SODIUM SULFOPROPYL OCTADECYL MALEATE - A LONG-CHAIN SURFACTANT WITH UNUSUAL AGGREGATION BEHAVIOR IN AQUEOUS-SOLUTION

The long chain surfactant sodium sulfopropyl octadecyl maleate (SSPOM) shows above the Krafft boundary (T-K approximate to 37 degrees C) wit h respect to its phase behavior in aqueous solution many similarities to other ionic surfactants. An isotropic liquid micellar phase L(1) fo llowed by the normal hexagonal H-alpha-phase are observed. Uncommon pr operties appear in the temperature range below T-K where crystals and a gel-like state with lamellar structure exist. The gel state is metas table but long-lived, so that its microstructure can be well-character ized by application of many different techniques. The Krafft boundary of mixtures of SSPOM and the C-14 homologue shows the typical chain le ngth dependence of the chain-melting phase transition of lipids. Wide- angle X-ray scattering data suggest the molecules within the bilayers of the gel state to be densely packed and interdigitated. The experime ntally estimated bilayer thickness is in well agreement with atomic da ta. The swelling of the gel state in water is nonlinear and shows that the system has to be viewed as a dispersion of lamellar aggregates.