Influence of ionic effects on the ordering and association phenomena in dilute and semidilute carrageenan solutions

A small angle X-ray scattering (SAXS) study was made on Na+-kappa- and Na+- or Li+-iota-carrageenan in aqueous solutions with and without added salt in the dilute (similar to 6 mg/ml) and semidilute (similar to 16 mg/ml) regime close to the critical concentration (similar to 9-11 mg/ml). This concentration should be understood to be determined by contacts between blobs rather than between isolated molecules. The SAXS data of carrageenan solutions are described in terms of the fluctuation model of Borue-Erukhimovich rather than by a particle-type model, although the latter form of scattering appears when aggregates of ordered fragments are formed. According to current models of carrageenan the ordered fragments are single or double chain helices. In conditions favorable for the appearance of the ordered conformation, two polymer subsystems with a different characteristic screening scale of Coulombic interaction for the ideal Gaussian chain are detected. The system with the larger characteristic screening scale corresponds to molecular fragments with an ordered conformation. The contribution of both systems to the scattering can be separated which allows for discussion of their behaviour in the framework of the fluctuation theory. The SAXS results suggest that in all dilute and in the semidilute solutions with added NaCl or LiCl for both kappa- and iota-carrageenans there is a transformation from short-length correlated (disordered) to longer-length correlated (ordered) molecular fragments followed by the association of the ordered fragments, whereas a transformation from an association of disordered fragments to a structure with ordered fragments is detected in semidilute solutions of Li+-iota-carrageenan in LiI and Na+-kappa-carrageenan in NaI. In the semidilute solutions of Na+-iota-carrageenan in NaI the formation of ordered fragments seems to follow an intermediate sequence.