Study of micellar solutions and microemulsions of an alkyl oligoglucoside via NMR spectroscopy

Abstract

The aggregation behavior of a commercial non-ionic surfactant, octyl diglucoside (C8G2), in water and when mixed with pentanol have been investigated using the multicomponent self-diffusion NMR method. The smallest C8G2 micelles have an apparent spherical hydrodynamic radius R of 1.9 nm. Micellar growth occurs progressively as surfactant concentration increases, unlike previously studied glycolipids which are structurally homogeneous. Long prolate-shaped aggregates form at higher concentrations. 13C-NMR chemical shift trends of C8G2 and pentanol carbons from L 2 solutions suggest a hydration number of 10 or 11 water molecules per glucose. Addition of water to L 2 solutions causes a decrease in extension of the C8G2 molecule, displacement of pentanol from near the hydrophilic head groups, and a reduction in the mean surfactant film curvature toward water. C8G2 hydrophilicity is decreased by adding the lipophilic cosurfactant pentanol to form microemulsions. Microstructural changes occurring within the L 2 regions of a four-component microemulsion system with decane are examined for two series of solutions selected to reveal the effect of water and pentanol. At 25% decane, an increase in pentanol content causes a progressive decrease in the mean curvature of the surfactant film toward oil, allowing the microstructure to change from oil-in-water to water-in-oil via a bicontinuous microemulsion. Water diffuses much faster than C8G2 in the L 2 solutions.