Physical entanglement governs the salt resistance of palygorskite dispersions: Decoupling surface charge from macroscopic stability

Gel stability is decoupled from surface charge modulation by electrolytes. • Physical entanglement of fibers dominates over electrostatic interactions. • Salt induces a transition from fine networks to massive fiber bundles. • Bundling triggers a shift from anti-thixotropic to thixotropic behavior. • High surface area keeps water dynamics constant despite network coarsening. Palygorskite exhibits unusual colloidal stability and favorable rheological properties in saline environments, yet the mechanisms governing its response to electrolytes over a wide concentration range remain unclear. We investigate the effects of mono- (Na + , K + ) and divalent (Ca 2+ , Mg 2+ ) cations across 10 −4 to 1 mol/L. The results show that the macroscopic gel stability of 5 wt% palygorskite dispersions remain unchanged across all salt conditions, despite pronounced electrolyte-dependent changes in surface charge, including charge reversal in divalent systems. Cryo-scanning electron microscopic results reveal a progressive transition from fine fiber networks to coarse bundle-based structures, with bundling occurring at lower concentrations for divalent salts. These structural changes influence rheological behavior, leading to enhanced viscosity at low-to-moderate salt concentrations and a shift from weak anti-thixotropic behavior to weak thixotropic behavior at high salinity. In contrast, low-field nuclear magnetic resonance shows that the water confinement remains under pervasive surface control despite large microstructural rearrangements. These findings support a decoupling between nanoscale surface-charge modulation and macroscopic gel stability, and indicate that physical entanglement and topological connectivity, rather than electrostatic interactions alone, govern the salt resistance of concentrated palygorskite dispersions. This work provides a mechanistic framework for the design of salt-resistant fibrous clay colloids for applications in complex saline environments. Download: Download high-res image (226KB) Download: Download full-size image