Zhou Ke, Xu Zhiping
Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China.
Nanoscale. 2020 Mar 19;12(11):6512-6521. doi: 10.1039/c9nr10731b.
Fluid transport confined in nanochannels shows ultrafast permeation and highly efficient separation performance. However, the size-controlled selectivity of hydrated ions with a similar valence and size, such as alkali ions, is well below 5. We propose in this work to boost ion selectivity through the interaction with the wall of flow channels, which can be enhanced by applying an external electric field across the channel. Molecular simulations show that for ions diffusing near the walls of a graphene nanochannel, the hydration shells are perturbed, endowing the contrast in ion-wall interactions to modify the ion-specific free energy landscape. The trapping/hopping nature of ion diffusion near the wall leads to the conclusion that the diffusivity depends on the free energy barriers rather than the hydration size. This effect can be magnified by elevating the field strength, yielding more than ∼10-fold enhancement in the diffusivity-specific selectivity. With recent experimental advances in external electric field control and local electric field modulation near the surface, this work demonstrates a possible route to achieve high selectivity of alkali ions in nanofluidics, and explore the molecular structures and dynamics of hydrated ions near a surface.
限制在纳米通道中的流体传输表现出超快的渗透和高效的分离性能。然而，对于具有相似价态和尺寸的水合离子（如碱金属离子），尺寸控制的选择性远低于5。我们在这项工作中提出通过与流动通道壁的相互作用来提高离子选择性，这可以通过在通道上施加外部电场来增强。分子模拟表明，对于在石墨烯纳米通道壁附近扩散的离子，水合壳层受到扰动，赋予离子 - 壁相互作用的差异以改变特定离子的自由能分布。壁附近离子扩散的捕获/跳跃性质导致扩散率取决于自由能垒而不是水合尺寸的结论。通过提高场强可以放大这种效应，使扩散率 - 选择性提高超过10倍。随着近期在外部电场控制和表面附近局部电场调制方面的实验进展，这项工作展示了在纳米流体学中实现碱金属离子高选择性的可能途径，并探索了表面附近水合离子的分子结构和动力学。
