Montmorillonite (MMT) nanosheets with large sizes is significant for controllable preparation of MMT functional nanocomposite materials. Compared with MMT, two-dimensional MMT nanosheets have a larger specific surface area, higher aspect ratio and richer hydroxyl groups at the edges. Therefore, MMT nanosheets are widely applied in the fields like environmental remediation, energy storage, flame retardant and membrane materials. The methods for preparing two-dimensional MMT nanosheets include mechanical milling, shearing, intercalation and ultrasonic exfoliation. However, the size of MMT nanosheets can be seriously damaged because of the higher external energy input of these exfoliation methods, thus affecting the applications. It is thus necessary to propose a new method for preparing MMT nanosheets with large sizes. In this paper, a method of MMT exfoliation was investigated based on its nature of osmotic hydration through weakening the interlayer binding energy of MMT. The exfoliation mechanism of MMT based on osmotic hydration was revealed via the conductivity analysis and molecular simulation. In addition, the regulation mechanism of exfoliation was also explored through the regulation of the water consumption and the concentration of MMT suspension during the osmotic hydration.

Sodium-exchanged MMT suspensions were stirred and dispersed. One end of a dialysis bag (a molecular weight cutoff of 3 000 Da) was sealed using a clamp. Subsequently, 100 mL of the suspensions was transferred into the dialysis bag, followed by sealing the other end with another clamp. Afterwards, the dialysis bag was placed in a beaker containing 500 mL of deionized water. The suspensions in the beaker was stirred in a magnetic stirrer at 200 r/min. Within the initial 12 h, the water in the beaker was refreshed every 3 h by pouring it out and replacing it with fresh deionized water. At 24 h and 36 h, the deionized water was replaced once again. At different time intervals, 2 mL samples were extracted from the dialysis bag using a pipette for subsequent test and analysis. These samples were labeled as MMT-t (where "t" represents the osmotic hydration time). The thickness of different two-dimensional MMT nanosheets was characterized by a model Multi Mode 8 atomic force microscope and a model JEM-F200 transmission electron microscope. Besides, the morphology of the two-dimensional MMT was determined by a model JSM 7100F scanning electron microscope.

MMT can be exfoliated into two-dimensional nanosheets during the osmotic hydration. The exfoliation degree of montmorillonite gradually increases with extending the osmotic hydration time to 48 h, which can achieve the thickness of MMT nanosheets similar to that of ultrasonic exfoliation. From the particle size distribution of MMT with dialysis time, the size distribution of MMT-48 is 48% in 0-300 nm and 30% in 300-600 nm, respectively. However, the size of MMT nanosheets using the same raw MMT material prepared by the ultrasonic exfoliation was distributed in the range of 100-300 nm. Therefore, the sizes of two-dimensional MMT nanosheets prepared by the osmotic hydration have more than twice greater than those by the ultrasonic exfoliation, demonstrating the effective preparation of two-dimensional MMT nanosheets with large sizes.

Furthermore, the exfoliation behavior of MMT through the osmotic hydration was investigated. The XRD patterns show that the a plane (001) of MMT appears a broad dispersion when the osmotic hydration time extends to 24 h, indicating that the laminar structure of MMT is destroyed and the exfoliation degree of MMT is increased. As the osmotic hydration time is 36 h or 48 h, the diffraction peaks corresponding to the crystal plane (001) of MMT disappear, exhibiting the emergence of an amorphous diffraction peak. This demonstrated that MMT is effectively exfoliated into nanosheets with a single layer or a few layers. The XRD patterns confirm that the exfoliation of MMT based on the osmotic hydration is achieved via promoting the hydration expansion. In addition, the exfoliation process is relatively slow and gentle, greatly protecting the size of MMT. There is osmotic pressure between MMT and the deionized water because of the concentration gradient between the interlayer cations of MMT and the deionized water in the surrounding environment. In osmotic pressure, more water molecules can penetrate into the interlayer of MMT, thereby promoting a further hydration exfoliation of MMT. The size of montmorillonite cannot be seriously damaged due to the increased hydration expansion force and low external input energy, resulting in the easier preparation of MMT nanosheets with large-sizes.

The thickness of MMT gradually decreased, but the size of MMT was barely damaged during the osmotic hydration, resulting in the effective preparation of two-dimensional MMT nanosheets with large sizes. The conductivity analysis and molecular simulation indicated that the exfoliation of MMT was controlled by the osmotic pressure between the interlayer cations and the external water. The exfoliation degree of MMT could be controllable with the water consumption and suspension concentration. The two-dimensional MMT nanosheets prepared by osmotic hydration had a thickness similar to that by ultrasonic exfoliation, and the size of MMT nanosheets prepared by osmotic hydration was twice greater than that by ultrasonic exfoliation.