1H NMR relaxation of water: A probe for surfactant adsorption on kaolin

In this study, 1H NMR is used to investigate properties of sodium dodecyl sulfate (SDS), tetradecyl trimethyl ammonium bromide (TTAB), and dodecyl trimethyl ammonium bromide (DTAB) adsorbed on kaolin by NMR T1 and T2 measurements of the water proton resonance. The results show that adsorbed surfactants form a barrier between sample water and the paramagnetic species present on the clay surface, thus significantly increasing the proton T1 values of water. This effect is attributed to the amount of adsorbed surfactants and the arrangement of the surfactant aggregates. The total surface area covered by the cationic (DTAB and TTAB) and anionic (SDS) surfactants could be estimated from the water T1 data and found to correspond to the fractions of negatively and positively charged surface area, respectively. For selected samples, the amount of paramagnetic species on the clay surface was reduced by treatment with hydrofluoric (HF) acid. For these samples, T1 and T2 measurements were taken in the temperature range 278–338 K, revealing detailed information on molecular mobility and nuclear exchange for the sample water that is related to surfactant behavior both on the surface and in the aqueous phase.

Adsorbed surfactants form a barrier that reduces effects on the proton resonance of water caused by paramagnetic species on kaolin.Figure optionsDownload high-quality image (107 K)Download as PowerPoint slideHighlights
► NMR on the water proton resonance as probe for surfactant adsorption on kaolin.
► Adsorbed surfactants shield the water from paramagnetic surface species.
► Water T1 relaxation is correlated with the amount of adsorbed surfactant.
► Studies on water dynamics (T1/T2) give information on adsorbed and non-adsorbed surfactant structure.