EFFECT OF ALKALI METAL IONS ON THE RHEOLOGICAL PROPERTIES OF κ-CARRAGEENAN AND AGAROSE GELS

Stress relaxation and dynamic measurements were carried out on κ-carrageenan and agarose gels to examine the effect of alkali metal ions. The elastic modulus of κ-carrageenan gels increase remarkably by adding alkali metal ions Li⁺, Na⁺, K⁺, Cs⁺. This was explained as follows: the electrostatic repulsion of sulfate groups in κ-carrageenan gels are prevented by the shielding action of the alkali metal ions stabilizing the double helical structure of κ-carrageenan gels. The increase of the modulus was in the order: Cs⁺ > K⁺≫ Na⁺ > Li⁺. The difference between these two groups (Cs⁺, K⁺) and (Na⁺, Li⁺) is that the former group belongs to the structure disordering ions while the latter group belongs to the structure ordering ions. Thus, the former group prevents the repulsion of sulfate groups more directly than the latter. Agarose gel, which lacks sulfate groups, is not influenced so much by adding the alkali metal ions.