SAXS study of ι-carrageenan-surfactant complexes

Small-angle X-ray scattering (SAXS) was used to study the structure of l-carrageenan gels formed in the presence of cationic surfactant cetylpyridinium chloride (CPC) at various concentrations of the polysaccharide and as a function of the surfactant concentration. A physical gel of low l-carrageenan concentration displays no characteristic peak at small angles. After addition of CPC to the l-carrageenan solution central scattering and equidistant characteristic peaks appear due to ordering in the gel structure. This ordering is the result of complex hydrophobic and electrostatic interactions between the polymer chains and the charged surfactants. The l-carrageenan gelation in the presence of CPC leads to self-assembly of surfactant molecules inside the physical gel similarly to micellization in a solution. In this case the shrinking or collapse of polyelectrolyte gels takes place. The additional central scattering emerges due to the process of surfactant micellization. The relative positions of the main and secondary peaks (1:2) correspond to the formation of a lamellar structure of the complexes. The mean long-range order dimensions, the radii of interaction, and the degree of disorder in the systems are calculated from the Bragg peaks on the SAXS patterns.