Viscosity of locust bean, guar and xanthan gum solutions in the Newtonian domain:: a critical examination of the log(ηsp)O-logC[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 [eta](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(sp))(o) to the reduced concentration C[eta](o), is obtained and allows tin good agreement with theoretical conjectures), two critical concentrations C* and C** to be defined, from which the value of the expansion coefficient or may be estimated. For xanthan, as expected for a polyelectrolyte, [eta](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(sp))(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(sp))(o)-C[eta](o) master curve for C>C** when [eta](o) is replaced by its unperturbed counterpart [eta](theta), a method for estimating [eta](theta) for the xanthan sample is proposed. In conclusion, the numerous exceptions to the widely accepted (eta(sp))(o) vs C[eta](o) "universal" behaviour are mainly ascribed to significant differences in expansion coefficient values which depend on both the polymer and the solvent. (C) 1998 Elsevier Science Ltd. All rights reserved.