SYNCHROTRON X-RAY-SCATTERING STUDIES ON THE SODIUM DODECYL SULFATE-WATER-PENTANOL-DODECANE L-3 SPONGE PHASE

High-resolution small-angle x-ray-scattering (SAXS) data in the quater nary sodium dodecyl sulfate-water-pentanol-dodecane membrane system in the L-3 phase were collected and analyzed. The SAXS data show a broad peak followed by q(-2) and q(-4) regions at higher q. The data an des cribed quantitatively by the product of a structure factor of 1 + C(1) arctan(q xi(1)/2)/q + C-2/[1/xi(2)(2)+(\q\ - q(c))(2)] and a form fact or of a randomly oriented disks. The C-1 term is the structure factor develop by Cates et al. [Europhys. Lett. 5, 733 (1988)], describing a bicontinuous phase with randomly connected bilayer sheets separating t he space at length scales larger than the typical cell size L. The C-2 term, introduced because of the observation of the broad peak centere d around q(c) in the SAXS data, describes the bilayer cell-cell correl ations. For high dilution samples, the form factor alone can describe the data very well. From our data of the water layer volume fraction a nd L = 2 pi/q(c), we have determined that for every volume of (2 pi/q( c))(3), the membrane has an area of (1.3-1.8)(2 pi/q(c))(2). This is c onsistent with a bicontinuous structure for the L-3 membrane system. T he logarithmic increase of the water layer area with 2 pi/q(c) is due to the membrane thermal undulations. The bending rigidity of the membr ane has been found to be (1.0+/-0.2)k(B)T.