Intrinsic viscosity and rheological properties of natural and substituted guar gums in seawater

• CMG showed lower [η] in seawater than in deionized water due to screening effect.
• GG and HPG showed higher [η] in seawater than in deionized water due to the kosmotropic nature of Na+, Mg2+ and Ca2+ in seawater.
• The rheological data of three samples in seawater was well characterized by both modified Cross model and Power–law model.
• The viscosifying efficiency of three samples gave an order of CMG > HPG > GG.
• CMG solution in seawater had the lowest Ea that showing lowest sensitivity to temperature and exhibited strong pH-dependent viscosity.

The intrinsic viscosity and rheological properties of guar gum (GG), hydroxypropyl guar (HPG) and carboxymethyl guar (CMG) in seawater and the effects of shear rate, concentration, temperature and pH on these properties were investigated. An intrinsic viscosity-increasing effect was observed with GG and HPG in seawater (SW) compared to deionized water (DW), whereas the intrinsic viscosity of CMG in seawater was much lower than that in DW due to a screening effect that reduced the repulsion between the polymer chains. Regardless of the functional groups, all sample solutions was well characterized by a modified Cross model that exhibited the transition from Newtonian to pseudoplastic in the low shear rate range at the concentrations of interest to industries, and their viscosity increased with the increase in their concentration but decreased with the increase in temperature. In contrast to nonionic GG or HPG, anionic CMG had a slightly decreased viscosity property in SW, exhibiting polyelectrolyte viscosity behavior. The α value in the zero-shear rate viscosity vs. concentration power–law equation for the samples gave the order of CMG > HPG > GG while the SW solution of CMG had the lowest viscous flow activation energy and exhibited a strong pH-dependent viscosity by a different shear rate.