Rheology of aqueous solutions of brea gum: Bimodal flow curves and (apparent) negative activation energies

Abstract

Rheological behavior of brea gum in aqueous solutions results from two particularities of this polysaccharide: (i) its ability to assume two different structures in solution: coils or self-assembled structures (probably helices) and (ii) the very dissimilar contribution of these conformations to the viscosity. The fraction of coils existing at given values of polymer concentration (c) and pressure increases with temperature and shear rate, where the transformation is taking place within a comparatively small temperature interval (approximately 20 °C) shifting towards higher temperatures as c rises. The observation that coils dissipate considerably more energy than helices is interpreted in terms of microphase equilibria leading to volume elements with reduced flow-through of the solvent. The bimodal flow curves of the present system reflect the presence of two different processes, the disentanglement of coils in the range of lower shear rates and the destruction of helical structures at higher ones.