Smectite phase separation is driven by hydration-mediated interfacial charge.

Area of Science:

• Geochemistry
• Colloid Science
• Materials Science

Background:

• Smectite clays exhibit significant swelling and exfoliation in response to hydration and ion exchange.
• Existing models fail to encompass the full spectrum of water, salt, and clay content in natural and engineered systems.
• Two swelling regimes, osmotic and crystalline, are traditionally identified based on water activity.

Purpose of the Study:

• To develop a unified model for smectite clay swelling across all water activities and electrolyte concentrations.
• To demonstrate that distinct colloidal phases, not separate swelling regimes, characterize clay behavior.
• To elucidate the role of ion hydration and interfacial charge in driving clay layer dynamics.

Main Methods:

• Development of an analytical model for intermolecular potentials involving water, salt, and clay.
• Prediction of swelling pressures across a range of water activities and electrolyte types (monovalent and divalent).
• Analysis of clay, ion, and water mobilities in relation to interfacial charge and hydration.

Main Results:

• Identified a rich array of distinct colloidal phases with varying water content, layer thickness, and curvature.
• Showed that all clay swelling can be described as osmotic, with interfacial osmotic pressure dominating at high clay concentrations.
• Observed hyperdiffusive layer dynamics driven by variable hydration-mediated interfacial charge, leading to long-lived intermediate states.

Conclusions:

• Smectite clay swelling is a unified osmotic process, with distinct colloidal phases emerging from interfacial phenomena.
• Ion (de)hydration at mineral interfaces is the primary driver of hyperdiffusive layer dynamics.
• The new model accurately predicts swelling pressures across diverse conditions, offering a more comprehensive understanding of clay behavior.