Capillary Forces in Nanochannels of Two-Dimensional Covalent Organic Frameworks during Water Sorption

Two-dimensional covalent organic frameworks (2D COFs) have been widely applied in liquid environments, where capillary forces inevitably arise when the well-defined nanochannels of 2D COFs contact liquids. These capillary forces may induce changes in pore size, surface area, and crystalline structure, thus altering the functions of 2D COFs. Therefore, it is crucial to understand capillary forces and their effects within 2D COF nanochannels. However, studies on this topic are still limited. In this study, we investigate capillary forces using water sorption as an example in three 2D COFs, which share identical backbones and pore sizes yet feature different side groups. Through a combination of experimental characterizations and computer simulations, we find that the capillary forces in these COFs range from 32 to 86 MPa. Such high pressures can distort the crystalline structures of the 2D COFs. Specifically, the highly crystalline structures of two 2D COFs became amorphous, while the third 2D COF retained its crystallinity but with altered crystalline phases. We attribute the varied performance of these 2D COFs to their subtly differing side groups (-H, -OH, and -OMe), which influence the hydrophilicity of the nanochannel surfaces, leading to distinct water condensation behaviors and differing magnitudes of capillary force. Additionally, the interlayer interactions between different side groups provide varying structural stability, influencing the 2D COFs' ability to withstand capillary forces. This study is the first to analyze capillary forces in the nanochannels of 2D COFs, providing insights into their interactions with liquids and highlighting their potential applications.