Concentration dependence of critical exponents for gelation in gellan gum aqueous solutions upon cooling

The sol-gel transition in aqueous gellan gum solutions induced upon cooling was investigated by rheology measurements. The gelation temperature was determined from the crossover point of storage and loss moduli, i.e., G' = G '' (T-c) and from the Winter's criterion (T-gel), respectively, which increased with gellan concentration. T-gel was higher than T-c and the difference became larger as the gellan concentration got higher. The relaxation critical exponent n was estimated with the Winter's method and the self-similarity was observed from the critical gel. The scaling for the zero-shear viscosity eta(o) before the gel point and the equilibrium modulus G(e) after the gel point was established against the relative distance epsilon from the gel point over the gellan concentration C-g of 1.0-2.5 wt%, giving the critical exponents k and z. The critical exponent n calculated from k and z agrees well with n from the Winter's criterion. However, no universal n was found for the gelation in aqueous gellan gum solutions, indicating that this gelation should be classified into the cross-linking category for the physical gelation. The critical exponent n decreased with increasing C-g for the gellan gum solution. The fractal dimension d(f) calculated from n with the screened hydrodynamic interaction and the excluded volume effect suggested a denser structure in the critical gel with higher C-g. (C) 2008 Elsevier Ltd. All rights reserved.