Liquid self-diffusion of H₂O and DMF molecules in Co-MOF-74: Molecular dynamics simulations and dielectric spectroscopy studies

In this work we use molecular dynamics simulations to study the diffusion of N,N-dimethylformamide (DMF) and H₂O as a function of temperature within the well-known metal-organic framework Co₂(dobdc)·[G] (G = 2DMF·1H₂O), also known as Co-MOF-74. The molecular dynamics simulations show that the diffusivity of guest molecules, which is almost negligible at low temperatures (T < 200 K), increases in the range of 200 < T (K) < 400 up to 3 and 4 orders of magnitude for DMF and H₂O, respectively. This molecular diffusion can be easily detected by dielectric spectroscopy as it gives rise to extrinsic interfacial polarization effects that result in an apparent "colossal" dielectric constant at room temperature, ϵ_r′ ∼ 42000 (T = 300 K, ν = 10 Hz). Furthermore, the measured dielectric constant exhibits a thermal dependence similar to that of the diffusion coefficient, revealing the parallelism of the dielectric response and the molecular diffusion as a function of temperature. These results highlight: (a) the great utility of the fast and non-destructive dielectric and impedance spectroscopy techniques for the study and detection of the molecular transport of small polar molecules within porous metal-organic frameworks and related materials; (b) the peculiarity and uniqueness of MOF materials with "medium" size nanopores containing guest molecules as they are solid materials in which the guest molecules display a liquid state-like behaviour close to room temperature; and (c) the potential of these materials for molecular transport applications.