A H-2 NMR-STUDY OF 2 CATIONIC SURFACTANTS ADSORBED ON SILICA PARTICLES

H-2 NMR spectra of dodecyltrimethylammonium bromide (DoTAB) and hexade cyltrimethylammonium bromide (CTAB), deuterium labeled in the moiety o f the charged headgroup and adsorbed on silica particles, have been re corded. DoTAB spectra show that the rate of the chemical exchange betw een the adsorbate and the solution depends on the packing of the parti cles; centrifuged suspensions exhibit slow exchange, while suspensions settled by gravitation show fast exchange. It was also found that two slowly exchanging molecular domains exist on the solid surface. The m olecular dynamics and local ordering are essentially equal in the two domains, causing the resonances and relaxation behavior to coincide. I t was concluded that a bilayer adsorbate structure is consistent with the experimental observations. Furthermore, it was shown that surfacta nt aggregation occurs already at approximately 20% coverage of the sur face. Relaxation rate measurements were also made. Reorientational mot ions of the surfactant headgroup adsorbed onto a solid surface were fo und to be strongly hindered, as compared to the dissolved state. The p resence of very slow motions was recognized. Multifield relaxation dat a were interpreted by means of a first-order motional model, including fast slightly anisotropic local motions superimposed by slow isotropi c overall motions. The obtained correlation times indicate that the lo cal chain motions are somewhat slower than in micelles, and that the s low motions are consistent with surfactant aggregates with a radius of curvature of the same magnitude as the particle radius.