SMALL-ANGLE X-RAY-SCATTERING OF KAPPA-CARRAGEENAN AND IOTA-CARRAGEENAN IN AQUEOUS AND IN SALT-SOLUTIONS

Aqueous and saline (0.03-0.08 M NaCl) solutions of the Na+-salt of kap pa- and iota-carrageenan (3-17 mg/ml) have been investigated using sma ll-angle X-ray scattering (SAXS). Investigation of the SAXS curves nor malized for concentration allows to detect the transition from the dil ute to the semidilute regime. Some of the experimental curves exhibit a clear maximum. In the case of kappa-carrageenan, the position of thi s maximum is proportional to the cube root of polysaccharide concentra tion. The fluctuation theory of Borue and Erukhimovich (BE theory), wh ich predicts a maximum in the scattering curves for low ionic strength , gives a good fit for all our experiments. An increase of the polymer concentration or the ionic strength causes the appearance of two subs ystems in solution exhibiting a different characteristic screening sca le of Coulombic interaction for ideal Gaussian chains (and two differe nt segment lengths), The condition for an increase of the relative con tribution of the subsystem with the larger segment length correlates w ith the one for the disorder-order transition of carrageenan. In the m ost concentrated (17.1 mg/ml) solution of Na+-L-carrageenan, the incre ase of NaCl molarity above 0.06 M gives rise to additional scattered i ntensity at the smallest angles, indicating molecular association.