Rheology of decolorized hsian-tsao leaf gum in the dilute domain

The polysaccharide extract of hsian-tsao leaves was treated with active carbon. Decolorized hsian-tsao leaf gum (dHG) solutions were then evaluated for their rheological properties as a function of concentration, ionic strength and ion types by using a glass capillary viscometer and dynamic rheometer. It was found that the weight average molecular weight of dHG in 0.1 M sodium chloride solution was about 550,000, with a Mark-Houwink constant and exponent of 9.32 x 10(-6) ml/g and 0.29, respectively, indicating a less extended conformation. The specific viscosity of dHG exhibited a power-law dependence on concentration. The exponent b ranged from 0.78 to 0.80 in the dilute domain and from 1.67 to 1.71 in-the semi-dilute domain. The coil overlap parameter of the dHG was found to be in the range of 13.32-13.42. The hydrodynamic behavior of dHG was strongly affected by ion types and ionic strength. The chain stiffness parameter of dHG was lower than that of several polysaccharides, indicating the stiff backbone of dHG. Dynamic rheological results indicated that the storage and loss modulus of dHG decreased with increasing sodium, potassium or magnesium ion concentrations, but increased with increasing calcium ion concentrations. The dHG solutions can be classified rheologically as those of dilute or concentrated solutions or weak gels, depending on the types and concentrations of ions presented. (C) 2002 Elsevier Science Ltd. All rights reserved.