Modeling of the Effects of Metal Complexation on the Morphology and Rheology of Xanthan Gum Polysaccharide Solutions

Polysaccharide solutions commonly contain metal ions, which form cross-linking complexes with polymer ligands and significantly affect the polymer morphology and solution rheology. Here, we study by multiscale computational modeling complemented with atomic force microscopy (AFM) measurements the molecular mechanisms of metal complexation with examples of xanthan gum (XG) solutions containing ZnCl2. We develop original atomistic molecular dynamics (MD) and coarse-grained dissipative particle dynamics (DPD) models for XG chains of different compositions and Zn complexation. MD simulations reveal that Zn dramatically affects conformations of individual XG chains, causing a reduction in the radius of gyration by the formation of various types of intrachain cross-linking that are enhanced by the increased pyruvate content in the chains. DPD simulations at the static conditions and under the shear flow show that salt addition leads to gelation in XG systems via a sol–gel transition due to enhanced interchain cross-linking. Two morphological transitions occur in the shear flow: an orientational transition to a nematic phase with XG chains aligned in the direction of flow and a bundling transition accompanied by a reverse gel–sol transition at high Zn concentrations. The presence of the increased pyruvate content in the chains leads to stronger aggregation and stiffer aggregates, in qualitative agreement with AFM results. The proposed computational methodology of modeling the effects of metal complexation and chain composition on morphological and rheological properties may be extended and applied to various polysaccharide systems.