Microstructure and thermodynamics of a lamellar phase with disrupted surfactant bilayers

Abstract

Intrinsic structural defects in the lamellar (L_α) phase of the system sodium dodecyl sulphate (SDS)/decanol/water are studied by a combination of NMR ²H quadrupole splittings of α-deuteriated SDS and small angle X-ray scattering (SAXS). The focus is on the variation of the density and size of the defects with the bilayer composition (decanol/SDS mol ratio 0.46–2.5), the bilayer volume fraction (0.27–0.64), and the temperature (20–40°C). It is found that bilayer defects are promoted by a small decanol/SDS ratio in the bilayers and by a low bilayer volume fraction, i.e. the same factors that drive the progression of phases towards microstructures with more highly curved interfaces. Variations of the extent of defects in the L_α phase reflect variations of the number of defects rather than changes of their size. The data are consistent with either slit defects (slit width ∼ 10 Å) or pore defects (diameter ∼ 25 Å). In either case, the interface separation across the defect is considerably smaller than the inter-bilayer separation. There is no evidence of transitions between L_α phases with different defect patterns. The defect variation with composition is analysed in terms of a simple thermodynamic model, showing that, apart from the electrostatic bilayer repulsion (which opposes defects), it is necessary to allow for considerable variations of the defect self-energy (which promotes defects).