Coupled Transport of Water and Ions through Graphene Nanochannels

Abstract

Water and ion transport through graphene nanochannels has attracted considerable attention thanks to the possibility of dimensional control of the channel sizes down to a single atomic layer. Using molecular dynamics simulations, we systematically analyzed the coupled transport of water and ions in the solutions of LiCl, NaCl, and KCl salts as a function of channel sizes, applied electric fields, and salt concentrations. A universal order of ion flux is found with K + > Cl − > Na + ≈ Li + , and the K + flux is twice as large as those of Na + and Li + , indicating the ion selectivity with such graphene channels. The local structures and transport dynamics within the channels show sensitive dependence on the channel height, forming two-dimensional hydration shells in the low-height limit. The hydration shells of the ions undergo transformation from three-to two-dimensional structures upon entering the narrow channel. A power law relationship between the ion translocation time and electric field is also found and can be well described by the one-dimensional Langevin equation. In addition, the linear relation between the ion flux and concentration agrees well with the one-dimensional Poisson−Nernst−Planck equation. Our results provide insights into the ionic transport through graphene channels and have implications for the design of novel nanofluidic devices for selective ion transport in future applications.

Rectification Correlation between Water and Ions through Asymmetric Graphene Channels

“…For D/d = 1.5, the Na + flux is very small, and the negative flux becomes larger (see Figure 2a inset), whose values are not sensitive to the positive field strength. This is similar to previous experiments 35,36 and simulations, 11 where the ion flux is nearly zero for single-layer graphene channels. This abnormal result can be attributed to the dehydration of Na + .…”

“…Actually, the ion hydration structures are highly relevant to their dynamics. For example, a recent experimental study found that on a solid surface, the migration of Na + strongly depends on its hydration number, while the dehydration plays a crucial role in determining the ion transport and selectivity in nanochannels. ,, For the present asymmetric graphene channel, the ion dehydration is simply due to the steric exclusion, where the ion hydration number changes differently in positive and negative directions. As seen in Figure a, under the positive E = 0.5 V/nm, Na + moves from the base to tip, where its hydration number decreases in a wide region for the case of D / d = 1.5.…”

“…However, it is believed that the ICR in a pure geometrical asymmetric channel should be more fundamental as it only considers the dehydration of ions due to confinement. Specifically, the two-dimensional graphene channels in recent experiments − and simulations show unique two-dimensional water or ion hydration structures, which should lead to distinct dehydration behavior that differs from the conical channels. Moreover, the ICR will also significantly affect the water dynamics due to the dynamical coupling between water and ions in electric fields , but still remains unexplored.…”

“…In recent years, different nanofluidic materials have attracted extensive attention due to their excellent performance and application, including glass, , polyethylene terephthalate, − mica, carbon nanotubes (CNTs), , and graphene. − Among them, graphene is a versatile two-dimensional channel or zero dimension nanopore with huge potential in desalination or ion separation . For example, a rotating nanoporous graphene membrane with pores of 2 to 4 nm in diameter can have almost 100% salt rejection even if the pore size is larger than that of hydrated ions …”

Asymmetric transport and desalination in graphene channels

“…To the best of our knowledge, previous studies mostly focused on nanocone channels for ionic transport, 10,[18][19][20][21] while recent experiments demonstrate unique transport behaviors in two-dimensional graphene channels. [22][23][24][25] In particular, inside such graphene channels, ions will have two-dimensional hydration structures 26 and exhibit ultrahigh diffusivity, 27 where the ions also can assemble into monolayer crystal structures 28 or elongated clusters under electric fields. 15 Furthermore, when the graphene channels are asymmetric, the water and ions exhibit specific rectification correlation in electric fields; 29 however, the desalination behaviors in such asymmetric graphene channels are still unexplored.…”