Selective Adsorption of Isopropyl Alcohol Aqueous Solution on

ARTICLE pubs.acs.org/JPCC

Selective Adsorption of Isopropyl Alcohol Aqueous Solution on Polypropylene Surfaces: A Molecular Dynamics Simulation Zheng-Wei Dai, Ling-Shu Wan, Xiao-Jun Huang, Jun Ling, and Zhi-Kang Xu* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China

bS Supporting Information ABSTRACT: Surface modification for hydrophilization is a versatile approach to significantly improve the separation performance of membranes prepared from hydrophobic polymers such as polypropylene (PP). This improvement is closely related to changes in the chemical characteristics of membrane surface and alterations in the adsorption behaviors of feed components. We simulated the interaction of isopropyl alcohol aqueous solution with nascent and hydrophilized PP surfaces by molecular dynamics to understand the molecular mechanism of these variations. The results were used to evaluate the selective adsorption of isopropyl alcohol on the model surfaces. The isopropyl alcohol molecule shows surfactant-like behaviors due to one hydroxyl group as the polar end and two methyl groups as the apolar end. For nascent PP surface, dispersive force plays a key role in the interaction of isopropyl alcohol molecules with model surface. Isopropanol is therefore selectively adsorbed from the aqueous solution, and water is repelled out of the interfacial layer. For the hydrophilized surfaces, however, the electrostatic interaction between water molecules and surface polar groups exceeds the effect of dispersive force. As a result, water molecules are attracted onto the modified surface and hydration layers are observed.

1. INTRODUCTION Surface hydrophilization is employed to improve the separation performance of membranes made from hydrophobic polymers such as polypropylene (PP), polyethylene (PE), and poly(vinylidene fluoride).1,2 The surface properties of a membrane can be significantly changed by introduction of carboxyl (
COOH), amino (
NH2), hydroxyl (
OH), and other polar groups. While affinity of the membrane surface with water molecules has been improved, its interactions with other components (including ions, biomacromolecules, and low molecular weight organic compounds) in the feed solution are also changed. Such difference is important for membrane processes such as pervaporation and desalination, in which composition of the feed solution at the solid/liquid interface directly affects the diffusion potential of each component for separation. Therefore, it is a challenge to understand the influence of surface modification on adsorption of various feed components onto the membrane surface from the principal aspect of molecular interactions. This knowledge is a base to optimize the hydrophilization method and achieve a membrane with robust separation performance. Adsorption at the solid/liquid interface has been the subject of various research fields. Experimentally, the properties of adsorbates can be investigated by surface potentiometric titration,3 zeta potential measurement,4 and ion strength dependency,5 as well as the more recently developed methods such as quartz crystal microbalance6 and surface plasma resonance.7 However, most of these methods are according to theories based on r 2011 American Chemical Society

material continuity. It still remains a challenge to understand atomistic details of the adsorbates such as molecular orientation or conformation. In this field, molecular simulation proves to be an indispensable tool and has been widely used to study various adsorbates such as ions,8
13 surfactants,14
16 polymer chains,17
20 biomacromolecules,21
27 and low molecular weight organic compounds.28
34 The types of solid surface include ideal surfaces,15,16 metallic surfaces,35 inorganic surfaces,24,36
38 and self-assembled monolayer surfaces.39
43 Previously, molecular dynamics (MD) was used to study the influence of surface hydrophilization on the interaction of PP surface with water.44 In this work, we investigated the impact of surface hydrophilization on the adsorption of isopropyl alcohol solution in water. Isopropyl alcohol was chosen as a representative of low molecular weight organic compounds because it is an important industrial solvent but forms an azeotrope at 87.5 wt % in aqueous solution.45,46 Pervaporation has been developed as a promising membrane technology for the dehydration of organic solvents such as isopropyl alcohol. Furthermore, it has been used for characterizing the maximum pore size in a given membrane by bubble-point method (ASTM F 316-03). Therefore, its adsorption on the membrane surface is of both scientific and industrial significance. We evaluate the interaction of isopropyl alcohol molecules with PP surfaces in the first part of this work. Received: July 15, 2011 Revised: October 2, 2011 Published: October 06, 2011 22415

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Figure 1. Periodic cell for the simulation of isopropyl alcohol/surface interaction.

Following that, we discuss the selective adsorption of isopropyl alcohol solution on these surfaces. Our results reveal that difference in the adsorption behaviors of low molecular weight organic compounds is caused by the surface hydrophilization of a membrane. We also prove the assumption of hydration layer on the hydrophilized membrane surfaces from an analysis on the molecular composition and orientation of interfacial liquid layer. This hydration layer has been taken as the theoretical foundation of surface modification so far but is still difficult to be directly validated experimentally.

2. SIMULATION METHODOLOGY The MD simulations were performed on NAMD47 with the all atomic force field CHARMM,48 which has been proved applicable for the simulation of organic molecules and polymeric systems. The simulation results were visualized by VMD.49 The molecular model of PP surface was constructed from the parallel arrangement of 10 PP chains in the same plane, each with 12 polypropylene units. To maintain its planar conformation during the simulation, the movement of methenyl carbons of PP chains was constrained near the plane of z = 0 by a harmonic constraint along the z axis with the force constant of 900 kcal/(mol 3 Å2). The force constant was chosen to make sure that the methenyl carbons could not move too far away from their equilibrium positions. The surface model was then subjected to simulated annealing to achieve the amorphous conformation (Supporting Information, Figure S1). Three different initial conformations were prepared independently, and all the following results were averaged over the models derived from the three initial conformations. The hydrophilized PP surfaces were constructed by replacing the hydrogen atoms of the methenyl groups with certain polar groups.44 Carboxyl groups and amino groups were chosen in this work due to their popularity in the hydrophilization modification of PP membranes. The model of unmodified PP surface is denoted by M
H; the hydrophilized PP model surfaces are denoted by M
COOH and M
NH2 according to the polar groups introduced. To investigate the influence of surface charge, the disassociated form of the polar groups, i.e., the carboxylate groups and the ammonium groups, were also introduced, and the resulting model surfaces are denoted by M
COOi and M
NH3i with sodium and chloride ions as counterions, respectively. All the hydrophilized model surfaces were further relaxed for 500 ps to eliminate the internal tension. The first part of the simulation was intended to investigate the interaction of isopropyl alcohol with the model surfaces. The simulation cell was illustrated in Figure 1. The height of

Figure 2. Snapshots of the interfacial isopropyl alcohol layer on the surfaces of (a) M
H, (b) M
NH2, and (c) M
NH3i. The red spheres represent the hydroxyl groups of isopropyl alcohol molecules.

the isopropyl alcohol box was 100 Å with an initial density of 0.786 g/cm3. The size of the periodic cell was set as 60  60  200 Å to avoid the interference of neighboring images in the vertical direction. The molecular simulation was carried out in the canonical ensemble (NVT) with the temperature of 298 K and time step of 2 fs. A 1000 ps run was required for the system to reach the equilibrium state. Following that, another 200 ps simulation was carried out with the trajectories recorded for further analysis and discussions. In the next part, we studied the interaction of isopropyl alcohol solution and model surfaces in a similar periodic cell with the isopropyl alcohol phase replaced by the mixture of isopropyl alcohol/water, in which the TIP3P water model was used.50 The molar ratio of water molecule to isopropyl alcohol molecule was 1:1. The simulation protocol was the same as that in the first part. To investigate the distribution of liquid molecules on the surfaces, the interfacial liquid layer was divided into slabs parallel to the surface with the thickness of 0.2 Å. The mass distribution profiles were calculated according to eq 1, in which dz represents the half thickness of the slab and Σmz(dz is the mass sum of the object molecules locating in the slab. The resulting mass distribution profile D(z) was achieved from the average of all the recorded frames and converted into the density distribution profile.   mz ( dz ð1Þ DðzÞ ¼ ÆDn ðzÞæ ¼ 2dz

∑

The mobility of water and isopropyl alcohol molecules was studied by mean square displacement (MSD), as illustrated in eq 2, where Na is the number of object molecules and rj(0), rj(t) are the initial and final coordinates of the mass center of the object molecule j over the time interval of t, respectively. MSD ¼

1 Na jrj ðtÞ
rj ð0Þj2 Na j ¼ 1

∑

ð2Þ

The determination of hydrogen bonds was according to the geometric criterion. According to the results from the ab initio 22416

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Figure 3. Density distribution profiles of interfacial isopropyl alcohol layer.

calculations, the distance cutoff between the donor atom and the acceptor atom is 3.10 Å, and the angle cutoff of donor
hydrogen
acceptor is 146.51,52

3. RESULTS AND DISCUSSION 3.1. Influence of Surface Hydrophilization on the Structure of Interfacial Isopropyl Alcohol Layer. Figure 2 shows the

typical snapshots of interfacial isopropyl alcohol layer on the nascent PP surface and the hydrophilized PP surfaces in an equilibrium state. From these snapshots it can be noticed that the structure of interfacial isopropyl alcohol layer is quite different from the bulk. Both the distribution of isopropyl alcohol molecules and their orientation are closely related with the chemical characteristics of the model surface. Figure 3 displays the density distribution profiles of isopropyl alcohol molecules on the studied surfaces. The density profiles were calculated by the accumulative mass of all the atoms in consideration. This atom-by-atom method offers density profiles more precisely presenting the mass distribution than that considering only molecular mass center. To make comparisons among the surfaces, the vertical coordinate (z) was calibrated to distance from the surface (δ), and the origin plane of the surface (where δ = 0) was defined as the plane beneath where 98 wt % of the atoms of PP chains distributed. Similar to the cases of water reported in our previous paper,44 the distribution profiles of isopropyl alcohol along the z axis show a damped oscillation pattern with a period of approximately 4
5 Å. This period of oscillation is larger than that of water (2
3 Å), indicating that it is related to the size of liquid molecules. Improvement in surface hydrophilicity not only increases peak height of the density profile but also shifts peak location toward the PP surface. However, the influence is not as significant as that of water, especially for the PP surfaces with charges. It is reasonable for this difference because isopropyl alcohol has weaker molecular polarity and larger molecular size than water. Molecular polarity restrains the disassociation of surface ions, whereas molecular size impedes the access of isopropyl alcohol molecules to the surface. On the other hand, due to the surfactant-like characteristics of isopropyl alcohol and better molecular alignment among layers, oscillation of the profiles extends more deeply into the bulk phase than that of water,44 resulting in an interfacial region with large thickness. More over, the difference of the molecular

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Figure 4. Distribution of the orientation angles of C
O bonds of interfacial isopropyl alcohol molecules.

polarity of water and isopropyl alcohol also has a significant influence on the distribution of surface ions (Supporting Information, Figure S2). We further investigated the structure of interfacial isopropyl alcohol molecules in the first peak of density distribution profiles. The molecular structure of isopropyl alcohol is composed of a polar hydroxyl group and two apolar methyl groups, endowing the isopropyl alcohol molecule with surfactant-like properties. We chose the vector of C
O bond (from the carbon atom to the oxygen atom), pointing from the hydrophobic end to the hydrophilic end, to indicate the alignment of interfacial isopropyl alcohol molecules. The orientation angle of isopropyl alcohol molecules was then defined as the angle between the vector of the C
O bond and the positive direction of the z axis. Distribution of the orientation angles is displayed in Figure 4. It can be seen that the distribution profiles exhibit considerable difference for various PP surfaces. On the hydrophobic surface of M
H, the orientation angles distribute mostly below 90, indicating that the isopropyl alcohol molecules are adsorbed to the surface by their hydrophobic end of methyl groups, as shown in Figure 4a. This reveals that the order of interfacial isopropyl alcohol molecules on the surface of M
H is determined by the hydrophobic interaction. When the surface is hydrophilized, as for M
NH2 and M
COOH, a substantial amount of interfacial isopropyl alcohol molecules are reversed in their orientation, as shown in Figure 4b, so that the hydroxyl groups could form hydrogen bonds with the polar groups on the hydrophilized PP surface. The orientation profile then turns to a bimodal pattern, with one peak above 90 and the other below 90. When the surface hydrophilicity is increased further by surface charges, as in the cases of M
COOi and M
NH3i, the peak below 90 disappears, indicating that the effect of hydrophobic interaction is inhibited. For M
COOi, the effect of Coulombic force is against the effect of hydrophilic interaction in direction, while for M
NH3i, the two effects are of the same direction, as displayed by parts c and d of Figure 4, respectively. As a result, the distribution profile for M
NH3i is more concentrated than that for M
COOi. 3.2. Influence of Surface Hydrophilization on the Interaction Potentials of Isopropyl Alcohol with the Model Surfaces. One could attribute the difference in molecular structures and behaviors of interfacial isopropyl alcohol to a change of isopropyl alcohol/surface interactions caused by surface hydrophilization. The interaction potential of the surface models with 22417

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Figure 5. Interaction potential between isopropyl alcohol and the model surfaces. Figure 7. Snapshots of the interfacial isopropyl alcohol solution layer in the equilibrium state, (a) for M
H, (b) for M
NH2, and (c) for M
NH3i. The blue lines represent isopropyl alcohol molecules, the red spheres represent the oxygen atoms of water, and the white spheres represent the hydrogen atoms of water.

Figure 6. Average hydrogen bond number per isopropyl alcohol molecule. Niib, isopropyl alcohol
isopropyl alcohol hydrogen bonds in bulk isopropyl alcohol; Niis, isopropyl alcohol
isopropyl alcohol hydrogen bonds in interfacial layer; Nis, isopropyl alcohol
surface hydrogen bond.

the liquid molecules in each system was calculated, and the results are displayed in Figure 5, where it is resolved into two components, i.e., the van der Waals (VDW) interaction and the electrostatic interaction. The VDW component is the major composition in hydrophobic interaction, while the electrostatic component is considered to be the indication of hydrophilic interaction. As shown in Figure 5, the VDW potential shows little variation among the neutral surfaces of M
H, M
NH2, and M
COOH. The effect of surface hydrophilicity is mainly on the electrostatic component, which is only a minor part of the isopropyl alcohol/surface interaction for M
H but becomes a substantial part for M
NH2 and M
COOH. However, the role of electrostatic interaction between the isopropyl alcohol molecules and the surfaces of M
NH2 and M
COOH is not as dominating as that in the water/surface interaction, indicating that the hydrophobic interaction still has a considerable influence on the interaction of isopropyl alcohol molecules with the hydrophilized PP surfaces. This fact can be confirmed by those results on the structure of interfacial isopropyl alcohol molecules. The hydrophilic attraction consists mainly of the interaction of dipoles, hydrogen bonding, and Coulombic force. Due to the weak molecular polarity of isopropyl alcohol, hydrogen bonding is a major component of the hydrophilic interaction between isopropyl alcohol molecules and the neutrally hydrophilized

Figure 8. Density distribution profiles of interfacial isopropyl alcohol solution on various surfaces, with the solid lines for water, the dashed lines for isopropyl alcohol, and the dotted lines for the mass ratio of water.

PP surfaces. The density of hydrogen bonds in the interfacial region and bulk phase is shown in Figure 6. The interfacial region 22418

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Figure 9. Illustration on the distribution and orientation of water and isopropyl alcohol molecules.

Figure 10. Mobility of water molecules and isopropyl alcohol molecules in the interfacial layer. The mean square displacement of the molecules (MSDxyz) is resolved to two components in the direction of x
y plane (MSDxy) and z axis (MSDz).

is defined as the first peak of density distribution profiles. For the same reason as that in the previous study,44 geometric criterion was employed for the determination of hydrogen bonds. Unlike water, however, interfacial isopropyl alcohol molecules show no obvious loss of hydrogen bonds on the hydrophobic surface of M
H compared with those in the bulk. This is caused by the highly ordered orientation, by which the isopropyl alcohol molecules in the interfacial layer are aligned with their hydroxyl groups exposed outward, forming hydrogen bonds with the isopropyl alcohol molecules in the outer layer. When PP surface is hydrophilized, some of the interfacial isopropyl alcohol molecules are turned over with their hydroxyl groups adsorbed on the surface, losing the hydrogen bonds formed with the outer layer. However, new hydrogen bonds are established between these adsorbed hydroxyl groups and the polar groups on the surface. The loss is thereby compensated, which is consistent with the cases of water. 3.3. Influence of Surface Hydrophilization on the Selective Adsorption of Isopropyl Alcohol Aqueous Solution. The previous results, together with our previous work,44 demonstrated the difference of water and isopropyl alcohol in the

interactions with the nascent PP surface and the hydrophilized PP surfaces. This difference induces selective adsorption of isopropyl alcohol from its aqueous solution. Figure 7 shows snapshots of interfacial regions of various surfaces, and Figure 8 displays the density distribution profiles of interfacial water molecules and isopropyl alcohol molecules on the surfaces. On the hydrophobic surface of M
H, isopropyl alcohol molecules are preferentially adsorbed on the model surface by their methyl groups and their hydroxyl groups point into the liquid phase. At the same time, water molecules are mostly repelled out of the interfacial region, and some of them aggregate outside the first adsorption shell of isopropyl alcohol, forming hydrogen bonds with the hydroxyl groups of isopropyl alcohol molecules. Therefore, the surface excess of water is negative while that of isopropyl alcohol is positive. With the increase of surface hydrophilicity, water molecules are pulled into the interfacial region by the intensive attracting force of surface polar groups, and a hydration layer is extracted from the homogeneous liquid phase of aqueous solution. The selectivity of surface adsorption is therefore reversed, with the surface excess of water turning positive and that of isopropyl alcohol turning negative, as revealed by the profiles of water mass ratio in Figure 8. The distribution and orientation of water and isopropyl alcohol molecules are illustrated in Figure 9. The mobility of interfacial liquid molecules is also greatly influenced by surface hydrophilization, which is illustrated in Figure 10 by mean square displacement. Only molecules in the first peak of density distribution profiles are considered. Water molecules on the hydrophobic surface of M
H show very high mobility, which is even higher than those in the bulk phase. The reason could be attributed to the weak affinity of M
H surface and the hydrophobic environment created by the adsorption shell of isopropyl alcohol molecules. Due to these factors, the binding force on water molecules from the surrounding hydrogen bonding network is released when they migrate into the interfacial region from the bulk phase, resulting in more freedom of molecular mobility. With the increase of surface hydrophilicity, interfacial water molecules adsorb onto the surface by the attraction of surface polar groups. The mobility of interfacial water molecules is therefore decreased significantly. The increase of surface hydrophilicity also has negative influence on the mobility of interfacial isopropyl alcohol molecules, but the effect is not less significant than water because of the intensive hydrophobic interaction between isopropyl alcohol molecules 22419

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4. CONCLUSION We investigated the influence of hydrophilization on the selective adsorption of isopropyl alcohol aqueous solution onto PP surface by MD simulations. Our results reveal the existence of intensive hydrophobic interaction between the methyl groups of isopropyl alcohol and the molecular chains of PP. Therefore, isopropyl alcohol molecules are preferentially adsorbed onto the hydrophobic PP surface, and water molecules are repelled out of the interfacial region. When the hydrophilicity of the model surface is improved by the introduction of polar groups, the influence of hydrophilic interaction exceeds that of hydrophobic interaction. Thus water molecules are attracted onto the hydrophilized surface, forming a hydration layer from the homogeneous liquid phase of isopropyl alcohol solution. The adsorption selectivity of PP surface is then completely reversed by hydrophilization modification. ’ ASSOCIATED CONTENT

bS

Supporting Information. Distribution of the dissociative ions of surface models in contact with water and isopropyl alcohol; illustration of various initial configurations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected].

’ ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (Grant no. 50933006) and the National Basic Research Program of China (Grant no. 2009CB623401) is gratefully acknowledged. ’ REFERENCES (1) Wan, L. S.; Liu, Z. M.; Xu, Z. K. Soft Matter 2009, 5, 1775–1785. (2) Korikov, A. P.; Kosaraju, P. B.; Sirka, K. K. J. Membr. Sci. 2006, 279, 588–600. (3) Seredych, M.; Bandosz, T. J. J. Phys. Chem. C 2010, 114, 14552– 14560. (4) Schnitzer, C.; Ripperger, S. Chem. Eng. Technol. 2008, 31, 1696– 1700. (5) Goldberg, S.; Johnston, C. T. J. Colloid Interface Sci. 2001, 234, 204–216. (6) Cooper, M. A.; Singleton, V. T. J. Mol. Recognit. 2007, 20, 154–184. (7) Smith, E. A.; Thomas, W. D.; Kiessling, L. L.; Corn, R. M. J. Am. Chem. Soc. 2003, 125, 6140–6148. (8) Meleshyn, A. J. Phys. Chem. C 2008, 112, 20018–20026. (9) Zhang, Z.; Fenter, P.; Cheng, L.; Sturchio, N. C.; Bedzyk, M. J.; Pedota, M.; Bandura, A.; Kubicki, J. D.; Lvov, S. N.; Cummings, P. T.; Chialvo, A. A.; Ridley, M. K.; Bnzeth, P.; Anovitz, L.; Palmer, D. A.; Machesky, M. L.; Wesolowski, D. J. Langmuir 2004, 20, 4954–4969. (10) Kerisit, S.; Parker, S. C. J. Am. Chem. Soc. 2004, 126, 10152–10161. (11) Predota, M.; Bandura, A. V.; Cummings, P. T.; Kubicki, J. D.; Wesolowski, D. J.; Chialvo, A. A.; Machesky, M. L. J. Phys. Chem. B 2004, 108, 12049–12060.

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