School of Chemical Engineering , The University of Queensland , Brisbane , QLD 4072 , Australia.
Solid State and Structural Chemistry Unit , Indian Institute of Science , C. V. Raman Road , Bengaluru , Karnataka 560012 , India.
Langmuir. 2018 Jul 10;34(27):8099-8111. doi: 10.1021/acs.langmuir.8b00616. Epub 2018 Jun 26.
Water displays anomalous fast diffusion in narrow carbon nanotubes (CNTs), a behavior that has been reproduced in both experimental and simulation studies. However, little is reported on the effect of bulk water-CNT interfaces, which is critical to exploiting the fast transport of water across narrow carbon nanotubes in actual applications. Using molecular dynamics simulations, we investigate here the effect of such interfaces on the transport of water across arm-chair CNTs of different diameters. Our results demonstrate that diffusion of water is significantly retarded in narrow CNTs due to bulk regions near the pore entrance. The slowdown of dynamics can be attributed to the presence of large energy barriers at bulk water-CNT interfaces. The presence of such intense barriers at the bulk-CNT interface arises due to the entropy contrast between the bulk and confined regions, with water molecules undergoing high translational and rotational entropy gain on entering from the bulk to the CNT interior. The intensity of such energy barriers decreases with increase in CNT diameter. These results are very important for emerging technological applications of CNTs and other nanoscale materials, such as in nanofluidics, water purification, nanofiltration, and desalination, as well as for biological transport processes.
水在狭窄的碳纳米管（CNTs）中表现出异常快速的扩散，这一行为在实验和模拟研究中都得到了再现。然而，关于体相水-CNT 界面的影响却鲜有报道，而这对于在实际应用中利用水在狭窄碳纳米管中的快速传输至关重要。本文采用分子动力学模拟研究了这种界面对不同直径扶手椅型 CNT 中水分子传输的影响。结果表明，由于孔口附近的体相区域，水在狭窄 CNT 中的扩散受到显著阻碍。动力学的减缓可归因于体相-CNT 界面处存在较大的能量势垒。在体相-CNT 界面处存在如此强烈的势垒是由于体相和受限区域之间的熵对比所致，水分子从体相向 CNT 内部进入时经历了高的平动和转动熵增益。这种能量势垒的强度随 CNT 直径的增加而减小。这些结果对于 CNT 及其他纳米尺度材料的新兴技术应用非常重要，例如在纳流控、水净化、纳滤和脱盐以及生物传输过程中。
