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.