Influence of molecular structure imaged with atomic force microscopy on the rheological behavior of carrageenan aqueous systems in the presence or absence of cations

Influence of microscopic molecular structure imaged using atomic force microscopy (AFM) on macroscopic rheological behavior of carrageenan was investigated in an aqueous system. Two types of carrageenan (iota and kappa) with equivalent average molecular weights, purified by alcohol precipitation and subsequently transformed to the sodium type, were subjected to the investigations at concentrations to form molecular assemblies; gel precursors or microgels (local networks and aggregates) in the presence or absence of cations. AFM observations elucidated the difference in the effect of cations on the (super-) molecular structure of carrageenan. In the case of iota, the addition of K+ increased the stiffness of the strands (composed of helices) and aligned them to a definite direction with some degree of branching, whereas that of Ca2+ mediated the associations between the strands into network-like structures (both of which might be an artifact during drying in preparing the AFM specimens). Each structural change involved, however, no or very low level of side-by-side aggregation between helices, which was in good agreement with rheological data, showing no hysteresis between sol-to-gel and gel-to-sol transitions. In the case of kappa, on the contrary, the addition of K+ or Ca2+ formed localized network through side-by-side aggregation between helices (more apparent with an addition of K+), which was consistent with the thermal hysteresis between sol-to-gel and gel-to-sol transitions, but this interhelical aggregation was not necessarily a prerequisite for gelation. The structure-function relationship and cation selectivity for each carrageenan were discussed.