SANS contrast in iota-carrageenan gels and solutions in D₂O

SANS of Na+-iota-carrageenan in D2O/saline solutions was measured as a function of concentration, temperature and type of counterions (K+ or Na+). High and low scattering-contrasted gels and solutions were detected. High contrast is caused by aggregation of low-hydrated chains at high concentration of carrageenan in the presence of gel-promoting counterions. The aggregates do not disappear at 60 degrees C, although at this temperature the molecules should partially lose their ordered conformation. The network formation, observed by SANS, does not happen in high-contrast system immediately after cooling and takes a few hours to be completed at 12 degrees C. The minimum concentration sufficient for the high-contrast regime in 0.1 M KCl lies between 15 and 25 mg/ml.