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2009, 113, 12622–12624 Published on Web 06/26/2009
Unique Hydrogen-Bonded Structure of Water around Ca Ions Confined in Carbon Slit Pores Tomonori Ohba,* Natsuko Kojima, Hirofumi Kanoh, and Katsumi Kaneko Graduate School of Science, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan ReceiVed: April 3, 2009; ReVised Manuscript ReceiVed: May 23, 2009
The ionic solution structure of Ca and Cl ions in slit-shaped hydrophobic nanopores of pore widths w of 0.6, 1.2, and 1.8 nm was studied by canonical Monte Carlo simulation. The ionic solution in the pore of w ) 0.6 nm shows unique hydration structure. A pentagonal hydration structure of a Ca ion is observed, being completely different from the bulk ionic solution. The Ca ions are surrounded by a more tightly hydrogenbonded water molecular shell than the bulk solution. On the contrary, the Cl ions have a more diffuse hydration shell than the bulk solution. Ionic solutions have played a key role in a whole range of chemistry and chemical technology, in particular electrochemistry and also in the battery technology.1-3 Chemistry on ionic solutions has been also indispensable to elucidate the mechanism of life activity.4-6 Accordingly great efforts have been done to clarify the hydration structure around ions using X-ray and neutron diffractions, X-ray absorption fine structure (XAFS) spectroscopy, and molecular simulations.7-14 The exact understanding of the relationship between the properties and structure of ionic solutions still need more studies. One difficulty in the research on the ionic solution is associated with the highly symmetrical structure around an ion; it is difficult to find out a slight structural difference in the water molecule-ion bonding. If we can expose the ions to an anisotropic molecular field, the slight coordination difference can be intensified, showing a clue to get a more deep understanding. Recent studies on water adsorbed in hydrophobic carbon nanopores of slit-shape showed that water molecules in the pore form a local ordered structure which sensitively depends on the pore width.15-18 Then, the hydrophobic carbon nanopore can affect efficiently the intermolecular structure of water. Ohkubo et al. introduced Rb and Br ions in the hydrophobic carbon nanopores, showing an evident dehydration around the Rb ion with the aid of XAFS analysis.19 Then, nanoconfinement of ionic solution can offer a new route to understand the ionic solution. Although XAFS spectroscopy is a powerful tool for structure analysis around heavy metal ions, it cannot be effectively applied to light metal ions. We need to study the hydration structures around Ca, K, and Na ions in nanopores, because these ions play an essential role in life activity, nature, and many technologies. Molecular simulation could be useful for studying the mechanism of ionic solution in nanopores.20 In this paper, we study the hydration structure of CaCl2 aqueous solution and the hydrogen bonding network of water around their ions in ideal slit-shaped hydrophobic nanopores having different nanopore widths. * E-mail:
[email protected].
10.1021/jp9030688 CCC: $40.75
Canonical ensemble Monte Carlo simulation of CaCl2 aqueous solution was conducted for assessment of the structures with 1.0 mol dm-3 Ca ion concentration in nanopores for the nanopore widths w of 0.6, 1.2, and 1.8 nm at 303 K. A nanopore is a slit space between the infinite parallel basal planes of graphite. Unit cell size is 6 × 6 × w nm3 and periodic boundary condition with Ewald correction was used in this calculation. We used the combined potential model of Lennard-Jones and Coulomb interactions for Ca and Cl ions. The optimum potential parameters were determined by fitting to the lattice energy of CaCl2 crystal; the Lennard-Jones parameters of Ca (σCa ) 0.3037 nm and Ca/kB ) 50.0 K) and Cl ions (σCl ) 0.4643 nm and Cl/kB ) 50.0 K), and the charges (qCa/e ) +2.0 and qCl/e ) -1.0) roughly agreed with the literature values.13,21 The TIP5P and 10-4-3 Steele potentials were used for water intermolecular interaction and water molecule-graphite surface interactions, respectively.22,23 The fundamental calculation procedure is almost similar to simulation of water adsorbed in carbon nanopore in the preceding article.24 The details of simulation are given in Supporting Information. Figure 1 shows snapshots of Ca and Cl ionic solutions in nanopores of w ) 0.6 (a), 1.2 (b), and 1.8 nm (c), which briefly correspond to the single layer, two to three layers, and four layers for ions, respectively. Ca and Cl ions mostly disperse in water but partially combine with each other. The hydration numbers, Nhyd, of Ca and Cl ions in the nanopore of w ) 0.6 nm were 5.4 and 9.3, respectively. The average hydrated ions in the nanopore are illustrated from the averaged snapshots; the presence of the hydrated Ca ion having a pentagonal structure only in the 0.6 nm pore should be noteworthy. The Nhyd for the Ca and Cl ions for w ) 0.6 nm was smaller than those for w ) 1.2 and 1.8 nm and the bulk ionic solution. In particular, the Ca ions in the nanopore of w ) 0.6 nm are partially dehydrated to have the hydration number smaller than the bulk Ca ions by 20%. The total coordination number of a Ca ion for w ) 0.6 nm was also decreased to 80% of the bulk one. Therefore, confinement in an extremely narrow nanopore prevents water molecules from coordinating Ca and Cl ions; the hydration 2009 American Chemical Society
Letters
Figure 1. Snapshots of Ca and Cl ionic solution in graphitic nanopores of w ) 0.6 (a), 1.2 (b), and 1.8 nm (c) (left side) and the typical hydration structure of ions (right side). Ca ions: Red spheres, Cl ions: yellow spheres, and water molecules: transparent blue spheres of oxygen with two red spheres of hydrogen. A pore wall surface represents the carbon hexagon network.
J. Phys. Chem. C, Vol. 113, No. 29, 2009 12623 in the nanopore, which shift to the shorter distance than those of the bulk ionic solution. Then, water molecules form a tightly hydrogen-bonded hydration shell around the Ca ion in the nanopore compared with the bulk solution. The two peaks at 0.26 and 0.36 nm in the nanopore are close to those of the pure water in the nanopore and thereby there should be an ordered hydrogen-bonded structure around the Ca ion in the nanopore. This is because the preceding in situ X-ray diffraction study on water adsorbed in carbon nanopores of w ) 0.7 nm clearly showed the presence of the solid-like water.15 On the other hand, only a single broad peak is observed for the Cl ion in the nanopore, being different from the bulk solution having two peaks. Accordingly, a tetrahedrally hydrogen-bonded shell of water molecules around the Cl ion is destroyed in the nanopore. The above results indicate that confinement of Ca and Cl ions in the hydrophobic carbon slit space of the w ) 0.6 nm enhances the difference in their hydration structures; a cation tends to form a more firmly hydrogen-bonded first hydration shell, whereas the first hydration shell around an anion becomes unstable. The nanoconfinement of ionic solutions can be applied to understand the essential nature and structure of hydrogen bonding around cations and anions. These fundamental facts should promote important studies on supercapacitors which are necessary for understanding the ionic solution structure confined in subnanoscale slit carbon nanopores.25-27 We are planning on studying the electrical double layer on the basis of the unique ionic structures elucidated in this research, which should promote the development of the high performance supercapacitor. Acknowledgment. This work was financially supported by the Kurata Memorial Hitachi Science and Technology Foundation, and Kurita Water and Environment Foundation. Supporting Information Available: Details of potential parameters and canonical ensemble Monte Carlo simulation. Density distributions of Ca and Cl ions, and water for w ) 0.6, 1.2, and 1.8 nm, and bulk. Radial distribution functions of water from these ions. Hydration and coordination numbers of these ions. The intermolecular O-H distribution between different water molecules around each ion. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 2. Local intermolecular O-H radial distribution functions around a Ca ion (red curve) and a Cl ion (green curve) for ionic solution, and the whole distribution of pure water (blue curve) in nanopore. Those for bulk solution and water are shown in black.
structure around ions should be slightly distorted in the nanopore, compared with the bulk one. The radial distribution functions of O and H atoms, and lone pair in the water molecule to Ca and Cl ions showed that the nearest peak of H atoms to a Ca ion was shortened by 0.02 nm, while that of H atoms to a Cl ion was lengthened by 0.03 nm. The others were not clearly changed, but the second neighbor peaks appeared distinctly. (see Supporting Information) These results suggest that adjacent water molecules are preferentially pressed to a Ca ion, while they avoid a Cl ion. Thus, a Ca ion and the adjacent water molecules are more strongly bound each other in the nanopore. The corresponding unique hydrogen bond network formation in the nanopore is observed. Figure 2 shows the local radial distribution functions of the intermolecular O-H bonds of water molecules around Ca and Cl ions in the ionic solution and the radial distribution in the pure liquid water for comparison. There are three distinct peaks around the Ca ion
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