Inhibited hydration of xanthan gum under synergistic high-pH/Ca2+ conditions: Rheological behavior and molecular dynamics simulations

Investigating the hydration behavior of xanthan gum (XG) under strongly alkaline and salt-containing conditions is essential for its reliable application in water-based drilling fluids and enhanced oil recovery (EOR) systems. In particular, the molecular mechanisms governing its hydration and rheological behavior under the coexistence of high-pH and Ca²⁺ conditions remain poorly understood. In this work, XG systems with varying pH and Ca²⁺ concentrations were examined through rheological measurements, structural characterizations, and molecular dynamics (MD) simulations. The results revealed that varying either pH or Ca²⁺ concentration alone exerted only limited effects on hydration and viscosity, whereas their coexistence above critical thresholds (pH > 12.0 and [Ca²⁺] ≥ 14 mM) led to a sharp viscosity loss and the formation of insoluble deposits. Remarkably, hydration and viscosity could be restored by chelating Ca²⁺ or lowering pH, indicating a reversible coordination-driven inhibition process rather than irreversible degradation of the XG backbone. Structural and MD simulation analyses jointly demonstrated that Ca²⁺ interacts with deprotonated carboxy groups on XG chains via electrostatic bridging and coordination, thereby restricting molecular extension and water accessibility. These findings provide molecular-level insights into the behavior of XG in complex ionic environments and hold important implications for its engineering applications.