A proton NMR study on aggregation of cationic surfactants in water: effects of the structure of the headgroup

Abstract

1HNMR chemical shifts of solutions of the following cationic surfactants in D2O were determined as a function of their concentrations: cetyltrimethylammonium chloride, CTAC1, a 1:1 molar mixture of CTAC1 and toluene, cetylpyridinium chloride, CPyCl, cetyldimethylphenylammonium chloride, CDPhACl, cetyldimethylbenzylammonium chloride, CDBzACl, cetyldimethyl-2-phenylethylammonium chloride, CDPhEtACl, and cetyldimethyl-3-phenylpropylammonium chloride, CDPhPrACl. Plots of observed chemical shifts versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. Satisfactory fitting was obtained for the discrete protons of all surfactants From these fits, we calculated the equilibrium constant for micelle formation, K, the critical micelle concentration, CMC and the chemical shifts of the monomer, δmon and the micelle δmic 1H NMR-based CMC values are in excellent agreement with those which we determined by surface tension measurements of surfactant solutions in H2O, allowing for the difference in structure between D2O and H2O. Values of K increase as a function of increasing the size of the hydrophilic group, but the free energy of transfer per CH2 group of the phenylalkyl moiety from bulk water to the micellar interface is approximately constant, 1.9 ± 0.1 kJ mor−1. Values of (δmic−δmon) for the surfactant groups at the interface, e.g., CH3−(CH2)15− N + (CH3)2 and within the micellar core, e.g., CH3−(CH2)15−N+ were used to probe the (average) conformation of the phenyl group in the interfacial region. The picture that emerges is that the aromatic ring is perpendicular to the interface in CDPhACl and is more or less parallel to it in CDBzACl, CDPhEtACl, and CDPhPrACl.