Selective mass transport mediated by two-dimensional confined water: A comprehensive review

Confined mass transport based on two-dimensional (2D) materials breaks the trade-off effect between permeability and selectivity, significantly enhancing the efficiency of mass transport. However, the prevailing view that mass transport performance is primarily determined by the structural design of molecules or ions within channels and the regulation of channel walls has led to the neglect of surrounding hydration layers. Recent studies indicate that the interactions between confined water and transport substances, particularly the formation of hydration shells, significantly influence the mass transport process. Therefore, a thorough investigation of the behavior and properties of confined water, especially its presence, regulation methods, and the enhanced mechanisms of mass transport in 2D channels is particularly urgent and constitutes an indispensable research direction for the future development of materials science and engineering technologies. This review summarizes the latest progress on 2D confined water including its structure, properties, and behavior under natural conditions or environmental influences, the mechanisms enhancing mass transport, and regulatory approaches, as well as multiple applications such as membrane separation, drug delivery, and confined reactions. Lastly, we present instructive perspectives on the current challenges and future directions in the study of confined water.