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 ad ded 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 c ontacts between blobs rather than between isolated molecules. The SAXS data of carrageenan solutions are described in terms of the fluctuati on model of Borue-Erukhimovich rather than by a particle-type model, a lthough the latter form of scattering appears when aggregates of order ed fragments are formed. According to current models of carrageenan th e 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 Coulombi c interaction for the ideal Gaussian chain are detected. The system wi th the larger characteristic screening scale corresponds to molecular fragments with an ordered conformation. The contribution of both syste ms to the scattering can be separated which allows for discussion of t heir behaviour in the framework of the fluctuation theory. The SAXS re sults suggest that in all dilute and in the semidilute solutions with added NaCl or LiCl for both kappa- and iota-carrageenans there is a tr ansformation from short-length correlated (disordered) to longer-lengt h correlated (ordered) molecular fragments followed by the association of the ordered fragments, whereas a transformation from an associatio n of disordered fragments to a structure with ordered fragments is det ected in semidilute solutions of Li+-iota-carrageenan in LiI and Na+-k appa-carrageenan in NaI. In the semidilute solutions of Na+-iota-carra geenan in NaI the formation of ordered fragments seems to follow an in termediate sequence.