Interplay of Confinement and Surface Energetics in the Interaction of Water with a Metal-Organic Framework

The enthalpy of water adsorption (Delta h) on the metal-organic framework (MOF) HKUST-1 has been determined directly by calorimetry. The most exothermic value of Delta h [-119.4 +/- 0.5 kJ/(mol of water)] occurs at zero coverage and perhaps represents water confinement in the smallest (4-angstrom) cages. An intermediate Delta h value of -50.2 +/- 1.8 kJ/(mol of water) at higher loading probably corresponds to the binding of water on the available Cu nodes and subsequent filling of the largest (11-angstrom) pores. The weakest interactions take place in the medium (10-angstrom) cages, showing weak inclusion of water clusters in a limited hydrophobic environment. By combining ethanol adsorption calorimetry, mathematical analysis of the slope of the water adsorption isotherm, and the differential enthalpy of water adsorption curve, we are able not only to develop an approach to separate energetically multistage guest-host interactions in complex MOF architectures but also to distinguish a sequence of interactions with very similar energetic effects.