MICROSTRUCTURE AND THERMODYNAMICS OF A LAMELLAR PHASE WITH DISRUPTED SURFACTANT BILAYERS

Intrinsic structural defects in the lamellar (L(alpha)) phase of the s ystem sodium dodecyl sulphate (SDS)/decanol/water are studied by a com bination of NMR H-2 quadrupole splittings of alpha-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 (dec anol/SDS mol ratio 0.46 2-5), the bilayer volume fraction (0.27-0-64), and the temperature (20-40-degrees-C). It is found that bilayer defec ts 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 prog ression of phases towards microstructures with more highly curved inte rfaces. Variations of the extent of defects in the L(alpha) phase refl ect variations of the number of defects rather than changes of their s ize. The data are consistent with either slit defects (slit width 10 a ngstrom) or pore defects (diameter approximately 25 angstrom). In eith er case, the interface separation across the defect is considerably sm aller than the inter-bilayer separation. There is no evidence of trans itions between L(alpha) phases with different defect patterns. The def ect variation with composition is analysed in terms of a simple thermo dynamic model, showing that, apart from the electrostatic bilayer repu lsion (which opposes defects), it is necessary to allow for considerab le variations of the defect self-energy (which promotes defects).