Sol–Gel Transition in Aqueous Alginate Solutions Induced by Cupric Cations Observed with Viscoelasticity

Abstract

Sol–Gel transition was occurred in 2 wt% aqueous solutions of four alginate samples with different molecular weight Mw and the ratio M/G of repeat unit mannuronate (M) to guluronate (G) induced by adding cupric ions. The transition was monitored with dynamic moduli G’ and G” in the linear region of viscoelasticity and the gel point was determined according to the Winter’s criterion where the loss angle tan δ became independent of frequency ω. The mole ratio of Cu2+ to the carboxyl group in alginate at gel point, fgel, was higher for the samples with lower molecular weight than those with higher molecular weight without obvious M/G dependence. This suggested there was no M/G selection of Cu2+ complex formation with alginate. The relaxation critical exponent n for alginate samples with higher G content was lower than 0.5, meaning G’ > G” for the critical gels with a denser network. The cupric ion number relevant to one alginate chain at the gel point, Ngel, was estimated from fgel Mn/M0 and reflected the gel elasticity.