Bridging the Gap Between Osmotic and Crystalline Swelling in Smectites Clays Minerals

The swelling behavior of smectite clay minerals has been extensively investigated due to their critical role in construction stability. Two distinct swelling regimes have long been recognized: crystalline swelling, involving the incorporation of a few water sheets between clay layers, and osmotic swelling, characterized by the complete delamination of individual clay layers. Crystalline swelling mechanisms have been well established through hydration techniques applied to dry clay in a humid environment, while osmotic swelling has been observed through the gradual concentration of a diluted clay suspension into a more compact, glassy state. Although a linear transition between these two regimes has been proposed, it has yet to be experimentally demonstrated. In this study, we investigate the complete dehydration process of a levitated smectite suspension droplet using in-situ Small Angle X-ray Scattering (SAXS). The results reveal a gradual shift from osmotic swelling to crystalline swelling, marked by a transition from a pure nematic glass to a coexistence zone where the nematic phase contracts and a saturated crystalline phase emerges, ultimately progressing to an unsaturated crystalline state. This transition occurs through a continuous process in which the interlayer space decreases from four water layers to zero water layers, forming stratified interlayers along the way.