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Smectite phase separation is driven by emergent interfacial dynamics

Smectite clay minerals have an outsize impact on the response of clay-rich media to common stimuli, such as water imbibition and ion exchange, motivating extensive effort to understand microscopic behaviors resulting from these processes such as swelling and exfoliation. Nonetheless, there is no general consensus about the microscopic forces that govern smectite properties, which are model systems for understanding colloidal and interfacial phenomena more generally. We find that the complex free energy surface arising from the interplay of at least four intermolecular forces and their nonlinear couplings that control local particle-particle interactions leads to dynamic, unstable equilibria between distinct phases. Mechanical disequilibrium arising from osmotic gradients between curved or interacting interfaces drive the dynamic exchange of layers and ions between dense and dilute phases via avalanche transitions that are sustained by thermal fluctuations. We suggest that the surprising interfacial dynamics displayed by smectite minerals, arising from the vastly different mobilities of water, ions and mineral, makes them fundamentally distinct from non-clay minerals because their structures are easily perturbed away from simultaneous chemical and mechanical equilibrium.