Bridging the Gap between Osmotic and Crystalline Swelling in 2D Layered Materials Using Levitated Droplets.

The swelling behavior of smectite clay minerals is known to have two distinct regimes. The crystalline swelling involves the incorporation of a few water sheets between clay layers while the osmotic swelling corresponds to the complete delamination of clay layers in a solvent. Although a linear transition between osmotic and crystalline swellings is proposed, a complete description of smectite swelling has never been achieved. Here, acoustic levitation coupled with Small Angle X-ray Scattering (LevSAXS) is proposed to follow the evolution of the interlayer space of a smectite in a single levitated droplet. The advantage is to track fast non-equilibrium phenomena over a wide concentration range, while avoiding anchoring effects during drying. The results reveal a gradual shift from osmotic 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. This transition occurs through a continuous process, forming interstratified structures ultimately progressing to an unsaturated crystalline state. Applied to the emblematic case of clay swelling, LevSAXS opens new perspectives to investigate or reconsider the swelling mechanisms of other low-dimensional 2D materials.