Critical exponents for sol–gel transition in aqueous alginate solutions induced by cupric cations

The sol–gel transition in aqueous alginate solutions of four alginate samples having different molecular weights (MW) and M/G ratios induced by cupric cations was monitored by rheology measurements. The gel point fgel and the relaxation critical exponent n were determined using the Winter’s criterion over the alginate concentration CAlg of 1–4 wt%. The scaling for the zero shear viscosity η0 before the gel point and the equilibrium modulus Ge after the gel point was established against the relative distance ε from the gel point at the concentration of CAlg = 1 wt%, giving the critical exponents k and z. The results indicated that fgel was almost independent of the alginate concentration and became higher for the sample with lower molecular weight. The critical exponent n decreased with the increase in CAlg for these four Cu-alginate samples and the fractal dimension df estimated from n suggested a denser structure in the critical gel with high G content. The critical exponent n evaluated from k and z agreed well with n determined from the Winter’s criterion.