posted on 2021-09-13, 19:06authored byKolattukudy
P. Santo, Kristina I. Fabijanic, Chi-Yuan Cheng, Andrei Potanin, Alexander V. Neimark
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.