Flexibility of Ordered Surface Hydroxyls Influences the Adsorption of

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Flexibility of Ordered Surface Hydroxyls Influences the Adsorption of Molecules in Single-Walled Aluminosilicate Nanotubes Ji Zang,† Shaji Chempath,‡ Suchitra Konduri,† Sankar Nair,† and David S. Sholl*,† †

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, and ‡Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

ABSTRACT Single-walled aluminosilicate nanotubes (NTs) are attractive for molecular separation applications because of their highly ordered structure, tunable dimensions, as well as their hydrophilic and functionalizable interiors. These NTs possess a pore surface consisting of an ordered array of silanol groups with flexible hydroxyls. We show that the flexibility of these hydroxyl groups is critical in the adsorption of hydrogen-bonding molecules. Specifically, we study the adsorption of water, methanol, CO2, and CH4 in the NT via grand canonical Monte Carlo (GCMC) simulations. The experimentally observed hydrophilicity of the surface can be captured in adsorption calculations only if the structural and orientational flexibility of the surface hydroxyls is incorporated. The adsorption selectivity of water over methanol is predicted to be larger than 100, which makes aluminosilicate NTs promising for dehydration of alcohols. Flexibility effects are less significant for the adsorption of non-hydrogen-bonding molecules. SECTION Nanoparticles and Nanostructures

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ingle-walled aluminosilicate and aluminogermanate metal oxide nanotubes (NTs) can be synthesized with tunable composition and functionality via lowtemperature liquid phase chemistry using inexpensive precursors.1,2 The diameters of synthetic aluminosilicate or aluminogermanate NTs are highly monodisperse.1,2 These NTs have a highly ordered wall structure with isolated silanol/ germanol groups bound on the inner surface of a nanotubular aluminum hydroxide wall. The presence of hydroxyl groups on this inner wall makes the interior of the pore hydrophilic as well as functionalizable by organic groups. These properties make the nanotubes attractive candidates for a variety of potential applications, including molecular separations, molecular encapsulation, and sensors. Unlike the case of carbon NTs, little is currently known about molecular separations with aluminosilicate or aluminogermanate NTs. An accurate description of adsorption properties is required for a reliable evaluation of molecular separations with the NTs. In simulations of adsorption in nanoporous materials, the solid adsorbent is often assumed to be rigid. Framework flexibility has been found to be important for molecules that are a tight fit to the pore structure, and relatively unimportant for molecules that are too small to fit tightly in the pores.3 This assumption may not be valid in all cases. For example, previous studies have discussed the guest-induced flexible behavior of materials such as metal-organic frameworks4,5 and zeolites.6,7 In these examples, the adsorbent structure undergoes deformation or even a phase transition upon adsorption of molecules. These

r 2010 American Chemical Society

effects;involving deformation of essentially the entire structure; can be captured in simulations by the use of techniques such as normal-mode Monte Carlo.8 In previous work, we examined the adsorption of water in hexagonally packed and structurally rigid aluminosilicate NTs using grand canonical Monte Carlo (GCMC) simulations, and compared our computational results with experimental adsorption isotherms.9 While the predicted saturation loading of water molecules was in reasonable agreement with experimental values, substantial quantitative differences (orders of magnitude) are observed between the measured and computed water uptake at low and intermediate pressures. The surface structure of the inner pore of this NT is an ordered array of silanol groups, as shown in Figure 1. The adsorption of water in zeolites has been found to be sensitive to the presence of silanol defects, even though the defects were assumed to have a rigid geometry.10,11 The motions of surface hydroxyl groups are known to play a critical role in the adsorption of amino acids on quartz surfaces12 and water on hydroxylated silica surfaces13 via hydrogen bonds. It is therefore reasonable to expect that the degrees of freedom in the hydroxyl groups are important in determining adsorption behavior of these molecules in aluminosilicate NTs. To explore this idea, we used GCMC calculations to study the adsorption

Received Date: February 17, 2010 Accepted Date: March 16, 2010 Published on Web Date: March 26, 2010

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DOI: 10.1021/jz100219q |J. Phys. Chem. Lett. 2010, 1, 1235–1240

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Figure 2. Calculated adsorption isotherms of water in the aluminosilicate NT with rigid and flexible surface hydroxyl groups, compared with experimental results (filled triangles: ref 8; halffilled triangles: ref 23; open triangles: ref 24).

Figure 1. (a) Atomic structure of an aluminosilicate NT viewed along the axial direction. (b) Side view of the aluminosilicate NT shell showing the tube interior with the silanol groups inside the pore shown in ball and stick format. The bridging-hydroxyl groups on the outer surface of the NT are not shown, for clarity. Blue: hydrogen; red: oxygen; pink: aluminum; gray: silicon.

single NT. The unit cell dimensions were not allowed to change. Before all adsorption simulations, the entire NT model was subjected to a potential energy minimization, to ensure that a physically realistic structure was used as the basis for comparing the rigid and flexible NT cases. The simple point charge (SPC) model was used for water.9,15,20 Using this force field, the heat of adsorption for water was calculated as 58.0 kJ/mol (at very low loading, 0.001 kPa) and 74.7 kJ/mol (at near-saturation loading, 1 kPa). These values show good agreement with sorption enthalpies of 53.0 and 83.7 kJ/mol at low and high loadings from experimental data fits (Supporting Information). To simulate methanol, we used the TraPPE force field,21 which accurately describes the vapor-liquid coexistence curves for primary, secondary, and tertiary alcohols. CH4 was treated as a spherical particle.22 CO2 was simulated using the three-site EPM2 model,23 which represents CO2 as a linear molecule with three LJ sites and a point charge on each site. Molecule-molecule and molecule-NT interactions were represented by LJ and Coloumbic potentials.9,15,20 The Lorentz-Berthelot combination rules were used for unlike LJ interactions. For efficiently handling Coulombic interactions, a pairwise damped shifted potential was used according to the method of Wolf.24,25 Trial adsorption simulations using a rigid NT model were used to verify that the Wolf method and the conventional (but more computationally expensive) Ewald summation yielded essentially the same results. A spherical cutoff distance of 1.2 nm was employed for all the interactions. To obtain each data point in the adsorption isotherm, the GCMC simulations included 1 109 and 4 108 Monte Carlo moves for adsorption in the rigid and flexible NT, respectively. The absorbed amount was obtained by averaging over data taken from the latter 70% of the configurations generated in each simulation. An individual Monte Carlo attempt included an insertion, deletion, translation, or rotation of a randomly selected molecule. The molecules were only allowed to be inserted within the NT pore, but not elsewhere in the unit cell. The fugacity coefficients of SPC water around 298 K are close to unity at pressures up to 4 kPa.26,27 We report our results by treating all vapors as ideal gases with the fugacity equal to the pressure.

of water in an aluminosilicate NT with all atoms fixed except the surface hydroxyl groups. Below, we use the term “flexible NT” for this model. Preliminary calculations showed that simulations with this model and with a model that allowed all atoms in the NT to move were in close agreement. Our results show that the flexibility of the hydroxyl groups plays a critical role in the adsorption of molecules that can form hydrogen bonds with the NT's inner surface. The details of construction and optimization of the NT structural models were described previously.9,14-16 The NT model used here has 12 gibbsite units in the circumference, which is the minimum energy structure observed both from experiments and simulations.14,15,17 Adsorption isotherms in rigid and flexible aluminosilicate NTs were calculated at 298 K for water and methanol and 273 K for CO2 and CH4 to compare with experimental results, using the GCMC method as incorporated in the MUSIC simulation code.18 The CLAYFF19 force field, which was well validated for a range of aluminosilicate layered minerals as well as for aluminosilicate/aluminogermanate NTs with Lennard-Jones (LJ) potentials and fractional charges assigned to each atom, has been used in our previous work on adsorption of water in a rigid aluminosilicate NT9 and diffusion of water and simple alcohol molecules in the aluminosilicate NT.9,16 The nonbonded interactions between the hydroxyl groups and the rest of NT allow us to treat the hydroxyl groups as another adsorbate in the system. In our Monte Carlo sampling procedure, a hydroxyl group can be chosen for a translational or rotational move, but not an insertion or deletion. These NVT Monte Carlo steps for hydroxyl groups make it possible to study their flexibility effects using the same CLAYFF19 force field. Although the hydroxyl groups were allowed to move, they did not detach from the NT, as also confirmed in our earlier molecular dynamics simulations with a fully flexible aluminosilicate NT9,14-16 with the same force field. The unit cell employed in our adsorption simulations had an axial dimension of 3.36 nm (four times the axial repeat distance of the NTstructure), a size of 3.0 nm in the other two dimensions, and contained a


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Figure 3. Energy-minimized configuration of a single water molecule in a (a) rigid and (b) flexible aluminosilicate NT with the water molecule and hydroxyl (-OH) groups emphasized. Blue: hydrogen; red: oxygen; gray: silicon. Selected O 3 3 3 H distances are shown in Å.

The effects of structural flexibility on water adsorption in the aluminosilicate NT are shown in Figure 2. An adsorption amount of 1 molecule/unit cell of water (methanol) corresponds to a content of 0.095 (0.17) wt % as used in Figure 2. In the rigid NT, the adsorption of water is insignificant until vapor pressures larger than 2 kPa are reached. This deviates considerably from the experimentally observed behavior, which shows that the NTs have a strong affinity for polar molecules.9,28,29 If the flexibility of the NT is considered, significant adsorption is observed at 0.1 kPa, i.e., an order of magnitude lower pressure than in the rigid NT. To probe the role of hydroxyl flexibility in the isotherms in Figure 2, we used the DLPOLY molecular simulation package30 to optimize the configuration of a single water molecule in rigid and flexible aluminosilicate NTs. The force field parameters were the same as those used in GCMC simulations. We then compared the total energy of these two systems. The optimization method was similar to our earlier work on calculating the potential energy surface of an isolated water molecule in the NT16 except that the whole water molecule was allowed to move in the present case. We found that the total minimized system energy decreases by 33.3 kJ/ mol of adsorbed water when flexibility effects are considered. The most stable structures of water in the rigid and flexible NT structures, as shown in Figure 3, are helpful to understand this energy difference. The NT's hydroxyl groups prefer to form hydrogen bonds with adsorbed water molecules, and thus have quite different orientations from their “ground states” in the absence of water molecules when they are flexible. To quantify this effect, a water molecule and the silanol group were considered to have formed a hydrogen bond if the O 3 3 3 O distance was less than 3.6 Å and the O 3 3 3 O-H angle (with the O-H atoms coming from either a water molecule or a hydroxyl group) was less than 30°.31 Awater molecule in the flexible NT (Figure 3b) can form up to three hydrogen bonds with the surface silanol groups. The distances between the acceptor O and donor H atoms are 1.75, 1.79, and 1.75 Å, respectively, and the three O 3 3 3 O-H angles are 6.4, 0, and 15.5°, respectively. For the rigid NT (Figure 3a), the corresponding values are 2.32, 2.04, and 1.81 Å, and 11.3, 21.2, and 41.7°, indicative of weaker hydrogen bonds. Figure 2 shows that the simulations of water adsorption in the flexible NTare in better agreement with experimental data in comparison to the simulations of water adsorption in the rigid NTat low and moderate loadings. At high loadings in real


Figure 4. Number of hydrogen bonds formed between water molecules and the inner-surface hydroxyl groups, with and without consideration of the O 3 3 3 O-H angle criterion for defining hydrogen bonds.

materials, water can absorb into the interstices between NTs and swell bundles of NTs. Because these effects were not considered in our calculations, our results cannot reproduce the entire adsorption isotherm observed experimentally. Using the criteria defined above, we calculated the number of hydrogen bonds formed between adsorbed water molecules and the silanol groups in rigid and flexible NTs. We also calculated the number of water molecules close to the hydroxyl groups, and thus available for forming hydrogen bonds, by only considering the O 3 3 3 O distance criterion. These results, normalized by the number of hydroxyl groups on the inner surface, are shown in Figure 4. For the flexible NT, the average number of hydrogen bonds formed by a hydroxyl group increases sharply at 0.1 kPa, and saturates to around 1.6. For the rigid NT, the number of hydrogen bonds is negligible until ∼2 kPa and then increases to 0.6 at higher pressures. Although the adsorbed amounts of water at high pressure are similar for rigid and flexible NTs (see Figure 2), the average number of hydrogen bonds formed is very different. Without considering the O 3 3 3 O-H bond angles, the calculated number of hydrogen bonds shows similar pressure dependence as when we considered both the distance and angle criteria. At higher pressures, both the rigid and flexible NTs have similar numbers of water molecules close enough to the inner surface and available for hydrogen bonding. The flexibility of the hydroxyl groups allows more than 70% of them to actually form hydrogen bonds with the NT surface, in comparison to only ∼30% for the rigid NT surface. A single hydroxyl group on the NT surface is surrounded by a hexagon of neighboring hydroxyl groups, and it therefore forms the common vertex for six triangles defined by three hydroxyl groups each (see Figure 1b). A hydroxyl group can form, at most, three hydrogen bonds as explained earlier. This will require three water molecules to be adsorbed near the hydroxyl group, thus occupying three of the six triangles. Near the saturation loading of water in the flexible NT, we find that there are ∼2.4 water molecules whose oxygen atoms are within 3.6 Å of the central hydroxyl oxygen atom. Water molecules cannot occupy all the available “triangular” surface sites even at the highest pressures. This observation is

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Figure 5. Binary adsorption isotherms of water (squares) and methanol (circles) with 1:9 bulk phase composition and adsorption selectivity of water (triangles) in rigid (open symbols) and flexible (filled symbols) aluminosilicate NTs.

Figure 6. Single component adsorption isotherms of CH4 and CO2 in rigid and flexible NTs at 273 K, and a comparison with experimental results.33

nanocomposite membranes incorporating aluminosilicate NTs. The technologically important separation of water from higher alcohols such as ethanol and butanol is also expected to be attractive with the present NT materials. We note that the bulk water/methanol mixtures considered here are in the vapor phase. A vapor phase mixture with 10 mol % of water and a partial pressure of 1.24 kPa is in equilibrium with liquid mixtures containing 36 mol % of water at 298 K, as calculated using van Laar activity coefficients.32 Single-component adsorption isotherms of the non-hydrogen-bonding molecules CO2 and CH4 in a rigid and flexible NT are shown in Figure 6. As expected, the flexibility effects are more important for CO2 than CH4, but the effects for both species are weaker than for water or methanol. Our adsorption data predict a lower sorption capacity than the limited experimental data available.33 This difference may arise from the NT packing in real materials. Our simulation set up allows sorption in the pore of an isolated NT only. The possible presence of water molecules even under degassed conditions may also enhance the adsorption of CO2 in the experimental data. These differences may also indicate the necessity of further optimization of the force field parameters used in the present study. The calculated binary adsorption of CO2/CH4 mixtures from an equimolar bulk phase composition in rigid and flexible aluminosilicate NTs, as well as CO2 adsorption selectivities, are shown in the Supporting Information. The hydroxyl flexibility enhances the CO2 selectivity from 5 in the rigid NT to about 9 in the flexible NT at high pressures. In conclusion, we have studied the effects of flexibility of the ordered silanol surface of single-walled aluminosilicate NTs on the adsorption of hydrogen-bonding and non-hydrogen-bonding molecules. We find that the surface structure of this NT necessitates the inclusion of hydroxyl flexibility for reliable adsorption predictions for hydrogen-bonding molecules. Specifically, the flexibility of the hydroxyl groups on the inner surface of the NT has a critical role in determining the adsorption behavior and in establishing the hydrophilic character of the NT. The arrangement of the silanol groups on the inner surface of the NT appears to lead to a “frustrated” hydrogen-bonding network. The NTs are predicted to have a high adsorption selectivity for water over methanol.

indicative of a “frustrated” system, wherein the strong orientational requirements for hydrogen bonding preclude configurations with the maximum number of hydrogen bonds for all the hydroxyl groups on the surface. Figure 5 shows the calculated adsorption isotherms of binary water/methanol mixtures with 1:9 bulk phase composition in rigid and flexible aluminosilicate NTs. This methanol-rich composition was chosen for its relevance in the dehydration of alcohols. In the rigid NT, the adsorption selectivity for water increases rapidly after the significant increase of water and methanol adsorption at 0.6 kPa partial pressure of water. With further increase in pressure, the adsorption of water continues to increase whereas that of methanol decreases, resulting in selectivities as high as 300 at the highest pressure we considered. When flexibility effects are considered, both the adsorption isotherms and selectivity curve show similar characteristics to those in rigid NT. The main difference is that all these curves shift toward lower pressures. This shift was also observed in the single component adsorption of water (Figure 2) and methanol, and in their ideal adsorption selectivity (Supporting Information). The simulations of mixture adsorption in the flexible NT experience difficulty in reaching equilibrium at higher pressures (see Supporting Information). In these cases, which are marked with arrows in Figure 5, our simulation data only provides a lower bound on the true equilibrium selectivity. Our results using a flexible NT indicate that the lowerbound water selectivity is more than 100 during mixture adsorption when the partial pressure of water is larger than 1 kPa. Our earlier work on water and methanol diffusion in the NT showed that the ratio of self-diffusivity of water to methanol in their mixtures is close to unity at high loadings.16 Thus, the flexibility of the NTsurface imparts a high selectivity due to preferential sorption, whereas our earlier work found that the ∼1 nm inner diameter of the NTallows a higher throughput in comparison to other nanoporous materials for water/alcohol separation such as small-pore zeolites.9,16 Our results present a strong motivation to experimentally fabricate and test


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SUPPORTING

INFORMATION AVAILABLE Calculated single-component adsorption isotherms of water and methanol, and ideal selectivity of water over methanol, in an aluminosilicate NT. Adsorbed amounts of water and methanol from a 1:9 mixture with 0.06 and 0.2 kPa partial pressure of water, as a function of the number of simulation steps, starting from different initial conditions. Calculated binary adsorption isotherms of CO2/CH4 mixtures with equimolar bulk phase composition, as well as CO2 adsorption selectivity over CH4 in an aluminosilicate NT. Sorption enthalpies of water from experimental data fits. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected].

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ACKNOWLEDGMENT This work was partially supported by the NSF under Awards NSF-#0846586 (CAREER) and CBET-0709090.

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REFERENCES (1)

Mukherjee, S.; Bartlow, V. A.; Nair, S. Phenomenology of the Growth of Single-Walled Aluminosilicate and Aluminogermanate Nanotubes of Precise Dimensions. Chem. Mater. 2005, 17, 4900–4909. (2) Mukherjee, S.; Kim, K.; Nair, S. Short, Highly Ordered, SingleWalled Mixed-Oxide Nanotubes Assemble from Amorphous Nanoparticles. J. Am. Chem. Soc. 2007, 129, 6820–6826. (3) Vlugt, T. J. H.; Schenk, M. Influence of Framework Flexibility on the Adsorption Properties of Hydrocarbons in the Zeolite Silicalite. J. Phys. Chem. B 2002, 106, 12757–12763. (4) Uemura, T.; Kadowaki, Y.; Yanai, N.; Kitagawa, S. Template Synthesis of Porous Polypyrrole in 3d Coordination Nanochannels. Chem. Mater. 2009, 21, 4096–4098. (5) Maji, T. K.; Kitagawa, S. Chemistry of Porous Coordination Polymers. Pure Appl. Chem. 2007, 79, 2155–2177. (6) Mentzen, B. F.; Lefebvre, F. Flexibility of the MFI Silicalite Framework Upon Benzene Adsorption at Higher Pore-Fillings: A Study by X-Ray Powder Diffraction, NMR and Molecular Mechanics. Mater. Res. Bull. 1997, 32, 813–821. (7) Kuznicki, S. M.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R. M.; Braunbarth, C. M.; Toby, B. H.; Tsapatsis, M. A Titanosilicate Molecular Sieve with Adjustable Pores for Size-Selective Adsorption of Molecules. Nature 2001, 413, 652–652. (8) Turaga, S. C.; Auerbach, S. M. Calculating Free Energies for Diffusion in Tight-Fitting Zeolite-Guest Systems: Local Normal-Mode Monte Carlo. J. Chem. Phys. 2003, 118, 6512–6517. (9) Konduri, S.; Tong, H. M.; Chempath, S.; Nair, S. Water in Single-Walled Aluminosilicate Nanotubes: Diffusion and Adsorption Properties. J. Phys. Chem. C 2008, 112, 15367– 15374. (10) Di Lella, A.; Desbiens, N.; Boutin, A.; Demachy, I.; Ungerer, P.; Bellat, J. P.; Fuchs, A. H. Molecular Simulation Studies of Water Physisorption in Zeolites. Phys. Chem. Chem. Phys. 2006, 8, 5396–5406. (11) Trzpit, M.; Soulard, M.; Patarin, J.; Desbiens, N.; Cailliez, F.; Boutin, A.; Demachy, I.; Fuchs, A. H. The Effect of Local Defects on Water Adsorption in Silicalite-1 Zeolite: A Joint Experimental and Molecular Simulation Study. Langmuir 2007, 23, 10131–10139.


(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28) (29)

(30)

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Han, J. W.; Sholl, D. S. Enantiospecific Adsorption of Amino Acids on Hydroxylated Quartz (0001). Langmuir 2009, 25, 10737–10745. Yang, J. J.; Meng, S.; Xu, L. F.; Wang, E. G. Water Adsorption on Hydroxylated Silica Surfaces Studied Using the Density Functional Theory. Phys. Rev. B 2005, 71, 035413–12. Konduri, S.; Mukherjee, S.; Nair, S. Strain Energy Minimum and Vibrational Properties of Single-Walled Aluminosilicate Nanotubes. Phys. Rev. B 2006, 74, 033401–4. Konduri, S.; Mukherjee, S.; Nair, S. Controlling Nanotube Dimensions: Correlation between Composition, Diameter, and Internal Energy of Single-Walled Mixed Oxide Nanotubes. ACS Nano 2007, 1, 393–402. Zang, J.; Konduri, S.; Nair, S.; Sholl, D. S. Self-Diffusion of Water and Simple Alcohols in Single-Walled Aluminosilicate Nanotubes. ACS Nano 2009, 3, 1548–1556. Guimaraes, L.; Enyashin, A. N.; Frenzel, J.; Heine, T.; Duarte, H. A.; Seifert, G. Imogolite Nanotubes: Stability, Electronic, and Mechanical Properties. ACS Nano 2007, 1, 362–368. Gupta, A.; Chempath, S.; Sanborn, M. J.; Clark, L. A.; Snurr, R. Q. Object-Oriented Programming Paradigms for Molecular Modeling. Mol. Simul. 2003, 29, 29–46. Cygan, R. T.; Liang, J. J.; Kalinichev, A. G. Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255–1266. Creton, B.; Bougeard, D.; Smirnov, K. S.; Guilment, J.; Poncelet, O. Molecular Dynamics Study of Hydrated Imogolite - 2. Structure and Dynamics of Confined Water. Phys. Chem. Chem. Phys. 2008, 10, 4879–4888. Chen, B.; Potoff, J. J.; Siepmann, J. I. Monte Carlo Calculations for Alcohols and Their Mixtures with Alkanes. Transferable Potentials for Phase Equilibria. 5. United-Atom Description of Primary, Secondary, and Tertiary Alcohols. J. Phys. Chem. B 2001, 105, 3093–3104. Skoulidas, A. I.; Ackerman, D. M.; Johnson, J. K.; Sholl, D. S. Rapid Transport of Gases in Carbon Nanotubes. Phys. Rev. Lett. 2002, 89, 185901–4. Makrodimitris, K.; Papadopoulos, G. K.; Theodorou, D. N. Prediction of Permeation Properties of CO2 and N2 through Silicalite Via Molecular Simulations. J. Phys. Chem. B 2001, 105, 777–788. Wolf, D.; Keblinski, P.; Phillpot, S. R.; Eggebrecht, J. Exact Method for the Simulation of Coulombic Systems by Spherically Truncated, Pairwise R-1 Summation. J. Chem. Phys. 1999, 110, 8254–8282. Fennell, C. J.; Gezelter, J. D. Is the Ewald Summation Still Necessary? Pairwise Alternatives to the Accepted Standard for Long-Range Electrostatics. J. Chem. Phys. 2006, 124, 234104–12. Wierzchowski, S. J.; Kofke, D. A. Fugacity Coefficients of Saturated Water from Molecular Simulation. J. Phys. Chem. B 2003, 107, 12808–12813. Wierzchowski, S. J.; Kofke, D. A. Fugacity Coefficients of Saturated Water from Molecular Simulation. J. Phys. Chem. B 2004, 108, 9375–9376. Wada, K.; Yoshinaga, N. Structure of Imogolite. Am. Mineral. 1969, 54, 50–71. Ohashi, F.; Tomura, S.; Akaku, K.; Hayashi, S.; Wada, S. I. Characterization of Synthetic Imogolite Nanotubes as Gas Storage. J. Mater. Sci. 2004, 39, 1799–1801. Smith, W.; Forester, T. R. DL_POLY, A Package of Molecular Simulation Routines; The Council for the Central Laboratory of the Research Councils: Daresbury Laboratory: Daresbury,


pubs.acs.org/JPCL

(31) (32) (33)

Warrington, U.K., 1996; http://www.cse.scitech.ac.uk/ccg/ software/DL_POLY/. Joseph, S.; Aluru, N. R. Why Are Carbon Nanotubes Fast Transporters of Water? Nano Lett. 2008, 8, 452-458. Wilson, E. D.; Ries, H. C. Principles of Chemical Engineering Thermodynamics; McGraw Hill: New York, 1956. Ackerman, W. C.; Smith, D. M.; Huling, J. C.; Kim, Y. W.; Bailey, J. K.; Brinker, C. J. Gas Vapor Adsorption in Imogolite: A Microporous Tubular Aluminosilicate. Langmuir 1993, 9, 1051–1057.


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Flexibility of Ordered Surface Hydroxyls Influences the Adsorption of

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