Intrinsic viscosities and Huggins constants of guar gum in alkali metal chloride solutions

Abstract

The effect of lithium, sodium, potassium, and cesium chlorides on the properties of dilute guar gum solutions was investigated through viscosity measurements. The results showed that the intrinsic viscosity of guar gum is not significantly affected by the salts up to an electrolyte concentration of 4.1 mol/L. In the same ionic strength range, however, clear differences were observed between individual salts in their ability to influence the Huggins constant. At even higher electrolyte concentrations, only lithium and sodium chlorides markedly increased the intrinsic viscosity of the polymer, pointing towards the formation of a new type of structure in those solutions. The observed trends were attributed to the chaotropic (K +, Cs +) or kosmotropic (Li +, Na +) properties of the background salts. It was suggested that chaotropic electrolytes were capable of enhancing the dissolution of colloidal guar gum aggregates normally present in polysaccharide solutions under ambient conditions. On the other hand, the presence of high concentrations of kosmotropic ions led to enhanced aggregation of guar gum due to competitive hydration for free water molecules between the polysaccharide chains and the strongly hydrated cations. Comparative tests with the use of urea showed that the solute affects the behavior of dilute guar gum solutions in a way similar to NaCl, so its dispersing/hydrogen bond-breaking capabilities towards the polymer were not very clear in the studied system.