MOLECULAR SEGREGATION AND AGGREGATE SHAPE IN A LYOTROPIC RECTANGULAR PHASE

The microstructure of the rectangular phase in the system sodium decyl sulphate/decanol/water is investigated by means of deuterium NMR. By a nalysing the lineshape from selectively deuteriated decylsulphate and decanol, we separate the effects of (i) the shape anisotropy of the ag gregate cross-section and (ii) the inhomogeneous distribution of the t wo surfactants within the aggregate. The aspect ratio of the cross-sec tion is determined to 1.39 +/- 0.01, substantially smaller than previo us estimates. We find no evidence for anisotropic growth of the aggreg ate cross-section in the hexagonal phase, as previously suggested. Rat her, the aggregate shape appears to change abruptly at the hexagonal-r ectangular phase transition with little change (with temperature) ther eafter. The distribution of decylsulphate and decanol within the aggre gates of the rectangular phase is highly non-uniform; the decanol conc entration is 3 times higher in the central lamellar region than in the curved edges, while the decylsulphate accumulates in the curved regio ns. This molecular segregation can be rationalized in terms of electro static interactions within and between the aggregates.