Structure of Sodium 4-(1'-Heptylnonyl)benzenesulfonate Adsorbed on Alumina Using 2H NMR

Abstract

Deuterium NMR has been used to probe the molecular dynamics and structure of the surfactant sodium 4-(1′-heptylnonyl)benzenesulfonate (SHBS) adsorbed from water solution onto alumina. In order to investigate the dynamics of the headgroup, the phenyl ring of the surfactant was specifically deuterated. Two different surface coverages were studied in the temperature range of -30 to 80°C. At the adsorption plateau, evidence for the presence of two distinct surfactant domains of approximately equal intensities, on the alumina surface, was found. One domain, the inner layer next to the solid surface, gave rise to a quadrupole splitting in the 2H NMR spectrum similar to that found for the surfactant in lamellar liquid crystals, whereas the second domain, the outer layer, gave rise to a broad unsplit resonance indicating a more isotropic environment. The chemical exchange between the two domains was found to be slow on the NMR timescale. At a lower coverage, a broad unsplit resonance appeared in the NMR spectrum, showing that the molecular motions are not sufficiently anisotropic to cause any observable quadrupole splittings. With increasing temperature, resonances for both samples were narrowed, and the quadrupole splitting was reduced, reflecting more rapid reorientations of the surfactant molecules.