Viscosity of locust bean, guar and xanthan gum solutions in the Newtonian domain: a critical examination of the log (ηsp)o-log c[η]o master curves

The viscosity in the low shear rate Newtonian domain of three biopolymers, locust bean gum, guar gum and xanthan gum was studied as a function of temperature and of polymer concentration in various aqueous solvents. The intrinsic viscosities [η]o of both galactomannans are not modified in the presence of 10 or 40% sucrose. In this case, a master curve relating the Newtonian specific viscosity (ηsp)o, to the reduced concentration c[η]o is obtained and allows (in good agreement with theoretical conjectures), two critical concentrations C∗ and C∗∗ to be defined, from which the value of the expansion coefficient α may be estimated. For xanthan, as expected for a polyelectrolyte, [η]o depends strongly on salt concentration and on added sucrose and the results did not obey the above-mentioned master curve. However, it is shown that (ηsp)o depends only on xanthan concentration when C > C∗∗, and then it is assumed that chain dimensions have attained their unperturbed values whatever the solvent. Considering that both types of chains, random coils (galactomannans) and semi-rigid (xanthan) should give the same (ηsp)o-C[η]o master curve for C > C∗∗ when [η]o is replaced by its unperturbed counterpart [η]θ, a method for estimating [η]θ for the xanthan sample is proposed. In conclusion, the numerous exceptions to the widely accepted (ηsp)o vs C[η]o “universal” behaviour are mainly ascribed to significant differences in expansion coefficient values which depend on both the polymer and the solvent.