Rheology of kappa-carrageenan in mixtures of sodium and cesium iodide: two types of gels

Recent studies on dilute solutions (Borgström et al. (1996), Int. J. Biol. Macromol. 18, 223) have shown that kappa-carrageenan helices associate into superhelical rigid rods in mixed 0.1 M aqueous solutions of NaI and CsI above a critical mole fraction (x_Cs = 0.4) of Cs. This work concerns the temperature-dependent rheology of more concentrated systems in mixed and pure solutions of the same salts. Gels with low moduli were even found in NaI alone, although this salt is known to impede the gelation of kappa-carrageenan, but only above 0.9% (w/w) of carrageenan. These gels were reminiscent of iota-carrageenan gels in two respects: the (low) magnitude of the shear storage modulus (G′), and the absence of hysteresis in the sol-gel transition. On the other hand, both the threshold concentration for gelation and the ratio between the loss and storage moduli were substantially higher for the kappa-carrageenan gels in NaI. In mixed solutions of CsI and NaI, two types of kappa-carrageenan gels could be distinguished, depending on the cesium content. The transition occurred at x_Cs = 0.4, as in the previous studies on dilute solutions. Below x_Cs = 0.4, the gels were similar to those in NaI alone. Above x_Cs = 0.4, the gels were similar to ‘conventional’ kappa-carrageenan gels, formed in salts such as KC1: a pronounced thermal hysteresis appeared in the sol-gel transition, the gels showed tendencies for syneresis, and G′ increased dramatically with increasing cesium content.