Di Trani Nicola, Pimpinelli Alberto, Grattoni Alessandro
Department of Nanomedicine, Houston Methodist Research Institute, Houston, Texas 77030, United States.
University of Chinese Academy of Science (UCAS), 19 Yuquan Road, Beijing 100049, Shijingshan, China.
ACS Appl Mater Interfaces. 2020 Mar 11;12(10):12246-12255. doi: 10.1021/acsami.9b19182. Epub 2020 Mar 2.
Molecular transport through nanofluidic structures exhibits properties that are unique at the nanoscale. The high surface-to-volume ratio of nanometer-sized confined spaces renders particle interactions with the surface of central importance. The electrical double layer (EDL) at the solid-liquid interface of charged surfaces generates an enrichment of counterions and the exclusion of co-ions that lead to a change in their diffusivity. In addition, the diffusive transport is altered by steric and hydrodynamic interactions between fluid molecules and the boundaries. An extensive body of literature investigates molecular transport at the nanoscale. However, most studies account for ionic species as point charges, severely limiting the applicability of the results to "large" nanofluidic systems. Moreover, and even more importantly, the change of pH in the nanoconfined region inside nanochannels has been completely overlooked. Corroborated by experimental data, here we present an all-encompassing analysis of molecular diffusion from the micro- to the ultra-nanoscale. While accounting for finite-size ions, we compute self-consistently the pH inside the channels. Surprisingly, we found that the concentration of ions H can change by more than 2 orders of magnitude compared to the bulk, hugely affecting molecular transport. Further, we found that counterions exhibit both enrichment and exclusion, depending on the size of nanochannels. Achieving a greater understanding of the effective transport properties of fluids at the nanoscale will fill the gap in knowledge that still limits development of innovative systems for medicine and industrial applications alike.
通过纳米流体结构的分子传输展现出在纳米尺度上独有的特性。纳米尺寸受限空间的高表面积与体积比使得粒子与表面的相互作用至关重要。带电表面固液界面处的双电层会导致抗衡离子富集和同离子排斥，进而改变其扩散率。此外，流体分子与边界之间的空间位阻和流体动力学相互作用会改变扩散传输。大量文献对纳米尺度的分子传输进行了研究。然而，大多数研究将离子物种视为点电荷，这严重限制了结果对“大型”纳米流体系统的适用性。而且，甚至更重要的是，纳米通道内纳米受限区域的pH变化完全被忽视了。在此，我们结合实验数据，对从微米到超纳米尺度的分子扩散进行了全面分析。在考虑有限尺寸离子的同时，我们自洽地计算了通道内的pH值。令人惊讶的是，我们发现与本体相比，氢离子浓度变化超过2个数量级，这对分子传输有极大影响。此外，我们发现抗衡离子会根据纳米通道的尺寸既表现出富集又表现出排斥。更深入地了解纳米尺度流体的有效传输特性将填补目前仍限制医学和工业应用创新系统发展的知识空白。
