In Situ Observation of Wetting and Drying of Montmorillonite in an Environmental Scanning Electron Microscope

Understanding the hydration dynamics of montmorillonite clay minerals is critical for predicting their behavior in geotechnical and environmental applications. This study employs in situ environmental scanning electron microscopy (ESEM) combined with X-ray diffraction (XRD) to directly observe and quantify the wetting and drying processes of montmorillonite SWy-1 under controlled pressure and temperature conditions. To characterize the real-time wetting and drying morphologies of montmorillonite and determine the relationship between water-induced swelling and relative humidity, ESEM enabled direct visualization of water-clay interactions by precisely controlling chamber pressure (4–5.3 Torr) and temperature (~2°C) to manipulate relative humidity and induce water condensation on mineral surfaces, while quantitative analysis of particle areas before and after hydration determined swelling percentages, XRD measured basal spacing (d₀₀₁) changes across relative humidity gradients, and water-adsorption isotherms were constructed from ESEM thickness measurements. ESEM revealed distinct wetting stages with water preferentially condensing on unsaturated edge sites and external surfaces at low pressures (<4.6 Torr), followed by rapid interlayer filling at elevated pressures with characteristic structural rounding and aggregate formation, while anisotropic swelling ocurred predominantly perpendicular to clay layers, with single water-layer hydration (1W) producing ~19% swelling and two-layer hydration (2W) yielding ~32% swelling, water-adsorption isotherms exhibited exponential swelling behavior with pronounced type H3 hysteresis, logarithmic analysis revealed steeper pressure dependency during hydration (slope = 2.7249) versus dehydration (slope = 1.6702) indicating thermodynamically driven water uptake but kinetically limited desorption, and rapid dehydration kinetics occurred within 3 minutes with complete equilibration by 15 minutes. ESEM successfully bridges microscale observations and molecular-scale understanding of smectite hydration, establishing practical timescales for clay equilibration and providing critical insights for predicting clay behavior in geotechnical engineering, soil stabilization, contaminant transport, and engineered barrier design.