Changing Water Affinity from Hydrophobic to Hydrophilic in Hydrophobic Channels

The behavior of water at hydrophobic interfaces can play a significant role in determining chemical reaction outcomes and physical properties. Carbon nanotubes and aluminophosphate materials have one-dimensional hydrophobic channels, which are entirely surrounded by hydrophobic interfaces. Unique water behavior was observed in such hydrophobic channels. In this article, changes in the water affinity in one-dimensional hydrophobic channels were assessed using water vapor adsorption isotherms at 303 K and grand canonical Monte Carlo simulations. Hydrophobic behavior of water adsorbed in channels wider than 3 nm was observed for both adsorption and desorption processes, owing to the hydrophobic environment. However, water showed hydrophilic properties in both adsorption and desorption processes in channels narrower than 1 nm. In intermediate-sized channels, the hydrophobic properties of water during the adsorption process were seen to transition to hydrophilic behavior during the desorption process. Hydrophilic properties in the narrow channels for both adsorption and desorption processes are a result of the relatively strong water–channel interactions (10–15 kJ mol<sup>–1</sup>). In the 2–3 nm channels, the water–channel interaction energy of 4–5 kJ mol<sup>–1</sup> was comparable to the thermal translational energy. The cohesive water interaction was approximately 35 kJ mol<sup>–1</sup>, which was larger than the others. Thus, the water affinity change in the 2–3 nm channels for the adsorption and desorption processes was attributed to weak water–channel interactions and strong cohesive interactions. These results are inherently important to control the properties of water in hydrophobic environments.