Studies on biocolloids and polyelectrolytes—the plant—gums viscosity of azrechtic acid

Abstract

AbstractAzrechtic acid (ARA) has been obtained by the electrodialysis of an aqueous solution of the gum of Azadirechta indica in an acidic medium. The viscosity of this gum acid has been investigated under different experimental conditions, i.e., with dilution in aqueous and in isoionic concentration of added electrolyte (HCl), and with change in pH. It is observed that the reduced viscosity of the aqueous solutions increases rapidly with dilution and the reduced viscosity—concentration curve is concave upwards instead of being linear as in the case of linear polymers. The viscosity of aqueous ARS solutions follows the equation, ηsp/C = A/(1 + B√C) where A, B, and D are constants (A = 0.03, B = 0.1845, and D = 0.05), as well as the equation ηsp/C = aCb, in which a and b are constants and equal to 0.2113 and −0.4375, respectively. It is observed that in isoionic concentrations of added electrolyte (HCl, 2.20 × 10−4 g.‐equiv./l.), the sharp rise in the ηsp/C versus C curve vanishes, and the curves show well defined maxima which shift to higher concentrations of the gum acid as the electrolyte concentration increases and ultimately vanish at sufficiently high concentration of the added electrolyte (HCl, 2.20 × 10−3 g.‐equiv./l.), and the ηsp/C versus C curves become linear. The viscosity of azrechtic acid at constant solute concentration increases rapidly when the pH is increased by the addition of NaOH, becomes maximum at a pH of about 6, after which it falls rapidly and becomes constant after a pH of 12. The viscosity also decreases when the pH is decreased on addition of HCl and becomes constant after a pH of 2.0. All these phenomena have been explained on the basis that azrechtic acid behaves as a polyelectrolyte.