Order parameters of gels and gelation kinetics of aqueous agarose systems: relation to the spinodal decomposition of the sol

Hydrogels of a significant biostructural polymer (agarose) were studied turbidometrically in the course of their isothermal self-assembly. Supramolecular order was characterized by an order parameter and associated correlation length. This type of characterization, in general, may help more in the quantitative measurement of order in biostructures. The picture of processes leading to isothermal gelation appears much richer than expected, and it is probably of more general validity, as it agrees with predictions based on the statistical model of site-bond correlated percolation. At the higher temperatures a spinodal decomposition occurs well before, and possibly triggers [P. L. San Biagio, et al. (1986) Biopolymers25, 2255–2269], the start of gelation. In this case, the spatial modulation of concentration, as well as topological features of the gel, appear to reflect those of the spinodally decomposed sol. When an upper limit is approached for the temperature of isothermal gelation, both the order parameter and the correlation length exhibit a critical-like behavior. At lower temperatures, the sequence of processes and the resulting features of the gel appear dictated by the (temperature- and concentration-dependent) interplay of spinodal decomposition and gelation processes and kinetics. The present results are believed to provide a basis for a deeper understanding of the relation between the processes of gelation and of spinodal decomposition, and between their time scales.