SAXS study of iota-carrageenan-surfactant complexes

Small-angle X-ray scattering (SAXS) was used to study the structure of l-ca rrageenan gels formed in the presence of cationic surfactant cetylpyridiniu m chloride (CPC) at various concentrations of the polysaccharide and as a f unction of the surfactant concentration. A physical gel of low l-carrageena n concentration displays no characteristic peak at small angles. After addi tion of CPC to the l-carrageenan solution central scattering and equidistan t characteristic peaks appear due to ordering in the gel structure. This or dering is the result of complex hydrophobic and electrostatic interactions between the polymer chains and the charged surfactants. The l-carrageenan g elation in the presence of CPC leads to self-assembly of surfactant molecul es inside the physical gel similarly to micellization in a solution. In thi s case the shrinking or collapse of polyelectrolyte gels takes place. The a dditional central scattering emerges due to the process of surfactant micel lization. The relative positions of the main and secondary peaks (1:2) corr espond to the formation of a lamellar structure of the complexes. The mean long-range order dimensions, the radii of interaction, and the degree of di sorder in the systems are calculated from the Bragg peaks on the SAXS patte rns.