Ice-like Water Structure in Carbon Nanotube (8,8) Induces Cationic Hydration Enhancement Zhongjin He, †,‡ Jian Zhou,* ,† Xiaohua Lu, § and Ben Corry* ,‡ † School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China § State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China ‡ Research School of Biology, The Australian National University, Canberra ACT 0200, Australia * S Supporting Information ABSTRACT: It is well recognized that ice-like water can be formed in carbon nanotubes (CNTs). Here, we perform molecular dynamics simulations of the hydration of Na + ,K + and Cl - in armchair CNT(n,n)(n = 6, 7, 8, 9 and 10) at 300 K to elucidate the effect of such water structures on ionic hydration. It is found that the interaction of Na + and K + with the water molecules is enhanced in CNT(8,8), but is similar or weaker than in bulk in the other CNTs. In bulk, water molecules orient in specific directions around ions due to the electrostatic interaction between them. Under the confinement of CNTs, the hydrogen bonds formed in the first hydration shell of Na + and K + disturb this orientation greatly. An exception is in CNT(8,8), where the dipole orientation is even more favorable for cations than in bulk due to the formation of a unique ice-like water structure that aligns the water molecules in specific directions. In contrast, the coordination number is more important than hydration shell orientation in determining the Cl - -water interaction. Additionally, the preference for ions to adopt specific radial positions in the CNTs also affects ionic hydration. ■ INTRODUCTION The hydration of ions under nanoscale confinement has attracted much attention for its critical role in a wide range of technological applications and in dictating transport in biological ion channels. Recently, carbon nanotubes (CNTs) have been proposed to be used in the construction of novel nanofluidic systems, high-performance membranes, electro- chemical capacitors and electrodes. These applications take advantage of the fast mass transport through the interior of the structure, the ease of chemical functionalization and the high surface area of CNTs. 1,2 Several CNT-based nanofluidic systems have been fabricated, which can be used as nanoscale flow sensors, 3 mass conveyors 4 and single ion detectors. 5 Pristine or modified CNTs with narrow diameter may be able to selectively transport water molecules while rejecting ions. 6 Therefore, narrow CNTs can be incorporated into reverse or forward osmosis membranes for efficient seawater desalina- tion. 7-9 In addition, a series of ion-selective CNTs, which mimic the function of biological systems, were designed via molecular dynamics (MD) simulations, 10,11 and the hydration structure of ions confined in the CNTs is essential to the remarkable ion selectivity. 12 CNTs are found to have great potential in methanol-water separation for the high selective adsorption of methanol. 13,14 The hydration of ions in the confines of biological ion channels is critical in dictating ion conduction and selectivity in narrow pores. For example, the different selectivity of potassium and sodium channels is at least partly due to differences in ion hydration. In the filter of K + -selective channel KcsA, K + is almost totally dehydrated and better coordinated by carbonyl groups than Na + . 15 The slightly modified NaK channel loses selectivity as ions can be hydrated by additional water molecules. 16 The selectivity of voltage gated sodium channels for Na + over K + is suggested to be dictated by the more favorable solvation structure of Na + confined in the charged pore. 17 Ion dehydration has also been proposed as playing an important role in channel gating, preventing the passage of ions in channels even when the pore is not completely occluded. 18 CNTs can be used as a simple model to investigate ionic hydration under nanoscale confinement, without the intrinsic complexity and flexibility of biological pores. A detailed understanding of ionic hydration in CNTs can help facilitate the development of novel CNT-based devices and may give insight into the mechanisms of ion permeation and selectivity taking place in biological ion channels. Ion hydration under nanoscale confinement is quite different from that in bulk. 19-26 Previous MD simulation results show that ions are desolvated and can easily form ion pairs in narrow CNTs; 27,28 however, the first hydration shell of ions in wide CNTs is almost bulk-like, and its size does not change. 29,30 Ions Received: March 13, 2013 Revised: April 27, 2013 Published: May 2, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 11412 dx.doi.org/10.1021/jp4025206 | J. Phys. Chem. C 2013, 117, 11412-11420