The gels from ι-carrageenan are less strong with less syneresis and need larger salt excess to be formed in direct relation with these differences, a polymer fractionation can be performed by gel-phase separation in the presence of KCl based on the different gel points for the κ- and ι-polymers. (b) Polymer concentrations: 20 g l −1 (solid squares) 10 g l −1 (solid triangles) 5 g l −1 (open triangles.) 144 (a) Polymer concentrations: 5 g l −1 (solid squares) 2.5 g l −1 (solid triangles) 1 g l −1 (solid stars). Syneresis (in % of volume change) and elastic modulus (10 4 Pa) as a function of time obtained on a κ-carrageenan prepared at different concentrations in 0.1 M KCl. Nevertheless, the stiff double helices are dispersed in solution, and in absence of aggregation they form a cholesteric liquid crystalline phase. The role of I − is very important 170,172–174 this anion stabilizes the double helix but prevents gelation the mechanism of its interaction with κ-carrageenan is not clearly established. It was shown that ι-carrageenan exhibits only very low ionic selectivity and when gelation occurs it may be due to κ-type impurities. For example, in 0.1 M salt, at 0.5 Hz and 20 ☌, 1 g l −1 potassium-κ-carrageenan gives a strong gel with an elastic modulus G′=57.45 Pa, while the tetramethylammonium form even at 10 g l −1 gives no gel at 20 ☌. The mechanical properties of the gels formed in the presence of different counterions follow the same order as for the stability of the double helices: potassium-κ-carrageenan gels form at a lower polymer and ionic concentrations and have a higher modulus and a higher melting temperature than tetramethylammonium-κ-carrageenan gels, which only form at much higher ionic concentrations and have a much lower modulus as determined in dynamic rheological experiments. 176 In this process, the electrostatic repulsion is counterbalanced by cooperative H-bond attractions. The ionic selectivity observed for the double-helix induction is also recognized for the gelation the ion pairs which reduce the net charge of the single coil play a role in the double-helix stabilization but also in the aggregation of the double helix. The stiffness of the gels formed in presence of K + counterions varies in the following order:Īgarose > κ -carrageenan > ι -carrageenanįor κ-carrageenan, the mechanism was described in detail and related to the solution behavior. The related rigidity of the gels follows the same trend. 160 The width of the hysteresis in temperature is directly related to the degree of aggregation of double helices and the charge density of the polymers (width of this hysteresis decreases from agarose to κ-carrageenan to ι-carrageenan). The aggregates or junction zones are the basis of the network formation: gelation proceeds in a two-step process as it has been clearly demonstrated for thermodynamic conditions around C T* and a polymer concentration larger than the overlap concentration (see Figure 23b).
![iota carrageenan iota carrageenan](https://produits.bienmanger.com/27085-0w0h0_Iota_Carrageenan.jpg)
It must be pointed out that Tm, the temperature for helix formation, is identical to T G over C T* this implies that the helices form in the same time as the gel is formed on cooling. Then, the hysteresis is characterized by two temperatures: T F, the melting temperature of the gel, and T G, the temperature for gel formation (with T F > T G).
![iota carrageenan iota carrageenan](https://i.ebayimg.com/images/g/JV0AAOSwyZhc0f1T/s-l400.jpg)
The aggregates are more stable when temperature increases than the isolated helices. From Figure 25, it can be seen that when the ionic concentration increases over C T*, the helix–coil transition is no more reversible, and that some hysteresis in relation with aggregation of double helices occurs.
![iota carrageenan iota carrageenan](https://1.bp.blogspot.com/-lQombvrudK4/YIm4n-cryxI/AAAAAAAAFfI/S8HUQQG2lc4L7DIpfwss6pau7wVYjRwswCNcBGAsYHQ/w1200-h630-p-k-no-nu/betadine%2Bcarragelose.jpg)
For these polymers, the physical thermoreversible gel formed is stabilized by H bonds between double helices.