Understanding the Structure and Rheology of Galactomannan Solutions with Coarse-Grained Modeling

Understanding and predicting the behavior of aqueous solutions of galactomannans is of significant interest for a variety of fields and applications, such as hydraulic fracturing in the petroleum industry and thickening in the food industry. Herein, we devise and validate a coarse-grained (CG) model of guar gum─a galactomannan─and study its structure and rheology in aqueous solution. The CG model is computationally efficient, utilizing an implicit solvent and one CG bead per monosaccharide, and exhibits good quantitative agreement with experiments with respect to the radius of gyration of the chain, aggregate structure in solution, and solution viscosity. Using both simulations and experiments, we show that the behavior of guar gum in water can be explained by scaling theory for polymers in good solvents. Namely, the radius of gyration of the chain shows a self-avoiding random walk behavior, and the solution specific viscosity η_sp exhibits the expected power-law scaling with the reduced concentration in different solution regimes (i.e., η_sp ∼ (c[η])^1.3 in semidilute and η_sp ∼ (c[η])^3.9 in entangled solutions). By extending our CG model to other galactomannans with different mannose-to-galactose ratios (M/G ratios) and different arrangements of unsubstituted mannose units, we show that high M/G ratios and long blocks of unsubstituted mannose units are responsible for aggregate formation in galactomannan solutions, which can reduce solution viscosity moderately.