纳米受限下溶质水化结构的分子模拟

利用分子动力学模拟研究了五种不同种类的溶质分子(K+, Mg2+, Cl-, K-和K0)在直径为0.60-1.28 nm的纳米碳管内的水化结构. 模拟结果揭示了单电荷溶质、双电荷溶质和中性溶质在受限条件下具有不同的水化行为. 单价溶质的配位数只有在直径不大于0.73 nm的纳米碳管内才会明显减少. 和带有电荷的溶质不同, 中性溶质的配位数对纳米碳管直径的改变非常敏感, 并且随着管径的减小而迅速减少. 模拟结果还表明带单价正电荷的溶质(K+)第一配位层水分子的取向结构会随着纳米碳管直径的改变发生变化, 而其他溶质配位层取向结构在本文所涉及的纳米碳管内都几乎和体相中一致. 在直径大于1.0 nm的纳米碳管中, K+的配位层取向结构有序度随着管径的减小而单调下降, 但是在直径小于1.0 nm的纳米碳管中, 随着碳管管径的减小而迅速上升. 在两个最窄的纳米碳管内, 其结构有度甚至高于体相. 双电荷溶质的水化结构在本文所研究的碳管直径范围内和体相完全一致, 即使在直径只有0.6 nm的碳管内也无任何改变.

Molecular Simulation of Solute Hydration Structure in Nanoscale Confinement

A series of molecular dynamics simulations were performed to investigate the effect of carbon nanotube diameter on the hydration structure of five different solutes (K+, Mg2+, Cl-, K- and K0) inside carbon nanotubes (CNTs). Simulation results reveal different hydration processes for monocharge, bicharge, and neutral solutes in CNTs. Coordination numbers of monocharge solutes decrease significantly only inside narrow CNTs with diameters less than 0.73 nm. The coordination number of neutral solute is, however, sensitive to the CNT diameter and decrease monotonically as CNT diameters decrease in all the CNTs used in this work. Only the positive monocharge solute has the order of its coordination shell structure vary considerably with a change in CNT diameter. The shells of the other solutes appear to be bulk like in all the CNTs used in this work. The shell order of K+ decreases as CNT diameter decreases for diameters larger than 1.0 nm, and increases as CNT diameter decreases for diameters less than 1.0 nm. Inside the two narrow CNTs with diameters of 0.6 and 0.73 nm, the shell order of K+ is even higher than that found in bulk solution. The hydration of bicharge solute is found to be identical to that in bulk solution in aU the CNTs used in this work, even in the narrow CNT with a diameter of 0.6 nm.