Molecular Insights into Electrostatically Modulated Transport of Ions Along The Graphene-Water Interface

Ion transport at charged interfaces is crucial for a wide range of applications across the energy, water, and biological sectors. Conventionally, ion permeation driven by a concentration gradient is thought to be reduced due to co-ion depletion and Donnan exclusion, a cornerstone principle in nanofluidics [1,2]. This phenomenon is also evident in widely used ion exchange membranes. Our experimental observations reveal that the ion permeation rate through water-mediated graphene membranes can be modulated unexpectedly higher than that in the bulk solution, upon applying a variable gate potential. This finding markedly contrasts with predictions, which anticipate suppression of ion permeation Nevertheless, a lack of molecular-level insights impedes a comprehensive understanding of this counterintuitive behaviour, as probing the ion structure and dynamic transport processes at electrified interfaces presents significant experimental challenges.<br/>Here, we conducted all-atom molecular dynamics simulations to explore the non-equilibrium ion transport behaviour in the water-filled graphene nanochannel, where the concentration gradient between the entrance and exit is meticulously maintained using a constant chemical potential algorithm [3]. Our results reveal that ion permeation fluxes in negatively charged graphene nanochannels significantly can surpass those in neutral nanochannels, with flux increasing in proportion to the surface charge density on the graphene, which is contradictory to classical mean-field theories yet consistent with our experimental observations. Furthermore, an increase in co-ion concentration within the negatively charged nanochannel, which exceeds that of the bulk solution and effectively amplifies the concentration gradient to the drain reservoir, plays a pivotal role in enhancing ion flux. Our dynamic analyses further reveal that the evolution of counter-ion hydration behaviour at the charged interface contributes to charge overscreening, thereby enhancing in-channel co-ion density. This structural arrangement also facilitates counter-ion mobility along the diffusion direction, comparable to that observed in the bulk solution. In contrast, this enhancement is absent in positively charged graphene nanochannels due to a differing evolution of the surface water dipole arrangement, an observation that is consistent with our experimental findings.<br/>Our molecular-level simulations highlight the critical role of the non-classic, short-range structure of interfacial water and ion solvation behaviour, factors often overlooked in classical theories, in governing the electrical response and dynamic transport behaviour of ions at the electrified solid-water interfaces. These insights reveal that complex, subtle interactions at the electrified surface enable the prompt modulation of ion transport in nanochannels by applying gate potential, paving the way for developing advanced applications in electrochemical systems, including energy harvesting and storage, energy-efficient ion separation, neuromorphic computing, and other emerging technologies.<br/><b>References</b><br/>[1] L. Bocquet, E. Charlaix, Nanofluidics, from bulk to interfaces, Chem. Soc. Rev. 39 (2010) 1073–1095. https://doi.org/10.1039/B909366B.<br/>[2] H. Daiguji, Ion transport in nanofluidic channels, Chem. Soc. Rev. 39 (2010) 901–911. https://doi.org/10.1039/B820556F.<br/>[3] C. Perego, M. Salvalaglio, M. Parrinello, Molecular dynamics simulations of solutions at constant chemical potential, The Journal of Chemical Physics 142 (2015) 144113. https://doi.org/10.1063/1.4917200.