UC San Diego

Water is essential to life, yet a majority of people in the world currently experience water scarcity. New methods of obtaining clean drinking water are required so that all people may have access to safe water. The process of atmospheric water capture, or harvesting water from the air, has emerged as one of the most promising methods for obtaining clean water. Metal-organic frameworks (MOFs) are materials that can operate at a wide range of temperatures and quantity of water in the atmosphere, being able to function in water harvesting. Therefore, it is necessary to understand the properties of water confined in MOFs to obtain an overall depiction of atmospheric water capture. In this work, we investigate the behavior of both bulk and confined water through molecular dynamics simulations. Developing accurate models for MOFs, computer simulations then provide a molecular-level picture of water present in various environments. In combination with experiment, we unravel the mechanism of pore filling in various MOFs. Systems that have open metal sites allow water to form strong interactions with the framework, serving as nucleation sites for water adsorption. Water also exhibits different properties in various regions of these MOFs with large pore spaces, having limited mobility near the MOF interface but resembling bulk water in the middle of the pore. On the other hand, MOFs that do not have open metal sites but contain hydrophilic groups allow water clusters to form in single pores, filling one pore at a time rather than all pores simultaneously. It is further investigated how the interactions between water and the MOF are critical for a correct description of these systems in order to achieve agreement with experimental properties. The comparison of water–water and MOF–water interactions reveals the strongest properties that contribute to water adsorption, which vary between different MOFs. Utilizing accurate models for both water and MOFs, we are able to obtain a deeper understanding of water confined in MOFs. With this information, future water harvesting MOFs can be developed that can provide a source of clean drinking water to the world.