Optically Triggered Nucleation Eliminates Hydration Hysteresis in Swelling Minerals

Hydration hysteresis is a ubiquitous feature of water exchange between porous solids and humid vapors. Whether one or both of the hydration hysteresis loop branches is metastable remains debated due to the many possible hydration mechanisms that can cause hysteresis. Here, we show that both endothermic and exothermic phase transitions among hydration states in smectites, model compliant porous media, can be optically activated during both hydration and dehydration, indicating that both branches are metastable. A net free energy difference of +15 kJ/mol of water = 6R at 298 K between adsorption and desorption branches is observed over a full hydration-dehydration loop in isothermal, isobaric hydration calorimetry. Hysteresis vanishes, and the reversible sorption energy is 5R per water molecule under continuous-wave laser illumination. In situ Raman spectra confirm that hydrogen bonding between water and structural hydroxyl in the metal oxide framework screens the interlayer charge, weakening interactions and releasing heat. The latent heat of hydration is stored in this immobilized water, which provides the mechanical dilation necessary to nucleate distinct hydration states when water vibrational modes are optically activated. Our findings show that water sorption hysteresis is caused by delicate interfacial phase transitions, which visible light at ambient levels is sufficient to promote. These findings may have important consequences for the fate of sorbents, such as carbon, mineral nutrients, and environmental contaminants associated with clays in soils since the retention and mobility of these species depends strongly on the hydration state of the mineral surface.