Viscosity of locust bean, guar and xanthan gum solutions in the Newtonian domain: A critical examination of the log ( eta sub(sp)) sub(o)-log C[ eta ] sub(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 [ eta ] sub(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 ( eta sub(sp)) sub(o) to the reduced concentration C[ eta ] sub(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 alpha may be estimated. For xanthan, as expected for a polyelectrolyte, [ eta ] sub(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 ( eta sub(sp)) sub(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 ( eta sub(sp)) sub(o)-C[ eta ] sub(o) master curve for C>C** when [ eta ] is replaced by its unperturbed counterpart [ eta ] sub( theta ), a method for estimating [ eta ] sub( theta ) for the xanthan sample is proposed. In conclusion, the numerous exceptions to the widely accepted ( eta sub(sp)) sub(o) vs C[ eta ] sub(o) `universal' behaviour are mainly ascribed to significant differences in expansion coefficient values which depend on both the polymer and the solvent.