Cooperative chemical-mechanical interactions during ion exchange promote rotational ordering in hydrated montmorillonite

Abstract

Ion exchange in clays plays a major role in water, nutrient, and contaminant storage and transport in clay-rich media including soils, sediments, and suspensions. Here, we show that ion exchange between sodium and potassium in hydrated montmorillonite is a cooperative process that couples ionic transport to interlayer forces that alter mesoscale particle structures. Fluctuations in cation concentration, hydration, and basal spacing within an interlayer initiate swelling or collapse that propagates once a local concentration threshold is exceeded. Coupling between adjacent interlayers then leads to collapse or swelling throughout an entire particle. Sodium- and potassium-rich particles are found to be distinct phases with a free energy difference comparable to the thermal energy, which dynamically equilibrate through the exchange of both ions and individual montmorillonite layers. This process promotes stacking order from initially turbostratic particles via (nearly-)oriented attachment. A far more dynamic picture of clay colloids than previously expected highlights the role of nanoscale chemical-mechanical feedback in natural ion exchange phenomena.