Beyond Salt Resistance: Ion-Specific Modulation of Microstructure and Rheology of Sepiolite Dispersions

Sepiolite, a fibrous clay mineral, is highly valued for its ability to form gel networks and control rheology in saline environments. However, the ion-specific effects governing this behavior are not fully understood. This study systematically investigates the influence of four common electrolytes (NaCl, KCl, CaCl₂, and MgCl₂) across a wide concentration range (10^-4 to 1 mol/L) on the colloidal stability, microstructure, and rheological behaviors of 5 wt % sepiolite dispersions. While all dispersions remained macroscopically stable against sedimentation, their rheological properties, including viscosity, yield stress, and thixotropy, showed a nonmonotonic trend. These properties initially decreased at low salt concentrations due to electrical double layer compression and subsequent network weakening, followed by a partial recovery at higher concentrations driven by van der Waals-induced aggregation. Cryo-scanning electron microscopy visualized the corresponding microstructural transition from a fine fiber network to an aggregated structure. The effects were ion-specific: divalent cations (Ca²⁺, Mg²⁺) caused more significant changes than monovalent cations (Na⁺, K⁺), with K⁺ proving more effective at screening charge than Na⁺. Low-field nuclear magnetic resonance showed that the mobility of water confined within interparticle pores remained constant. These findings explain the system's ability to retain water and maintain a gel-like structure despite microstructural changes. This work provides a framework that links ion-specific interactions to multiscale structural and rheological changes, offering insights into the formulation of sepiolite-based fluids.