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

Abstract

ABSTRACT:Sol-Gel transition was occurred in 2 wt% aqueous solutions of four alginate samples with different molecular weight M w 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 Cu 2+ to the carboxyl group in alginate at gel point, f gel , 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 Cu 2+ 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, N gel , was estimated from f gel M n /M 0 and reflected the gel elasticity.KEY WORDS Alginate / Cupric Cations / Sol-Gel Transition / Critical Exponent / Dynamic Viscoelasticity / Alginate is a natural polysaccharide consisting of β-D-mannuronate (M) and its C-5 epimer α-L-guluronate (G) residues (Figure 1) (1→4) linked in a nonregular blockwise pattern along the linear chain and is mainly used as a gelling agent in numerous food and pharmaceutical applications. [1][2][3][4] There are three types of dyad sequential blocks as MM, GG, and MG. These residue sequences endow with the alginate chain different stiffness, e.g., that the mean square end-to-end distance per uronate residue for G component is 2.2 times larger than that for the M component. 5 The chemical composition and sequence of the M and G residues depend on the biological source and growth and seasonal conditions. 1,2 The most attractive ability of alginate is the gel formation induced by adding various divalent cations, except Mg 2+ . 6 The gelation behavior and gel strength of aqueous alginate solutions strongly depend on the content of guluronate residues and also on the molecular weight and molecular weight distribution of alginates. [6][7][8] Matsumoto and Mashiko investigated the influence of added salts on the viscoelastic properties of aqueous alginate solution and argued that the interaction between alginate and metal cations did not work as the cross-linking point. 8 Matsumoto et al. also followed the gelation process in sodium alginate solutions induced by increasing polymer concentration without any divalent cations and found the promoting effect to the gelation of chain stiffness, i.e., the alginate with higher G residue content formed gel at lower concentration with more perfect structure. 9, 10 By viscometry, † To whom correspondence should be addressed.

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“…5 for the copper experiment and Fig. 6 for the calcium experiment clearly show that the transition from Region 2 to Region 3 is a type of ‘critical’ transition with rapid changes in MR measured parameters as the gel spans the sample 34–36, 44…”

“…Mikkelsen and Elgsaeter21 describe this concentration regime as the ‘dilute regime’ where the gel species are below some critical concentration level 34. Region 2 is considered a fluid region though the presence of an increasing level of gel in the region implies that the viscosity of the region would be changing quite rapidly but sample spanning clusters which arrest fluid‐like motions are not yet present 34–36…”

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“…Sodium alginate with abundant carboxylate and hydroxyl groups [ 71 ], reacts with divalent cations such as Cu(II) [ 72 ], and others [ 73 , 74 , 75 ], to form cross-linked hydrogels, existing in reticular structures called “egg box” structures [ 76 , 77 ]. In transition metal-alginate systems, the sol-gel transition is characterized by a complex formation in which only the carboxyl groups in both M and G residues are coordinated to the metal ions [ 78 ].…”

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“…Sodium alginate gels ionotropically at constant temperature upon addition of divalent cations, such as Ca 2+ , Sr 2+ , and Cu 2+ [6][7][8][9]. Ionotropic gelation follows the egg-box modelinterchain associations of G-block sequences of 20 or more residues with arrays of site bound ions sandwiched between the participating chains [10,11].…”