Nanochannel characteristics contributing to ion/ion selectivity in two-dimensional graphene oxide membranes

Selective ion/ion transport is of great importance in environmental and energy applications. Nanofiltration membranes possessing sub-nanometer pore sizes and charge characteristics have garnered significant attention in effectively separating target ions. Nevertheless, the contributions of pore size, charge, and their combined impact on ion/ion separation have remained relatively unexplored. In this study, a well-defined example is presented using multilayered graphene oxide membranes with ordered interlayer nanochannels. These membranes were utilized to prepare diverse nanochannel types, each possessing distinct channel sizes and charge properties. A systematic investigation was conducted to discern their separation selectivity for mono-/di-valent ion pairs. The dominant factor governing mono-/di-valent ion selectivity was not found to be the channel pore size within the sub-nanometer scale. This could be because nanochannels are flexible and cannot strictly sieve ions based on their specific size. Instead, the difference in electrostatic effects between mono- and di-valent ions plays a more significant role in enhancing ion/ion selectivity within the subnanometer channel. This study offers certain insights into selective ion transport in two-dimensional membrane nanochannels, potentially assisting in the design of nanochannels with single-ion selectivity.