title: Swollen micelles and alcohol-surfactant co-adsorption: structures and mechanisms from liquid- and solid-state1H-1H NMR spectroscopy.
abstract: Mixtures of surfactants and medium chained alcohols display an anomalous phase behaviour, with the formation of swollen micelles in mid-range surfactant concentrations, which transition into larger non-swollen aggregates when the surfactant concentration increases above a critical point. These alcohols also affect the adsorption behaviour of the surfactants. In this study, intermolecular proximities are measured for such systems by1H-1H NMR dipolar correlation experiments, giving molecular localizations. The medium chained 1-heptanol and an anionic surfactant sodium dodecyl sulphate (SDS) are studied, both solubilized and adsorbed on alumina. Nuclear Overhauser Effect Spectroscopy (NOESY) shows that 1-heptanol localizes in both the palisade layer and in the core of SDS micelles when the 1-heptanol : SDS mole ratio increases beyond 2. The micelle diameter then increases with increasing 1-heptanol : SDS mole ratios due to more 1-heptanol partitioning in the micelle interior. When the micelle diameter increases beyond ∼6 nm, some SDS moves into the micelle interior, which may be a driving force for the structural transition at higher SDS concentrations. After being adsorbed on alumina,1H-1H double-quantum magic angle spinning (DQ MAS) shows that SDS/1-heptanol bilayers are formed where 1-heptanol localizes in the palisade layer only, but with slightly different localizations compared to that in micelles. Three different 1-heptanol environments are identified on the surface by2H NMR using2H labelled 1-heptanol. However, in contrast to in solution, no 1-heptanol adsolubilizes in the bilayer interior.
authors.affiliation: Department of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway. christian.totland@uib.no.
authors.forename: Christian
authors.identifier: http://orcid.org/0000-0002-1457-9012
authors.initials: C
authors.lastname: Totland
authors.affiliation: Department of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway. christian.totland@uib.no.
authors.forename: Anne Marit
authors.initials: AM
authors.lastname: Blokhus