RHEOLOGY OF KAPPA-CARRAGEENAN IN MIXTURES OF SODIUM AND CESIUM IODIDE- 2 TYPES OF GELS

Recent studies on dilute solutions (Borgstrom et al. (1996), Int. J. B iol. Macromol. 18, 223) have shown that kappa-carrageenan helices asso ciate 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 wo rk concerns the temperature-dependent rheology of more concentrated sy stems in mixed and pure solutions of the same salts. Gels with low mod uli were even found in NaI alone, although this salt is known to imped e the gelation of kappa-carrageenan, but only above 0.9% (w/w) of carr ageenan. These gels were reminiscent of iota-carrageenan gels in two r espects: the (low) magnitude of the shear storage modulus (G'), and th e absence of hysteresis in the sol-gel transition. On the other hand, both the threshold concentration for gelation and the ratio between th e loss and storage moduli were substantially higher for the kappa-carr ageenan gels in NaI. In mixed solutions of CsI and NaI, two types of k appa-carrageenan gels could be distinguished, depending on the cesium content. The transition occurred at x(Cs) = 0.4, as in the previous st udies on dilute solutions. Below x(Cs)=0.4, the gels were similar to t hose in NaI alone. Above x(Cs)=0.4, the gels were similar to 'conventi onal' kappa-carrageenan gels, formed in salts such as KCl: a pronounce d thermal hysteresis appeared in the sol-gel transition, the gels show ed tendencies for syneresis, and G' increased dramatically with increa sing cesium content. (C) 1997 Elsevier Science Ltd.