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Despite water intrusion in microporous materials being extensively investigated, obtaining a detailed overview of the intrusion mechanism in materials with more complex morphology, topology, and physical–chemical characteristics, such as metal–organic frameworks (MOFs), is far from trivial. In this work, we present a qualitative study on the mechanism of water intrusion in a crystallite of hydrophobic Cu2(tebpz) (tebpz = 3,3′,5,5′-tetraethyl-4,4′-bipyrazolate) MOF. This MOF is characterized by a complex morphology; it consists of primary (main channels) and secondary (lateral apertures) porosities. This is similar to some zeolites, such as the so-called ITT-type zeolite framework, but it presents the additional characteristics of high flexibility of the material and non-uniform hydrophobicity. Interestingly, in Cu2(tebpz), water intrusion occurs first for some of the channels lying tangent to the surface of the MOF's crystallite. This is due to hydrogen bonding bridging with bulk water across the (thin) lateral apertures of these channels. In macroscopic terms, this can be understood as a local reduction of hydrophobicity favoring intrusion. Temperature and pressure influence the average number of hydrogen bonds and the number of intruded water molecules, explaining the effect of these thermodynamic parameters on the intrusion/extrusion characteristics of this porous material. Molecular dynamics simulations allowed us to glimpse liquid intrusion in this complex hydrophobic material, highlighting how the classical models valid for mesoporous systems, namely, Young–Laplace's law, are not quite appropriate to describe intrusion in such materials. [ABSTRACT FROM AUTHOR]

Liquid porosimetry experiments reveal a peculiar trend of the intrusion pressure of water in hydrophobic Cu 2 (3,3',5,5'-tetraethyl-4,4'-bipyrazolate) MOF. At lower temperature (T) range, the intrusion pressure (P i ) increases with T. For higher T values, P i  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 Cu 2 (tebpz) pores, the physical intuition suggests that to a reduction of their hydrophobicity corresponds a reduction of the P i . Molecular dynamics simulations and sychrothron experiments allowed to clarify the mechanism of the peculiar trend of P i 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 P i 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 P i inverts its trend with T., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)