Water and mass transport in low-dimensional confined structures

Abstract

Water and mass transport in low-dimensional confined structures is of great importance in solving many challenging problems in interface chemistry and fluid mechanics,and presents versatile applications including mass transport,catalysis,chemical reaction,and nanofabrication.Recent achievements of water and mass transport in low-dimensional confined structures are summarized.Water flow confined in nanochannels with different wettability reveals the viscosity in the interface region increases as the contact angle decreases,whereas the flow capacity of confined water increases as the contact angle increases.Small difference in the nanochannel size has a big effect on the confined water flow,especially for nanochannels with a diameter smaller than 10 nm.The phenomena of ultrafast mass transport are universal in the nanochannels with smaller diameter (<10 nm),e.g.,ultrafast ionic transport across the biological and artificial ionic channel;ultrafast water flow through aligned carbon nanotube (CNT) membrane;ultrafast water permeation through GO membranes with hydrophilic end-group.From the classical hydrodynamics,the penetration barrier in such a small channel in both biological and artificial systems is huge,which is contradictory with the actual phenomena.Thus,we propose a concept of quantum-confined superfluid (QSF) to understand this ultrafast fluid transport in nanochannels.Molecular dynamic simulations of water confined in 1D nanochannel of CNTs (with diameter of 0.81 nm) and 2D nanochannel of graphene (two graphene layers distance <2 nm) demonstrate ordered chain of water molecules and pulse-like transmission of water through the channel,further provide proof for the QSF concept.Reversible switching of water wettability in the nanochannel via external stimuli (temperature and voltage) are presented,raising the temperature causes water wettability switching from hydrophilic to hydrophobic state,while increasing the voltage induces water wettability change from hydrophobic to hydrophilic state.The ultrafast liquid transport performance promotes the application of nanochannels in separation.There exist an upper limit for the surface tension of the liquid (≈ 180mN/m) below which the nanochannels of CNTs can be wetting.Then,we summarized versatile applications of low-dimensional confined structures in catalysis,chemical reaction,nanofabrication,and battery.Despite considerable advances over the last few decades,many challenging issues on water and mass transport in low-dimensional confined structures are still unresolved.The biggest obstacle is focused on understanding the physical origin of the non-classical behavior of liquid under confinement.In this situation,our proposed QSF concept will provide new ideas for the fluidic behavior in the nanochannels,and the introduction of QSF concept might create QSF-based chemistry.By imitating enzyme synthesis,the reactant molecules can be arranged in a certain order,and the reaction barrier will be greatly reduced to achieve highly efficient and selective chemical synthesis.Some previous works including organic reaction and polymeric synthesis have approached the example of QSF-like chemical reactions.On the other hand,the advances in nanomechanical techniques such as surface forces apparatus,atomic force microscope,and sum-frequency vibrational spectroscopy will provide useful experimental approaches to understand the mechanism of water and mass transport in low-dimensional confined structures,and promote wider application of nanoconfined structures.