Colloidal Stability and Solubility of Metal-Organic Framework Particles

Chemical separation membranes, drug delivery agents, and other nascent applications of metal-organic frameworks (MOFs) benefit from preparing MOFs as nanoparticles (nanoMOFs) and by controlling their particle surfaces. Despite the lack of deliberately added surface ligands, or surfactants, common examples of nanoMOFs exhibit multiweek colloidal stability in a range of polar solvents. Whereas nanocrystal colloidal stability in general arises from a combination of electrostatic repulsion, steric hindrance between surface species, and favorable interactions with solvent, nanoMOFs present the unusual combination of interior and exterior surfaces for these interactions to transpire. Here, we demonstrate that nanoMOFs suspend only in solvents that dissolve the constituent MOF linkers. Moreover, the maximum “solubility” of nanoMOFs, i.e., the concentration of saturated particle suspensions, correlates with the solubility of the linkers in the same solvent. Calorimetry measurements indicate that nanoMOF immersion enthalpies resemble the solvation enthalpies of the linkers, suggesting solvent-linker interactions dictate nanoMOF colloidal stability. As a proof-of-concept, whereas nanoMOFs generally suspend only in polar solvents, we achieve nanoMOF suspensions in toluene by identifying linkers soluble in the same solvent. Furthermore, atomistic molecular dynamics simulations reveal that solvents best at dissolving nanoMOFs are those that pack densely into the pores and interact with the MOF linkers. These results provide a predictive tool for achieving nanoMOF colloidal stability and highlight the uniqueness of defining a MOF “surface”, where solvents access both interior and exterior surfaces.