Field- and Confinement-Modulated Unconventional Electroosmotic Flow in Two-Dimensional Graphene Nanochannels

Precise control of water and ion transport in nanochannels is fundamental to the development of functional nanofluidic devices. Recent experimental advances have enabled the exploration of nanoscale transport phenomena through the construction of two-dimensional channels with atomically tunable widths. Here, we show that unconventional electroosmotic flow occurring without an electric double layer (EDL) in two-dimensional nanochannels can be modulated by both the applied electric field and the channel width. Significant electroosmotic flow is observed in narrow channels when a single ion permeates under an external field. Free energy calculations of ion permeation events reveal that the flux is induced by channel selectivity for chloride ions over sodium ions. The flux undergoes an unexpected significant decrease when the channel broadens to a width of about 16 Å. This transition is attributed to the reduced permeation events of Bjerrum pairs and to altered free ion mobility by water orientational ordering under an electric field. These results offer new insights into the mechanisms of field-controlled transport in nanoscale channels, informing the design of molecular transport systems and energy-efficient separation technologies.