Gel point studies for chemically modified biopolymer networks using small amplitude oscillatory rheometry

Solutions of xanthan gum form a strong gel upon the addition of aluminum ions (Al(III)). Previous studies have reported that the addition of chromium and ferric ions induces gelation in xanthan solutions; this study presents a characterisation of the transition from solution to gel for xanthan in the presence of AI(III). Xanthan-Al(III) solutions are shown theologically to undergo a sol-gel transition upon heating, the extent of gelation is controlled by the temperature at which the system equilibrates. The rapid thermal equilibration of xanthan-Al(III) provides an ideal system to test the applicability of two established gelation criteria [1,2]. At the gel point, congruency in G' and G is observed over 2-3 decades of frequency demonstrating the applicability of the method of Winter and Chambon [1]. The difficulties associated with determining an equilibrium relaxation modulus (E) as a criterion for gelation [2] are discussed. The gelation behaviour of xanthan-Al(III) gels is characterised for a range of polymer crosslinker combinations (stoichiometric ratios) and a limiting stoichiometric ratio below which gelation does not occur is determined. The relaxation exponent (n) is determined at the limiting stoichiometric ratio to be n = 0.22. A value of 0.22 for n is not in agreement with previous work for systems at a limiting stoichiometric ratio, The relaxation exponent is found to vary between 0.22 and 0.16 for a 0.5 wt% xanthan and a range of Al(III) concentrations. The temperature at which the gel point is observed, T-g, follows an asymptotic relationship with stoichiometric ratio suggesting the importance of the order-disorder transition of xanthan molecules on the gelation process of xanthan-Al(III) gels. (C) 2000 Elsevier Science Ltd. All rights reserved.