Abstract
Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. Here, we combine infrared spectroscopy and many-body molecular dynamics simulations to probe the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with ordered cobalt open coordination sites. Building upon the agreement between experimental and theoretical spectra, we demonstrate that water at low relative humidity binds initially to open metal sites and subsequently forms disconnected one-dimensional chains of hydrogen-bonded water molecules bridging between cobalt atoms. With increasing relative humidity, these water chains nucleate pore filling, and water molecules occupy the entire pore interior before the relative humidity reaches 30%. Systematic analysis of rotational and translational dynamics indicates heterogeneity in this pore-confined water, with water molecules displaying variable mobility as a function of distance from the interface.
Highlights
Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate
The uniformity of the hydrophobicity or hydrophilicity associated with these materials engenders mostly predictable water–substrate interactions that may differ significantly from those observed in heterogeneous environments, such as aquaporins and other structures found in biological systems[3], where both hydrophilic and hydrophobic patches coexist and lead to a variety of competing H-bonding domains[1,2,3,4,5,6,7]
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were measured at 20 °C under variable relative humidity (RH) (Fig. 2b and Supplementary Fig. 8)
Summary
Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. We combine infrared spectroscopy and many-body molecular dynamics simulations to probe the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with ordered cobalt open coordination sites. Metal-organic frameworks (MOFs) have recently received attention as water containers exhibiting tunable hydrophilicity of potential use in adsorption heat pumps[23,24,25,26,27,28] and for atmospheric water harvesting[29,30] In this regard, we posit that a MOF termed Co2Cl2BTDD (H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]) dibenzo[1,4]dioxin)), which was recently investigated for its record reversible water uptake (Fig. 1), will provide a relevant crystalline analog for investigating the H-bonding structure of water in heterogeneous confinement[30]. As the relative humidity (RH) increases, these 1-D chains template the subsequent formation of cylindrical water shells that extend along the hydrophobic pore channels and exhibit progressively faster rotational and translational mobility as a function of the distance from the pore surface
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