Helicity degree of carrageenan conformation determines the polysaccharide and water interactions

Abstract

The polysaccharide in solution at critical concentration, Cc (g/L), is assimilated to a nano hydrogel (nHG) made of a single polysaccharide chain. Taking as reference the characteristic temperature of 20 ± 2 °C at which kappa-carrageenan (κ-Car) nHG swelling is greater with a Cc = 0.55 ± 0.05 g/L, the temperature of the minimum deswelling in the presence of KCl was found at 30 ± 2 °C for 5 mM with a Cc = 1.15 ± 0.05 g/L but not measurable above 100 °C for 10 mM of which Cc = 1.3 ± 0.05 g/L. Lowering the temperature to 5 °C, contraction of the nHG and further coil-helix transition with self-assembly increases the sample's viscosity, which steadily evolves with time in a logarithmic scale. Accordingly, the relative increment of the viscosity per unit of concentration, Rv (L/g), should increase in agreement with increasing polysaccharide concentration. But the Rv decreases for κ-Car samples above 3.5 ± 0.5 g/L in the presence of 10 mM KCl under steady shear 15 s−1. This reflects a decrease of κ-Car helicity degree knowing that the polysaccharide is rather hydrophilic when its helicity degree is the lowest.