The structure and dynamics of locust bean gum in aqueous solution

Abstract

Locust bean gum (LBG) is an industrially important polysaccharide used widely in food, cosmetics, textiles, and biopharmaceutical applications, where understanding its solution behaviour is important for formulation design. To address this, we investigate the structure and dynamics of LBG in aqueous solution using steady state shear rheology, static light scattering (SLS), ultra-small-angle light scattering (USALS), and dynamic light scattering (DLS). We find that steady state shear rheology mainly probes the well-dispersed LBG fraction, whereas the scattering response is dominated by supramolecular LBG aggregates. We identify three viscosity-concentration regimes (dilute, semidilute unentangled, and semidilute entangled), with scaling behaviour largely consistent with predictions for neutral flexible polymers. SLS and USALS provide evidence for the existence of two separate populations of aggregated structures, where SLS mainly probes the internal structure of the smaller aggregate population. The solution dynamics are dominated by a ‘slow’ decay mode, which is consistent with Zimm dynamics and attributed to relaxations within the aggregates. In dilute solutions, the apparent viscosity associated with this mode is greater than the solution viscosity, but the two converge as concentration is increased. An additional ‘fast’ mode is observed within the semidilute range, associated with the dispersed polymer fraction, and attributed to cooperative diffusion. Our study provides comprehensive insight into the structure and dynamics of aqueous LBG solutions—paving the way for a greater fundamental understanding of galactomannans in solution, and for increased control over LBG-based formulations of industrial interest.