Encapsulated Water Inside Mo132 Capsules: The Role of Long-Range Correlations of about 1 nm

The dynamics of water encapsulated in the well-known polyoxomolybdate nanocapsules of the type Mo₁₃₂ is studied through molecular dynamics simulations. Two different ligands, namely, formate and sulfate ligands, are considered as decoration of the inner surface of the capsule. In both cases it is found that 172 water molecules are trapped inside, 72 of which are coordination water molecules, and the remaining 100 form a water nanodrop whose properties are studied. We find that the dynamic behavior of the nanodrop is significantly different between the two types of capsules considered as they have different interiors. We argue that the commensurability of the sulfate ligand nanocapsule imposes a high degree of tetrahedrality that confers rigidity to the three-dimensional water network affecting the whole water nanodrop in the cavity. The formate ligand capsule, instead, permits additional degrees of freedom that confer the water nanodrop a more liquid-like behavior. The overall size of the inner cavity is of the order of 1 nm which is comparable to the crossover length observed for effects related to the rigidity of water layers in contact with hydrophobic molecules.1 The results could stimulate the investigation of encapsulated water in the presently used capsule, the interior of which can be widely tuned (even from hydrophilic to hydrophobic) in context with the fact that knowledge about water under confined conditions is of extreme importance.