Structure of cationic starch (CS)/anionic surfactant complexes studied by small-angle X-ray scattering (SAXS)

The structure of lyotropic liquid crystalline or gellike phases formed in cationic starch (CS)/anionic surfactant/water systems in the temperature range 25-80 degreesC has been investigated by small-angle X-ray scattering. The surfactants were sodium dodecyl sulfate (SDS), sodium decanoate (NaDe), sodium dodecanoate (NaDod), sodium palmitate (NaPal), sodium oleate (NaOl), and sodium erucate (NaEr). The phases were in equilibrium with aqueous solutions at 60 degreesC and contained 15-25 wt % of CS, and 10-30 wt % of surfactant, depending on the charge density of the CS and the chain length of the surfactant. The phases consist of CS/surfactant aggregates arranged in long-range structures similar to lyotropic mesophases formed by the pure surfactants alone, but they separate from aqueous solutions at much lower surfactant concentrations. When the charge density of the CS is low or the surfactant hydrocarbon chain is short, cubic or hexagonal phases are formed. As expected, the formation of lamellar phase is promoted by increasing these parameters. Temperature affects the stability of the phases and their structure. At high temperatures the long-range order breaks down, and the phases are akin to concentrated micellar CS/surfactant solutions.