kJ (moles of CH2)(-1) to 0.29 kJ (moles of CH2)(-1) in going from the trimethyl-to the tri-n-propyl, and the tri-n-butyl analogue, respectively. Micellization affects the chemical shifts of the groups at, or near to, the interface mainly because of differences in hydration of the surfactant molecule in its monomeric and aggregated forms. This conclusion is supported by comparing (delta(mic)-delta(mon)) with (delta(CDCl3)-delta(mon)), where delta(CDCl3) is the chemical shift of the surfactant in CDCl3, i.e., in absence of hydration. For series II surfactants, the chemical shifts versus [surfactant] plots are qualitatively similar to those for series I, this shows that substitution of the 2-hydroxyethyl group for the ethyl group in the hydrophilic part of the surfactant does not seem to change the properties of the formed aggregate.