The world is currently in a water crisis; today, 1 in 9 people lack access to water. Coming into graduate school at UC San Diego, I knew that I wanted to do theoretical chemistry with environmental applications. While I was interested in computational chemistry, I had always been drawn towards the inorganic and materials side of chemistry as well. In a conversation with my advisor Francesco Paesani one day, we talked about having me work on simulations of metal-organic frameworks (MOFs) for atmospheric water capture. I was excited about this idea as it would allow me to combine my interests in computational chemistry and inorganic chemistry with environmental applications.
MOFs are of interest in many branches of science due to their applications in liquid and gas adsorption, catalysis, chemical separation, proton conduction, and numerous other topics. MOFs combine a metal center with an organic linker, and this metal center and linker can be exchanged to create a vast number of MOFs, which have large surface areas and high porosity.
My first project investigating MOFs led to a collaboration with Adam Rieth and Mircea Dincă at MIT. Adam and Mircea had previously synthesized a MOF, Co2Cl2BTDD, that had a record water uptake capacity and could provide a source of clean drinking water. Since our group has expertise in the computational study of water using our MB-pol water model, we became interested in investigating the structure and dynamics of water in confinement. MOFs were the ideal system to study confined water as they have both hydrophobic and hydrophilic regions in their structure. Studying water confined in Co2Cl2BTDD provided an intriguing idea since this MOF has an amphipathic structure along with one of the highest water capture abilities.
To begin investigating water confined in Co2Cl2BTDD, we calculated infrared (IR) spectra of water both in experiment and theory. The agreement we found was astounding. To our knowledge, this was the best agreement between experiment and theory for confined water. The frequencies of the peaks in the IR spectra agree very well with each other, providing validation for the structure of the hydrogen-bonding network of water in confinement.
Having agreement between experiment and theory, we wanted to see if we could elucidate the mechanism of pore filling. Doing simulations of one and two water molecules per pore, we were able to determine that water first interacts with the cobalt metal center of the framework, starts to form hydrogen bonds with itself, and forms one-dimensional chains along the interior of the pore interface. This water chain then nucleates pore filling as water fills the interior of the pore.
Furthermore, we were interested in the dynamics of water inside the MOF pore and were curious if confined water could resemble bulk water in any way. What we found was a vast degree of dynamical heterogeneity in the pore interior, with water near the framework interface being almost immobile while water in the middle of the pore has very fast dynamics that closely resemble bulk water.
We believe that this study can aid in the development of future water-harvesting materials, providing a source of clean drinking water. We are not only able to determine the mechanism of pore filling, but the level of agreement between experiment and theory is unprecedented.
By Kelly Hunter