The 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, common examples of nanoMOFs exhibit multi-week colloidal stability in a range of polar solvents, in stark contrast with most conventional nanoparticles that require surface functionalization with bulky ligands. And yet, the origin of this stability remains unknown. Although nanoMOF zeta potentials exceed |±20 mV|, electrostatics alone cannot explain colloidal stability. 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 toluene nanoMOF suspensions by identifying linkers soluble in toluene. 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.