The nature of alcohol co-adsorption sites and their effects on surfactant aggregate structure on silica investigated by 2H and 13C NMR

Abstract

The effects of medium- and long-chain alcohols on surfactant aggregation in solution are well understood; however, our molecular level understanding of alcohol interaction with adsorbed surfactant aggregates is comparatively scarce. Here we use 2H and 13C NMR spectroscopy to study the system tetradecyltrimethylammonium bromide (TTAB) adsorbed on silica at the plateau level, where co-adsorption of 1-butanol and 1-heptanol is investigated. It is found that 13C and 2H NMR spectra of samples where the alcohol is labelled with deuterium in the α-segment give clear indications about the nature of the alcohol co-adsorption sites. 1-Butanol and 1-heptanol co-adsorb at a common co-adsorption site. However, 1-heptanol co-adsorbs at a second site as well. The common co-adsorption site is interpreted as being the alcohol in the contact area between the hydrophobic TTAB tail and the bulk solution. For higher concentrations of 1-heptanol, co-adsorption at the second site is interpreted as being 1-heptanol molecules intercalated between adsorbed TTAB molecules, presumably with the hydroxyl group positioned between TTAB head groups. NMR results, adsorption densities and zeta potentials further show that the differences in 1-butanol and 1-heptanol co-adsorption are related to structural changes of the adsorbed TTAB aggregate.