A carbon-13 and proton nuclear magnetic resonance study of hydrocarbon chain order in isotropic and anisotropic phases formed in the sodium p-octylbenzenesulfonate-short-chain alcohol-water system

Abstract

The order of the hydrocarbon chain and the phenyl ring of sodium p-octylbenzenesulfonate (SOBS) in some anisotropic phases consisting of SOBS, pentan-1-ol, and water is investigated with deuterium NMR and compared with the order in some two-, three-, and four-component isotropic solutions consisting of SOBS, water, butan-1-ol, and toluene, as extracted from /sup 13/C NMR relaxation measurements. These latter measurements also give information on the dynamic state of the SOBS molecules in these microemulsion systems. It is found that a short-chain alcohol has the effect of compressing the bilayers of the lamellar phase below the maximum thickness. The odd-even effect clearly seen in the order parameter profile for the lamellar phase is interpreted as a result of the C/sub 2/ axis of the phenyl ring not being parallel to the symmetry axis of the hydrocarbon chain. The order parameter profile of a rectangular liquid-crystalline phase together with X-ray findings is claimed to be a result of the rods constituting the mesophase having a biaxial cross section. From the present relaxation results and earlier NMR self-diffusion measurements the following structural and dynamic picture of microemulsions of the present type of systems emerges: the hydrophobic-hydrophilic interface imposes a local anisotropy which ismore » similar in magnitude to the one in the lamellar phase but has a very short lifetime. Ill-defined and highly flexible aggregates with rather large radii of curvature are present. 29 references, 3 figures, 3 tables.« less