NUCLEAR-MAGNETIC-RESONANCE AND X-RAY STUDY OF A RECTANGULAR PHASE

The rectangular phase in the lyotropic system sodium decyl sulphate (S dS)-decanol-water was studied by H-2 NMR and small-angle X-ray scatter ing. To investigate the variation in unit cell dimensions, the shape o f the ribbons, and molecular segregation within the ribbons with compo sition, nine samples were prepared. The aspect ratio of the ribbon cro ss section was found to be independent of composition, rho = 1.42 +/- 0.02, whereas the aspect ratio of the unit cell varied with compositio n, b/a = 2.7 - 3.0. In general, the molecular distribution of SdS and decanol is highly nonuniform; the decanol concentration is 2.5 to 3.0 times higher in the central lamellar region of the ribbons than in the ir curved edges, while SdS accumulates in the curved regions. On addit ion of SdS, a vertical compression of the unit cell occurs (a decrease s and b remains unchanged). However, the absolute dimensions of the ri bbons remain constant and the added SdS accumulates in the central lam ellar region of the ribbons. Addition of decanol yields horizontal exp ansion of the unit cell (a decreases and b increases) and the absolute dimensions of the ribbon cross section increase slightly. The added d ecanol distributes evenly in the aggregate. (C) 1996 Academic Press, I nc.