A PROTON NMR-STUDY ON AGGREGATION OF CATIONIC SURFACTANTS IN WATER - EFFECTS OF THE STRUCTURE OF THE HEADGROUP

H-1 NMR chemical shifts of solutions of the following cationic surfact ants in D2O were determined as a function of their concentrations: cet yltrimethylammonium chloride, CTAC1, a 1:1 molar mixture of CTAC1 and toluene, cetylpyridinium chloride, CPyCl, cetyldimethylphenylammonium chloride, CDPhACl, cetyldimethylbenzylammonium chloride, CDBzACl, cety ldimethyl-2-phenylethylammonium chloride, CDPhEtACl, and cetyldimethyl -3-phenylpropylammonium chloride, CDPhPrACl. Plots of observed chemica l shifts versus [surfactant] are sigmoidal and were fitted to a model based on the mass-action law. Satisfactory fitting was obtained for th e discrete protons of all surfactants. From these fits, we calculated the equilibrium constant for micelle formation, K, the critical micell e concentration, CMC and the chemical shifts of the monomer, delta(mon ) and the micelle delta(mic). H-1 NMR-based CMC values are in excellen t agreement with those which we determined by surface tension measurem ents of surfactant solutions in H2O, allowing for the difference in st ructure between D2O and H2O. Values of K increase as a function of inc reasing the size of the hydrophilic group, but the free energy of tran sfer per CH2 group of the phenylalkyl moiety from bulk water to the mi cellar interface is approximately constant, 1.9 +/- 0.1 kJ mol(-1). Va lues of (delta(mic)-delta(mon)) for the surfactant groups at the inter face, 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 o f the phenyl group in the interfacial region. The picture that emerges is that the aromatic ring is perpendicular to the interface in CDPhAC l and is more or less parallel to it in CDBzACl, CDPhEtACl, and CDPhPr ACl.