Counterintuitive Trend of Intrusion Pressure with Temperature in the Hydrophobic Cu₂(tebpz) MOF

Liquid porosimetry experiments reveal a peculiar trend of the intrusion pressure of water in hydrophobic Cu₂(3,3',5,5'-tetraethyl-4,4'-bipyrazolate) MOF. At lower temperature (T) range, the intrusion pressure (Pⁱ) increases with T. For higher T values, Pⁱ first reaches a maximum and then decreases. This is at odds with the Young-Laplace law, which for systems showing a continuous decrease of contact angle with T predicts a corresponding reduction of the intrusion pressure. Though the Young-Laplace law is not expected to provide quantitative predictions at the subnanoscale of Cu₂(tebpz) pores, the physical intuition suggests that to a reduction of their hydrophobicity corresponds a reduction of the Pⁱ. Molecular dynamics simulations and sychrothron experiments allowed to clarify the mechanism of the peculiar trend of Pⁱ with T. At increasing temperatures the vapor density within the MOF' pores grows significantly, bringing the corresponding partial pressure to ≈5 MPa. This pressure, which is consistent with the shift of Pⁱ observed in liquid porosimetry, represents a threshold to be overcame before intrusion takes place. Beyond some value of temperature, the phenomenon of reduction of hydrophobicity (and water surface tension) dominated over the opposite effect of increase of vapor pressure and Pⁱ inverts its trend with T.