Thermoreversible gelation of k-Carrageenan: relation between conformational transition and aggregation.

We have studied, by optical rotation dispersion, light scattering and rheology, the kappa-Carrageenan system to elucidate the processes involved in gel formation (on decreasing the temperature) and gel melting (on increasing the temperature). Our results show that, on decreasing the temperature, a conformational transition from coils to double helices first occurs, followed by aggregation of the double helices into domains and gel formation at appropriate polymer concentration. Structural details of this sequence are better revealed by re-heating the system. Melting appears as a two-step process characterized by first a conformational change of helices involved in junction zones between aggregates, followed by the conformational transition of the helices inside the aggregates. These helices can regain the coil conformation only when the aggregates melt at higher temperature, in full agreement with the old 'domain' model. The full description of the sol-gel mechanism of this system can be useful in the search for new methods to control the gel texture, a relevant property for many industrial applications.