SOLID-STATE H-1 AND C-13 NMR INVESTIGATIONS OF DODECYL SULFATE-ALUMINA INTERFACIAL INTERACTIONS USING HIGH-SURFACE-AREA PSEUDO-BOEHMITE SOLIDS CONTAINING ADSORBED SURFACTANTS

Solid-state H-1 NMR using CRAMPS (combined rotation and multiple pulse spectroscopy) technique and C-13 cross-polarization magic angle spinn ing (CP/MAS) NMR studies of the interactions between a high surface ar ea alumina material and various surfactants adsorbed at the solid/liqu id interface in aqueous media systems are reported. These studies have focused on ''wet'' and evacuated alumina-surfactant solids obtained b y adsorption of surfactant ions (dodecyl sulfate (DDS), oleate, and do decylammonium) from aqueous media (pH 3 and 6.5), over surfactant load ing concentrations (C-0) from 1 x 10(-3) to 1 x 10(-2) M in 2% w/v sus pensions of a high surface area pseudo-boehmite material. H-1 CRAMPS a nd H-1 single-pulse (SP)/MAS NMR were used to identify the relative pr oton populations associated with surface Al - OH groups, ''physisorbed '' water, and the various protons of adsorbed surfactant ions of ''wet '' and evacuated solids. 1H CRAMPS data and adsorption densities show that the adsorption of the dodecyl sulfate ion on the alumina surface is increased as C-0 is increased from 1 x 10(-2) to 1 x 10(-3) M, at b oth pH 6.5 and 3.0. Evacuation was found to eliminate the ''physisorbe d'' water (4-6 ppm), permitting the observation of both ''clustered'' surface and/or internal Al-OH sites (7.0-7.5 ppm) and surfactant proto ns (1-3 ppm). Increases in the relative peak intensity of the surfacta nt protons as C-0 was increased from 1 x 10(-3) to 1 x 10(-2) M, toget her with a decrease in the ''physisorbed'' water peak intensity, sugge st that a competition occurs between the ''physisorbed'' water and the surfactant ions for the surface hydroxyl sites on the alumina materia l. Dipolar dephasing experiments show that the surfactant ions are qui te mobile in ''wet'' DDS-Al2O3 solids, while the removal of water lead s to increased surfactant rigidity. Several of the DDS-Al2O3 solids, p repared by equilibration in 2 x 10(-3) to 1 x 10(-2) M DDS loading con centration, were also examined by solid-state C-13 CP/MAS NMR. While t he C-13 CP/MAS NMR spectra revealed numerous resonances that were assi gned to various carbons of the adsorbed surfactant ion DDS, and that t he intensity of these peaks are dependent on the surfactant loading le vels, no significant changes were observed in the chemical shifts or l ine widths of the C-13 NMR peaks to provide additional information abo ut the mode of attachment and mobility of these adsorbed species on th e alumina surface.