Effect of Entropy and Hydrogen Bonding on Water Configuration in Sub-Nanometric Active Confinements

Water in sub-nanometeric confinements demonstrates properties that are absent in its bulk counterpart. In this study, we have conducted reactive molecular dynamics (MD) simulations to study the role of water entropy and hydrogen bond (HB) strength on its structure when confined inside boron nitride nanotubes (BNNTs) of different diameters. The nitrogen atoms of the BNNT participate in such HB formation making the BNNT an active confinement. The ReaxFF force field parameters used here were shown to accurately capture water structure and predict a water-hexagonal-Boron-Nitride (water-hBN) contact angle of ~70° that agrees with the experimental results. Most previous works involving MD simulations of water confined in BNNT have underestimated the water-hBN contact angle (~40°) resulting in a significant overestimation of energetic contribution to the water free energy inside the BNNTs. As such, our study shows that water does not form a continuous single-file inside sub-nanometer BNNT at 1 atmospheric pressure and 300K temperature which is at odds with results from previous MD simulations: we associate such behavior to an intricate interplay of the entropic effects (associated with the entry of the water molecules into the BNNT) and the corresponding HB formation dynamics.