Proton magic-angle spinning-NMR investigation of surfactant aqueous suspensions

In this Note we present the advantages of 1H magic-angle spinning nuclear magnetic resonance (MAS-NMR) for the investigation of surfactant suspensions via transverse relaxation rate (R2) measurements. 1H-relaxation rates can be determined by the classical CPMG method from high-resolution spectra obtained either under conditions of liquid-state NMR for monomers and small spherical micelles or by using MAS-NMR for larger aggregates. For a mixture of alkyl dioxyethylene sulfate and alkylbetaine (80:20, w/w), up to a percentage of surfactant in water of 20%, we found that R2 increased, in accordance with an increased micellar size and very likely the formation of an HI phase. However, above 25%, R2 decreased. This result suggests a change from a hexagonal to a lamellar phase that would be difficult to observe by proton NMR without magic-angle spinning because the lines would be very broad, or by light scattering because of sample opacity. This NMR approach seems to have been overlooked by the community of surfactant physical chemists. It can be complementary to other analytical techniques and presents the advantage of not requiring isotopic labeling.