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Water Mobility: A Bridge between the Hofmeister Series of Ions and the Friction of Zwitterionic Surfaces in Aqueous Environments Yi He,† Qing Shao,† Shengfu Chen,†,‡ and Shaoyi Jiang*,† † ‡
Department of Chemical Engineering, University of Washington, Seattle, Washington 98195, United States Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang, 310027 China
bS Supporting Information ABSTRACT: In this work, we systematically studied the effects of monovalent ions in the Hofmeister series on the friction of zwitterionic surfaces with carboxybetaine self-assembled monolayers (CB-SAMs) as model surfaces using molecular dynamics simulations. The friction coefficients between two CB-SAM surfaces under shear were calculated in the presence of LiCl, NaCl, KCl, CsCl, CsF, CsBr, or CsI solutions at 1 M. Results show that there is a strong correlation between the order of ions in the Hofmeister series and the friction of the CB-SAM surfaces. For salt solutions with the same anion, the friction of the surfaces in a solution with kosmotropic cations has larger friction than that in a solution with chaotropic ones. The same relationship between the order of ions in the Hofmeister series and the friction of the surfaces can be found for the salt solutions with the same cation but different anions. The analysis of water near the surfaces further suggests that these surfaces are hydrated with a similar amount of water molecules and the mobility of interfacial water is the key factor that bridges the relationship between the order of ions in the Hofmeister series and the friction of zwitterionic surfaces. High water mobility promotes the lubrication of zwitterionic surfaces.
’ INTRODUCTION Ultralow fouling and biocompatible zwitterionic materials and their derivatives have attracted a great amount of attention in the past few years.16 More recently, this family of materials has also been found to have superlow friction properties in aqueous solutions,4,7,8 which expands a new horizon for their applications. Moro et al. found that the friction coefficient of the polyethylene (PE) plate grafted with zwitterionic 2-methacryloyloxy ethyl phosphorylcholine (MPC) was roughly 1/4 of that of the unmodified PE plate.4 Using molecular simulations, we showed that phosphorylcholine self-assembled monolayer (PC-SAM) surfaces provide a superb low friction property comparable to that of an artificial joint.7 Friction in water-based systems can be affected by salt solutions.9 The simulations also predicted that the friction coefficient of the PC-SAM surfaces increases when a NaCl or KCl solution is used for lubrication instead of pure water.7 Moreover, the simulations show that higher NaCl concentration leads to a larger friction of the PC-SAM surfaces. Interestingly, another study reported that the Na+ concentration of synovial fluid is increased two to three-fold in people with arthritis as compared with healthy individuals.10 This suggests that a large ionic concentration is not preferred for joint because it increases friction. Klein et al. showed experimentally that mica sheets coated with brushes of MPC could have friction coefficients as low as 0.0004 at pressures as high as 7.5 MPa.8 Their experiments confirmed that salt solutions are detrimental to the lubrication of zwitterionic surfaces, as found in our previous r 2011 American Chemical Society
simulation studies. Although our previous studies have indicated some correlations between the hydration number of the surfaces and the friction for the zwitterionic PC-SAM systems, fundamental questions regarding the exact role of ions in friction remain unanswered. For instance, it is not clear how ions will interact with the charged groups on the zwitterionic surfaces. If strong binding between ions and surfaces occurs, then the hydration of these bound ions should be taken into account toward the hydration number of the surfaces as well. Furthermore, in addition to the hydration number, the relationship between the friction and the mobility of water molecules near the surfaces under shear also needs to be investigated. A fundamental understanding of the questions will facilitate the effort to develop ultralow friction and biocompatible zwitterionic-based materials needed for diverse applications such as artificial joint replacements and marine coating. The Hofmeister series11 ranks the relative impact of ions on a wide range of aqueous processes.1214 Many studies have shown that kosmotropic and chaotropic ions in the series exhibit significant quantitative differences in their hydration coordination numbers, nearest neighbor distances, and Jones-Dole viscosity B coefficients.15,16 Collins discussed the difference in the hydration of various kosmotropic and chaotropic inorganic Received: May 19, 2011 Revised: June 25, 2011 Published: July 07, 2011 15525
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The Journal of Physical Chemistry C ions.15 He concluded that kosmotropes tightly bind nearby water molecules and immobilize them, whereas chaotropes “free up” nearby water molecules, allowing them to move more rapidly than in bulk solutions. Collins further investigated interactions among kosmotropic and chaotropic ions in the Hofmeister series.15 He proposed a “law of matching water affinities”, which relates the tendency of oppositely charged ions to associate and form ion pairs in aqueous solutions to the level of the free energies of their ionic hydration. This theory is supported by many experimental results measured with solution neutron and X-ray diffraction, nuclear magnetic resonance, and gel sieving chromatography.15 This theory was also confirmed by simulation results from Fennell et al.17 They concluded that (1) small small ion pairs tend to associate in water as a result of strong electrostatic interactions because water caging stabilizes the contact ion pair, whereas largelarge ion pairs prefer to associate because of weak electrostatic interactions and hydrophobic-like water caging and (2) small-large ion pairs easily dissociate in water because the smaller ion interacts with water more strongly than ionion or waterwater interactions, leading to a solventshared ion pair that is more stable than a contact ion pair. Vlachy et al. extended the study on the Hofmeister series of ions to organic ions.18 The interactions between ions and organic ions such as sulfonates and carboxylates show a Hofmeister-like ordering.18 Zwitterionic surfaces contain a great number of charged groups similar to those organic ions. If these charge groups on the surfaces bind to ions from solutions at the shearing interface, then the binding may have significant impacts on the lubrication of zwitterionic surfaces. To reveal the role of ions in the Hofmeister series on the friction between zwitterionic surfaces in aqueous environments, we selected two series of salt solutions based on Hofmeister series. One contains different cations (LiCl, NaCl, KCl, CsCl), whereas another contains different anions (CsF, CsBr, CsCl, CsI). Carboxybetaine self-assembled monolayers (CB-SAM) were used as model zwitterionic surfaces because they contain carboxylate groups and choline groups, which are known to have preferred interactions with kosmotropic cations or chaotropic anions, respectively.18 We systematically studied the friction of the model surfaces in the presence of these salt solutions using molecular dynamics (MD) simulations. The friction coefficients of the surfaces in each salt solution were calculated as lateral forces divided by vertical forces. The ionsurface interactions were analyzed. The hydration of the surfaces was investigated to elucidate the lubrication mechanism of zwitterionic surfaces in salt solutions.
’ SIMULATION MODEL AND PROTOCOL The structure of CB-SAMs is unknown. Therefore, the same 19 was used to protocol as our previous work√for PC-SAMs √ determine that a CB-SAM has a 7 7 R19 lattice structure shown in the Supporting Information. The simulation box for tribological studies is shown in Figure 1. The origin was defined as the center of the box. This simulation box was constructed by placing a bulk solvent reservoir on each side of the surfaces in the y direction. There were also solvent molecules present between the two CB-SAM surfaces. The top and bottom surfaces had opposite sliding directions along the x axis. The surfaces were infinite in the x direction but finite in the y direction. The confined solvent was in contact with bulk water on each side. For tribological studies, a CB-SAM containing 32 (4 chains in the x direction and 8 in the y direction) CB chains was first built
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Figure 1. Snapshot of the simulation box from the MD simulation, showing the arrangement of the two CB-SAM surfaces and the bulk water regions. The origin is located at the center of the box. The two surfaces are sheared against each other in the y direction. This system is immersed in 1.0 M KCl. Red, gray, blue, and yellow represent oxygen, carbon, nitrogen, and sulfur atoms, respectively. Water molecules are shown with green wireframe. Purple and green balls denote potassium and chloride ions, respectively.
and energy minimized for 5000 steps with the conjugate gradient algorithm, whereas the positions of the sulfur atoms of the CBSAM were fixed. The minimization was performed in a continuous medium, with a distance-dependent dielectric constant (ε = ε0 3 r) that mimics water. The lattice structure of the CB-SAM surface was obtained from the previous step. Second, the CBSAM was duplicated to obtain a second CB-SAM. The second surface was inverted and moved to a nominal separation distance of 10 Å between the two surfaces. This distance between the two surfaces is defined by the minimum separation between any atoms on either surface. Both surfaces were then energy minimized for 5000 steps with the conjugate gradient algorithm, whereas the positions of the sulfur atoms of the CB-SAMs were fixed. The two surfaces were then solvated with pre-equilibrated three-site point charge model (TIP3P) water.20,21 Two regions of bulk water were placed on either side of the surfaces in the y direction to ensure surfacesolution equilibrium. The length of the water box in the x and y directions was 26.4 and 117.2 Å, respectively. The length in the z direction was 28.3 Å, which corresponds to surface separation distances of 10 Å. All of the water molecules within 2.2 Å of any SAM atoms were removed. The SAMwater systems were subsequently energy minimized for 5000 steps with the conjugate gradient algorithm while fixing the sulfur atoms of the CB-SAMs. For all salt solutions in the simulations, the concentration of salt is at 1 M, containing 36 cations and 36 anions. After the ions were inserted, the SAMwater systems were again energy minimized for 5000 steps with the conjugate gradient algorithm while the sulfur atoms of the CB-SAMs were fixed. Finally, the system underwent a heating protocol that included 20 ps of temperature increase from 50 K to the final temperature of 300 K and 50 ps of an equilibration period without any constraints on the SAMs except for the fixed sulfur atom constraint. All of the initial structures were built using the CHARMM22 molecular simulation package (version c30b1). For production MD, the starting configuration and velocities of the CB-SAMs, water, and ions were taken from the final frame of the equilibration MD simulation. The velocity Verlet method was used for the integration of the Newtonian equations in the NVT ensemble with a time step of 1.0 fs. The Berendsen method was used to maintain a constant temperature of 300 K with a coupling constant of 0.1 ps. In the MD simulations, the total number of solvent molecules in each system was kept constant, and the solvent molecules confined between the monolayers were allowed to exchange freely with those in the bulk water region on either side. The number of solvent molecules in each 15526
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bulk water region was controlled by scaling its volume of so that the liquid density there remained at 0.997 g/cm3 for water. To simulate the friction of various interfaces immersed in a liquid environment, it is essential to ensure that the liquid density in the bulk region is equal to its experimental bulk density at the simulation temperature and pressure. Each simulation system was placed in a rectangular box. The periodic boundary condition and minimum image convention were applied to the x and y directions only. Z direction is nonperiodic. There is a hard wall on both the top and bottom of the simulation box, and a reflective boundary condition was applied. All of the sulfur atoms of the SAM surfaces were fixed, and all of the covalent bonds involving hydrogen atoms were constrained using the SHAKE algorithm. As before,7,2325 normal loads and frictional forces were calculated by averaging the forces exerted on the monolayers in the z and x directions, respectively. Friction coefficients were then obtained by dividing the friction force by the normal load. The CHARMM22 parameter set,26 an all-atom potential force field, was used to model the Na+, K+, Cs+, and Cl ions and the CB-SAMs. The Lennard-Jones and electrostatic parameters for Li+, F, Br, and I ions were from Aqvist’s work.27 All of van der Waals (VDW) parameters for the relevant ions were listed in the Supporting Information. Water molecules were treated with the TIP3P model.20,21 The total length of each individual MD simulation run was 5.0 ns. Configurations were saved every 1.0 ps after 1 ns for analysis. The short-range VDW interactions were calculated by the switch function at a twin range cutoff between 8 and 10 Å. The long-range electrostatic interactions were calculated by the force-shifting function at a cutoff distance of 12 Å. Previous studies have shown that the atom-based force-shifting function can conserve energy, correctly predict experimental data, and generate stable trajectories.2830 The atom neighbor list with a cutoff range of 13.2 Å was employed to accelerate the simulation and was updated automatically whenever necessary (heuristic test). MD simulations for tribological studies were performed on our Beowulf Linux cluster using the in-house developed BIOSURF program.31
’ RESULTS AND DISCUSSION In this work, we studied the friction between two CB-SAM surfaces immersed in different salt solutions or water to correlate the order of ions in the Hofmeister series with the friction between zwitterionic surfaces. The interactions between the ions and the charged groups on the CB-SAM surfaces were studied to see whether there is a preferred binding between them and if so what the strength of the binding is. Such binding can affect CBSAM surfaces in two ways. One way is that when ions bind to a surface they may enhance the overall surface hydration number with water molecules around them, even though their binding occupies the positions of some interfacial water molecules. Another way is that they alter the structure of interfacial water by either enhancing or destructing it, depending on whether ions are kosmotropic or chaotropic. In other words, the former affects the quantity of the hydration water molecules, whereas the latter affects the dynamics of hydration water molecules. To elucidate these two effects on friction, finally, the characteristics of water molecules near the surfaces were investigated, and the key that bridges the order of ions in the Hofmeister series and the friction of zwitterionic surfaces was discussed. Effect of Ions on Friction: General Trend. To elucidate the relationship between ions and friction, we investigated two sets
Figure 2. Friction coefficients for CB-SAM surfaces from simulations with a shear velocity of 0.1 Å/ps and a surface separation distance of 10 Å were compared when they were immersed in (A) LiCl, NaCl, KCl, or CsCl solutions or water (as reference) and (B) CsF, CsCl, CsBr, or CsI solutions, or water. The concentration of all salt solutions is 1.0 M.
of salt solutions in the Hofmeister series in this work. One set contains Li+, Na+, K+, or Cs+ as cations and Cl as anions (referred to as the cation series). The other set contains Cs+ as cations and F, Cl, Br, and I as anions (referred to as the anion series). Both Figure 2A and 2B suggest that a strong correlation between the order of an ion in the Hofmeister series and the friction between the zwitterionic CB-SAM surfaces. For the salt solutions in the cation series, as shown in Figure 2A, the more kosmotropic a cation is, the larger the friction coefficient is. Therefore, the surfaces in the LiCl solution have the largest friction, whereas the surfaces in the CsCl solution have the lowest friction in the cation series. Figure 2A also shows that all of the surfaces in the salt solutions have relatively higher friction than the surfaces in pure water. This result is consistent with our previous findings7 that the friction of PC-SAMs in pure water is lower than that in either NaCl or KCl solutions. The results in Figure 2B confirmed the same trend as that observed in Figure 2A. For the anion series, the surfaces in the CsF solution 15527
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Figure 3. (A) Cation coordination number on CB-SAM surfaces when the surfaces were immersed in 1.0 M LiCl, NaCl, KCl, or CsCl solution. (B) Anion coordination number on CB-SAM surfaces when the surfaces were immersed in 1.0 M CsF, CsCl, CsBr, or CsI solution. The cutoff distances used to calculate the coordination numbers of ions were obtained from the first minimums of the radial distribution functions of cations around carboxylate groups or anions around choline groups. The carbon atoms of carboxyl groups or the nitrogen atoms of choline groups were taken as the centers for the radial distribution calculations.
have the highest friction, whereas the surfaces in the CsI solution have the lowest friction in the anion series. This is because the F ions are the most kosmotropic anions, whereas the I ions are the most chaotropic ones in the anion series. Interestingly, the surfaces immersed in the CsI solution can actually achieve a friction coefficient even lower than that in pure water. To understand these results and elucidate the key mechanism of salt effects on friction, a close examination of the roles of ions and water is taken in the following two sections. Analysis of IonSurface Interactions. The analysis of friction coefficients clearly shows the importance of the nature of ions. To reveal further the effect of ions on friction, it is necessary to examine interaction behaviors between different ions and the charged groups of the surfaces. If ions bind to the charged groups, then they can cause considerable changes on the sheared surfaces
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Figure 4. (A) Survival time autocorrelation function of cations on CBSAM surfaces when the surfaces were immersed in 1.0 M LiCl, NaCl, KCl, or CsCl solution. (B) Survival time autocorrelation function of anion density on CB-SAM surfaces when the surfaces were immersed in 1.0 M CsF, CsCl, CsBr, or CsI solution.
and thus affect friction. Results in Figures 3 and 4 show that the Collin’s theory is applicable to explain the interactions between the ions and the charge groups of the surfaces. The CB-SAM surfaces have positively charged choline groups and negatively charged carboxylate groups. The former is regarded to be chaotropes, whereas the latter is considered to be kosmotropes.32 Therefore, according to Collin’s theory, chaotropic anions prefer to bind to chaotropic choline groups, whereas kosmotropic cations prefer to bind to kosmotropic carboxylate groups. Both of these patterns were observed in this study. For the cation series in Figure 3A, the coordinate number of ions near the surfaces demonstrates that kosmotropic cations tend to bind to kosmotropic carboxylate groups on the CB-SAM surfaces. The binding affinity between the cations and the carboxylate groups follows an increasing order: Cs+ < K+ < Na+ < Li+. Similarly, Hess et al. also observed that an order of increasing binding affinity among K+, Na+, Li+, and carboxylate anions.33 For the anion series in Figure 3B, likewise, the coordinate number of ions 15528
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Figure 5. Coordination number of water on CB-SAM surfaces when the surfaces were immersed in 1.0 M LiCl, NaCl, KCl, CsCl, CsF, CsBr, or CsI solution or water.
near the surfaces shows that chaotropic anions tend to bind to chaotropic choline groups on the CB-SAM surfaces. Kosmotropic F ions have almost no affinity to the choline groups on the surfaces, whereas chaotropic I ions have strong affinity to the choline groups on the surfaces. This observation is also consistent with those from Vacha et al.,34 which show that I exhibits a genuine affinity for the choline group on phosphatidylcholine lipid membranes. To evaluate the strength of the binding between ions and the charge groups on the surfaces, we calculated the survival time autocorrelation function of the ions. The function is defined as Cr ðtÞ ¼
1 Np ÆPð0ÞPðtÞæ Np i ¼ 1 ÆPð0Þæ2
∑
where P(t) is a binary function that equals 1 if an ion is near any oppositely charged group on the surfaces at time t; otherwise, P(t) equals 0. Np is the number of the ions found near the oppositely charged group on the surfaces. The broken brackets (Ææ) denote the ensemble average. The definition is similar to the definition for the survival time autocorrelation of hydrogen bonding. A slower decay in a survival time autocorrelation function indicates a more stable association between ions and oppositely charged groups. In Figure 4A,B, we plotted the survival time autocorrelation function for ions and the oppositely charged groups on the CB-SAM surfaces. Because the positively charged choline groups on the CB-SAM surfaces are chaotropic, whereas the negatively charged carboxylate groups are kosmotropic, results for the cation series in Figure 4A show that kosmotropic Li+ ions stay for the longest time near the carboxylate groups of the surfaces. This indicates that Li+ ions have the strongest binding strength with the carboxylate groups on the surfaces. On the contrary, results for the anion series in Figure 4B show that chaotropic I ions stay for the longest time near the choline groups of the surfaces. This indicates that I ions have the strongest binding strength with the choline groups on the surfaces. Results in Figure 4 are consistent with those in Figure 3 for the coordination numbers of ions near surfaces.
Figure 6. (A) Survival time autocorrelation function of water near CBSAM surfaces when the surfaces were immersed in 1.0 M LiCl, NaCl, KCl or CsCl solution or water. (B) Survival time autocorrelation function of water near CB-SAM surfaces when the surfaces were immersed in 1.0 M CsF, CsCl, CsBr, or CsI solution or water.
Relationship between Water and Friction. Elucidating the behavior of ions in the friction of the CB-SAM surfaces is an important step toward a complete understanding of salt effects on friction. The strong binding of certain ions on the surfaces indicates that the hydration of the zwitterionic surfaces may be substantially disturbed, which can lead to the change in both the number of the hydration water molecules and the dynamics of hydration water molecules. Surface hydration is important for various systems35,36 and has been found to be critical to friction in several studies.8,3740 However, its exact role is still unclear. To take into account the overall effects of ions on surface hydration, we calculated the coordination number of the water molecules on the CB-SAM surfaces for each simulation case. To be considered as a water molecule near the surfaces, the water molecule must be either in the hydration layer of the surfaces or in the hydration layer of an ion which binds to the surfaces. The thicknesses of the hydration layers for the surfaces or ions are based on the first minima of the 15529
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water mobility has a dominate effect on the friction of the surfaces. The friction of the surfaces in a salt solution will be considerably higher than that in water if the salt solution contains typical kosmotropic ions, for example, Li+ or F. An interesting finding from this work is that the zwitterionic surfaces immersed in the CsI solution shows even lower friction than that in pure water. The underline reason of the observation can be boiled down to the enhanced mobility of interfacial water molecules (Figure 7) as well as a slightly increased surface hydration number (Figure 5).
Figure 7. Correlation between friction coefficient and residence time of water near CB-SAM surfaces when the surfaces were immersed in 1.0 M LiCl, NaCl, KCl, CsCl, CsF, CsBr, or CsI solution or water.
radial distribution functions of water around the surfaces or water around ions. This definition takes into account possible enhanced hydration by absorbed ions on the surfaces, which is not considered in our previous studies.7 The results in Figure 5 show that most ions in either the cation or anion series do not significantly affect the overall hydration of the surfaces. LiCl and CsI have slightly enhanced surface hydration numbers as compared with other systems. This occurs for different reasons. For LiCl, its cation (Li+) strongly binds to the CB-SAM surfaces. For CsI, however, its anion (I) has strong surface binding, as shown in Figures 3 and 4. The strong binding of these ions onto the surfaces causes the increase in the overall surface hydration because they bring more water molecules to the surfaces than the water molecules the surfaces lose. To understand the dynamics of hydration water molecules under the influence of ions, we measured the survival time autocorrelation function of water near the CB-SAM surfaces. This function is directly related to the mobility of water molecules near the surfaces. The faster decay of the function indicates higher water mobility. Both Figure 6A and 6B show that typical kosmotropic ions, such as Li+ and F, slow the mobility of interfacial water molecules whether or not they bind to the surfaces. The decrease in the water mobility leads to the enhancement of water structure due to kosmotropic ions. For comparison, typical chaotropic ions, such as I, can make water around them even more mobile than in bulk because they can destruct water structure. Weak kosmotropic or chaotropic ions do not show substantial impact on the mobility of the water molecules near the surfaces because their ability to affect water structure is less significant. Therefore, the types of ions in the Hofmeister series have a close relationship with the water mobility. After elucidating the correlation between the water mobility and the Hofmeister series, we further examined the relationship between water mobility and friction. To quantify how long water molecules stay near the surfaces, we fitted the survival time autocorrelation function of water to obtain the residence time of interfacial water.36 A smaller residence time implies higher water mobility. Figure 7 shows a strong correlation between friction and the residence time of interfacial water, which suggests that
’ CONCLUSIONS In this work, we found a close relationship between the Hofmeister order of ions and the friction between zwitterionic surfaces. For salt solutions containing the same anion, the friction of the surfaces immersed in a solution with kosmotropic cations shows larger friction than that in a solution with chaotropic ones. The same relationship applies for the salt solutions with the same cation but different anions. A further analysis of interfacial water molecules reveals that the amount of surface hydration remains nearly invariant and the mobility of those molecules is the key that bridges the relationship between the order of ions in the Hofmeister series and the friction between zwitterionic surfaces. The water mobility is determined by the nature of ions in a salt solution. If ions are kosmotropic, the water mobility will be decreased, resulting in an increased friction. If the ions are chaotropic, then the water mobility will be increased, leading to a decreased friction. The ionsurface interactions can be explained with Collin’s ion-pairing theory. The CB-SAM surfaces are composed of both kosmotropic carboxylate groups and chaotropic choline groups. Therefore, a significant amount of kosmotropic cations was found to bind strongly to the carboxylate groups of the CB-SAM surfaces, whereas chaotropic anions were found to bind strongly to the choline groups instead. The binding affinity between the ions and the surfaces follows the Hofmeister order. ’ ASSOCIATED CONTENT
bS
Supporting Information. Force fields of various ions, friction coefficient of CB-SAM surfaces, residence times of water near the surfaces, and the optimal lattice structure of CB-SAM and its chain orientation. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 206-616-6509. Fax: 206-685-3451. E-mail: sjiang@ u.washington.edu.
’ ACKNOWLEDGMENT We would like to thank the National Science Foundation under CMMI 0758358 and the American Chemical Society, Petroleum Research Funds under ACS PRF no. 48096-AC7 for financial support. ’ REFERENCES (1) Jiang, S. Y.; Cao, Z. Adv. Mater. 2009, 21, 1. (2) Muro, E.; Pons, T.; Lequeux, N.; Fragola, A.; Sanson, N.; Lenkei, Z.; Dubertret, B. J. Am. Chem. Soc. 2010, 132, 4556. 15530
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The Journal of Physical Chemistry C (3) Yameen, B.; Ali, M.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. J. Am. Chem. Soc. 2009, 131, 2070. (4) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U. I.; Nakamura, K.; Kawaguchi, H. Nat. Mater. 2004, 3, 829. (5) Kim, C. K.; Ghosh, P.; Pagliuca, C.; Zhu, Z. J.; Menichetti, S.; Rotello, V. M. J. Am. Chem. Soc. 2009, 131, 1360. (6) Holmlin, R. E.; Chen, X. X.; Chapman, R. G.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 2841. (7) He, Y.; Chen, S.; Hower, J. C.; Bernards, M. T.; Jiang, S. J. Chem. Phys. 2007, 127, 084708. (8) Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Science 2009, 323, 1698. (9) Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Langmuir 2005, 21, 1834. (10) Deamer, D. W. J. Bioenerg. Biomembr 1985, 17, 395. (11) Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 19. (12) Zhang, Y. J.; Cremer, P. S. Annu. Rev. Phys. Chem. 2010, 61, 63. (13) Pegram, L. M.; Record, M. T. J. Phys. Chem. B 2008, 112, 9428. (14) Zhang, Y. J.; Cremer, P. S. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15249. (15) Collins, K. D.; Neilson, G. W.; Enderby, J. E. Biophys. Chem. 2007, 128, 95. (16) Collins, K. D. Biophys. J. 1997, 72, 65. (17) Fennell, C. J.; Bizjak, A.; Vlachy, V.; Dill, K. A. J. Phys. Chem. B 2009, 113, 6782. (18) Vlachy, N.; Jagoda-Cwiklik, B.; Vacha, R.; Touraud, D.; Jungwirth, P.; Kunz, W. Adv. Colloid Interface Sci. 2009, 146, 42. (19) Zheng, J.; He, Y.; Chen, S.; Li, L.; Bernards, M. T.; Jiang, S. J. Chem. Phys. 2006, 125, 174714. (20) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (21) Neria, E.; Fischer, S.; Karplus, M. J. Chem. Phys. 1996, 105, 1902. (22) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.; Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983, 4, 187. (23) Zhang, L. Z.; Balasundaram, R.; Gehrke, S. H.; Jiang, S. Y. J. Chem. Phys. 2001, 114, 6869. (24) Zhang, L. Z.; Jiang, S. Y. J. Chem. Phys. 2002, 117, 1804. (25) Zhang, L. Z.; Jiang, S. Y. J. Chem. Phys. 2003, 119, 765. (26) MacKerell, A. D.; Bashford, D.; Bellott, M.; Dunbrack, R. L.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S.; JosephMcCarthy, D.; Kuchnir, L.; Kuczera, K.; Lau, F. T. K.; Mattos, C.; Michnick, S.; Ngo, T.; Nguyen, D. T.; Prodhom, B.; Reiher, W. E.; Roux, B.; Schlenkrich, M.; Smith, J. C.; Stote, R.; Straub, J.; Watanabe, M.; Wiorkiewicz-Kuczera, J.; Yin, D.; Karplus, M. J. Phys. Chem. B 1998, 102, 3586. (27) Aqvist, J. J. Phys. Chem. 1990, 94, 8021. (28) Steinbach, P. J.; Brooks, B. R. J. Comput. Chem. 1994, 15, 667. (29) Norberg, J.; Nilsson, L. Q. Rev. Biophys. 2003, 36, 257. (30) Beck, D. A. C.; Armen, R. S.; Daggett, V. Biochemistry 2005, 44, 609. (31) Zheng, J.; Li, L. Y.; Chen, S. F.; Jiang, S. Y. Langmuir 2004, 20, 8931. (32) Garcia-Celma, J. J.; Hatahet, L.; Kunz, W.; Fendler, K. Langmuir 2007, 23, 10074. (33) Hess, B.; van der Vegt, N. F. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 13296. (34) Vacha, R.; Jurkiewicz, P.; Petrov, M.; Berkowitz, M. L.; Bockmann, R. A.; Barucha-Kraszewska, J.; Hof, M.; Jungwirth, P. J. Phys. Chem. B 2010, 114, 9504. (35) Giovambattista, N.; Lopez, C. F.; Rossky, P. J.; Debenedetti, P. G. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2274. (36) He, Y.; Chang, Y.; Hower, J. C.; Zheng, J.; Chen, S. F.; Jiang, S. Phys. Chem. Chem. Phys. 2008, 10, 5539. (37) Leng, Y. S.; Cummings, P. T. J. Chem. Phys. 2006, 124, 074711. (38) Zhang, Z.; Morse, A. J.; Armes, S. P.; Lewis, A. L.; Geoghegan, M.; Leggett, G. J. Langmuir 2011, 27, 2514.
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(39) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J. F.; Jerome, R.; Klein, J. Nature 2003, 425, 163. (40) Kim, S. H.; Opdahl, A.; Marmo, C.; Somorjai, G. A. Biomaterials 2002, 23, 1657.
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