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 c ationic starch (CS)/anionic surfactant/water systems in the temperature ran ge 25-80 degreesC has been investigated by small-angle X-ray scattering. Th e surfactants were sodium dodecyl sulfate (SDS), sodium decanoate (NaDe), s odium dodecanoate (NaDod), sodium palmitate (NaPal), sodium oleate (NaOl), and sodium erucate (NaEr). The phases were in equilibrium with aqueous solu tions at 60 degreesC and contained 15-25 wt % of CS, and 10-30 wt % of surf actant, depending on the charge density of the CS and the chain length of t he surfactant. The phases consist of CS/surfactant aggregates arranged in l ong-range structures similar to lyotropic mesophases formed by the pure sur factants alone, but they separate from aqueous solutions at much lower surf actant concentrations. When the charge density of the CS is low or the surf actant 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 struc ture. At high temperatures the long-range order breaks down, and the phases are akin to concentrated micellar CS/surfactant solutions.