Effect of alkali metal ions on the viscoelasticity of concentrated kappa-carrageenan and agarose gels

Abstract

The effect of the addition of the monovalent cations Li⁺, Na⁺, K⁺, and Cs⁺ on the gelation of agarose and kappa-carrageenan aqueous gels has been studied by the measurement of longitudinal vibration. The dynamic Youngs's modulusE′ of 2% w/w agarose and 0.4–6% w/w kappa-carrageenan gels containing the alkali metal salt LiCl, NaCl, KCl or CsCl of various concentrations from 0 to 4.5 mol/l has been measured at various temperatures. By the addition of the alkali metal salt, the value ofE′ for agarose gels is influenced only slightly, while for kappa-carrageenanE′ is increased substantially. Kappa-carrageenan has many sulphate groups. The addition of the alkali metal ions screens the electrostatic repulsion between these groups. As a result of this, the helical structure of kappa-carrageenan is stabilised and the helices may form densely packed aggregates, so increasingE′. In contrast, agarose has a naturally stable molecular structure and therefore, the structure and henceE′ is not sensitive to added ions. The K⁺ and Cs⁺ ions increaseE′ more than Li⁺ and Na⁺ for kappa-carrageenan gels. This is interpreted on the basis that these ions are either structure ordering or structure disordering ions for water.