Rheology characterization of sol–gel transition in aqueous alginate solutions induced by calcium cations through in situ release

Four alginate gels were prepared by in situ release of calcium cations from Ca-EDTA with gradual hydrolysis of d-glucono-δ-lactone (GDL) in 3 wt% of aqueous solutions of four alginate samples with different M/G compositions, molecular weights, and molecular weight distributions. The sol–gel transition was monitored by measuring the dynamic mechanical spectroscopy in the linear region with variation in the mole ratio f of Ca2+ to the carboxyl group in the alginate. fgel, the critical value of f at the gel point, was determined according to the appearance of power law relaxation for the storage and loss moduli as proposed by Winter and Chambon. The critical exponent of relaxation moduli n for the power law was estimated from the loss tangent tan δ=G″/G′ at the gel point which is independent of angular frequency. fgel for the two samples with higher M/G ratio is about 0.037 while that for the other two samples with lower M/G ratio is 0.049 and 0.060, suggesting that more Ca2+ cations are chelated into the ‘egg-box’ cavities formed by the G sequence to cross-link alginate chains with higher G content. The n value is higher than 0.5, showing that the Ca2+ induced alginate network is not perfect due to its low concentration.