Solid-State 1H and 13C NMR Investigations of Dodecyl Sulfate−Alumina Interfacial Interactions Using High Surface Area Pseudo-Boehmite Solids Containing Adsorbed Surfactants

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Abstract

Solid-state 1H NMR using CRAMPS (combined rotation and multiple pulse spectroscopy) technique and 13C cross-polarization magic angle spinning (CP/MAS) NMR studies of the interactions between a high surface area alumina material and various surfactants adsorbed at the solid/liquid interface in aqueous media systems are reported. These studies have focused on “wet” and evacuated alumina−surfactant solids obtained by adsorption of surfactant ions (dodecyl sulfate (DDS), oleate, and dodecylammonium) from aqueous media (pH 3 and 6.5), over surfactant loading concentrations (C0) from 1 × 10-3 to 1 × 10-2 M in 2% w/v suspensions of a high surface area pseudo-boehmite material. 1H CRAMPS and 1H single-pulse (SP)/MAS NMR were used to identify the relative proton 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 C0 is increased from 1 × 10-2 to 1 × 10-3 M, at both pH 6.5 and 3.0. Evacuation was found to eliminate the “physisorbed” water (4−6 ppm), permitting the observation of both “clustered” surface and/or internal Al−OH sites (7.0−7.5 ppm) and surfactant protons (1−3 ppm). Increases in the relative peak intensity of the surfactant protons as C0 was increased from 1 × 10-3 to 1 × 10-2 M, together with a decrease in the “physisorbed” water peak intensity, suggest that a competition occurs between the “physisorbed” water and the surfactant ions for the surface hydroxyl sites on the alumina material. Dipolar dephasing experiments show that the surfactant ions are quite mobile in “wet” DDS−Al2O3 solids, while the removal of water leads to increased surfactant rigidity. Several of the DDS−Al2O3 solids, prepared by equilibration in 2 × 10-3 to 1 × 10-2 M DDS loading concentration, were also examined by solid-state 13C CP/MAS NMR. While the 13C CP/MAS NMR spectra revealed numerous resonances that were assigned to various carbons of the adsorbed surfactant ion DDS, and that the intensity of these peaks are dependent on the surfactant loading levels, no significant changes were observed in the chemical shifts or line widths of the 13C NMR peaks to provide additional information about the mode of attachment and mobility of these adsorbed species on the alumina surface.

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