Liu Hongmei, Murad Sohail, Jameson Cynthia J
Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA.
J Chem Phys. 2006 Aug 28;125(8):084713. doi: 10.1063/1.2337289.
Molecular dynamics simulations are carried out to investigate the permeation of ions and water in a membrane consisting of single wall carbon nanotubes possessing no surface charges connecting two reservoirs. Our simulations reveal that there are changes in the first hydration shell of the ions upon confinement in tubes of 0.82 or 0.90 nm effective internal diameter. Although the first minimum in the g(r) is barely changed in the nanotube compared to in the bulk solution, the hydration number of Na(+) ion is reduced by 1.0 (from 4.5 in bulk to 3.5 in the 0.90 nm tube) and the hydration number is reduced further in the 0.82 nm tube. The changes in the hydration shell of Cl(-) ion are negligible, within statistical errors. The water molecules of the first hydration shell of both ions exchange less frequently inside the tube than in the bulk solution. We compare ion trajectories for ions in the same tube under identical reservoir conditions but with different numbers of ions in the tubes. This permits investigation of changes in structure and dynamics which arise from multiple ion occupancy in a carbon nanotube possessing no surface charges. We also investigated the effects of tube flexibility. Ions enter the tubes so as to form a train of ion pairs. We find that the radial distribution profiles of Na(+) ions broaden significantly systematically with increasing number of ion pairs in the tube. The radial distribution profiles of Cl(-) ions change only slightly with increasing number of ions in the tube. Trajectories reveal that Na(+) ions do not pass each other in 0.90 nm tubes, while Cl(-) ions pass each other, as do ions of opposite charge. An ion entering the tube causes the like-charged ions preceding it in the tube to be displaced along the tube axis and positive or negative ions will exit the tube only when one or two other ions of the same charge are present in the tube. Thus, the permeation mechanism involves multiple ions and Coulomb repulsion among the ions plays an essential role.
进行分子动力学模拟以研究离子和水在由无表面电荷的单壁碳纳米管组成的膜中的渗透，该膜连接两个储液器。我们的模拟表明，当离子被限制在有效内径为0.82或0.90纳米的管中时，其第一水合层会发生变化。尽管与本体溶液相比，纳米管中g(r)的第一个最小值几乎没有变化，但Na(+)离子的水合数减少了1.0（从本体中的4.5减少到0.90纳米管中的3.5），并且在0.82纳米管中，水合数进一步减少。在统计误差范围内，Cl(-)离子水合层的变化可以忽略不计。两种离子第一水合层中的水分子在管内的交换频率低于本体溶液。我们比较了在相同储液器条件下但管内离子数量不同时，同一管中离子的轨迹。这允许研究在无表面电荷的碳纳米管中由于多个离子占据而引起的结构和动力学变化。我们还研究了管的柔韧性的影响。离子进入管中形成一串离子对。我们发现，随着管中离子对数量的增加，Na(+)离子的径向分布轮廓会系统地显著变宽。随着管中离子数量的增加，Cl(-)离子的径向分布轮廓变化很小。轨迹显示，在0.90纳米的管中，Na(+)离子不会相互通过，但Cl(-)离子会相互通过，相反电荷的离子也是如此。进入管中的离子会导致管中位于其前面的同电荷离子沿管轴移动，并且只有当管中存在一两个其他相同电荷的离子时，正离子或负离子才会离开管。因此，渗透机制涉及多个离子，离子之间的库仑排斥起着至关重要的作用。
