Pauli Repulsion Enhances Mobility of Ultraconfined Water

Water is ubiquitous on Earth and dominates chemical and biological processes in daily life. However, how water behaves under some critical conditions is not fully understood. In this paper, we employed quantum first-principles calculations and dynamics simulations to reveal the unexpectedly high mobility of water molecules in ultraconfined spaces. The water molecules rotated more freely in the (4, 4) carbon nanotube than in the (5, 5) carbon nanotube, which is induced by the Pauli repulsion from the wall of the narrower channel when reducing the size of the channel from general confinement to ultraconfinement. Moreover, this quantum effect facilitates the transport of water molecules into the space within their van der Waals diameter easily, which is in contrast to the general understanding. Thus, the conventional concept that the tighter the confined space, the more difficult the motion of the confined object is not always correct. This quantum-induced enhancement of water mobility by Pauli repulsion calls us to pay more attention to the existence and the function of water in neglected ultraconfined spaces (e.g., cells and the Earth's crust) in the future.