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2008, 112, 6296–6300 Published on Web 04/29/2008
On the Manifestation of Hydrophobicity at the Nanoscale Niharendu Choudhury* Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: March 03, 2008; ReVised Manuscript ReceiVed: April 09, 2008
The manifestation of hydrophobicity at the nanoscale has been shown to depend on the topology of the solute. Using various nanoscopic hydrophobic plates, molecular dynamics simulation has been employed to explore the hydration and dewetting at the nanoscale. The topology of the solute regulates the behavior of nanoconfined water, resulting in any of the wet, dry, and intermittent wet-dry intersolute states. The present result reconciles apparently contrasting literature reports on how water behaves at extended hydrophobic surfaces and sheds light on the mechanism of dewetting. Introduction The issue of hydrophobicity1–4 at the nanoscale has significant implications4,5 in many processes and phenomena such as protein folding, colloidal stability, and micelle formation. It is believed5 that the manifestation of hydrophobicity is length scale dependent. However, the literature on the same at the nanometer length scale is conflicting.6,7 Theoretical and computational studies have delivered contrasting results, with many favoring5,8–12 the concept of dewetting at the intersolute region of large hydrophobic solutes, while many others are against10,13–16 it. Experimental observations on the same are also conflicting, with many supporting7,17,18 a depleted water layer around extended hydrophobes and many others depicting a wet19,20 hydrophobic interface. Although intersolute dewetting has been predicted from theoretical and computational investigations5,8–11,21,22 with idealized purely repulsive model solutes, consideration of a realistic model solute with van der Waals interaction has either resulted in a wet10,13–16 intersolute state or reduced12,23 the critical distance for the dewetting transition. From the hydration study10,11,14 of graphene plates in water, it has been demonstrated10 that if the dispersion interaction is taken into account properly, the stability of the contact pair state is governed by the solute-solute van der Waals interaction, not by the solventinduced interaction. Also, a transition from the vapor to liquid phase passing through an intermediate phase, where the intersolute region oscillates between the vapor and liquid states, has been observed14 for a pair of plates with a fixed graphene-like topology when the solute-water dispersion interaction for the solute is varied. However, the existence of intersolute dewetting has been observed even in the case of realistic models.7,24–26 Koishi et al.24 have found intersolute dewetting even though the individual attractive solute-water interaction is stronger than the same used by Choudhury et al., who found10,14 a completely wet intersolute state. The existence of drying in the interdomain region of a protein,26 on one hand, and the disappearance of dewetting in * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected] and
[email protected].
10.1021/jp801852v CCC: $40.75
the case of another protein23 creates puzzlement in understanding the role of attractive interactions. It indicates that apart from the influence of an attractive solute-water dispersion interaction, the topology of the solute might be playing a very important role in the hydration and dewetting at the nanoscale. In this Letter, we have demonstrated how the topology of the solute can be tuned to control the behavior of water and, therefore, the manifestation of hydrophobicity at the nanoscale by employing atomistic molecular dynamics (MD) simulations. Models and Methods The MD simulations of the model hydrophobic solute plates in water were performed at constant temperature and pressure with a target pressure of 1 atm and target temperature of 298 K. The solute was modeled as paraffin-like plates, each of which was constructed by arranging n-C18H38 molecules in parallel in such a way that all of the carbon atoms were on the same plane. Potential parameters for the alkane were kept fixed for all of the plates studied here and were taken from united atom OPLSUA force field.27 The topology of the plate was varied by changing the intermolecular spacing, a0, between different C18 molecules in a plate. Three plates with a0 values of 4, 5, and 6 Å were used. The sizes of the above three plates as measured by the center-to-center distances between the end CH3 or CH2 groups were 16, 20, and 24 Å, corresponding to the edge-to-edge distances (using van der Waals radii) of approximately 20, 24, and 28 Å, respectively. For all of the plates, potential parameters of the CH3 and CH2 groups remained the same. Water was modeled by the standard SPC/E28 potential. For the solute-water interaction, the cross parameters for the LJ potential were obtained from the Lorentz-Berthelot mixing rule. Two solute plates were placed symmetrically around the middle of a water box containing around 2400 water molecules with the two plates parallel to each other as well as with the xy-plane of the box at an intersolute separation of r0. In the case of the hydration study of the single plate, one plate was placed in the middle of the box and was kept in parallel with the xy-plane of the box. The simulations were performed in an isothermal-isobaric (NPT) ensemble with the molecular dynamics extended system approach of Nose and Anderson. 2008 American Chemical Society
Letters
Figure 1. Internal potential energy per unit area (Ep/A) of the model paraffin plates as a function of intermolecular spacing a0.
Figure 2. (a) The normalized single-particle density F(z)/F0 of water oxygen in and around the plates with intermolecular spacing a0 ) 5 Å at an intersolute distance of r0 ) 7.2 Å. The two arrows correspond to the positions of the two plates. (b) Plot of the instantaneous number density of water, Fc(t), in the intersolute region corresponding to (a). (c) Same as (a), but with r0 ) 7.5 Å. (d) The corresponding instantaneous intersolute density.
Results and Discussions In order to have an idea about the relative stability of the plates, the energy of the plate with fixed topology in vacuum has been calculated and the energy (Ep) per unit area (A) of the plate as a function of the a0 values is shown in Figure 1 for the plates with different a0 values. It reveals that the plate with a0 ) 5 Å has the minimum energy. We have presented here the behavior of water in and around the energy-minimized plates (a0 ) 5 Å) along with two other plates with a0 values of 4 and 6 Å, those lying on either side of the energy minimum. In most of the cases, two different types of simulations were performed: one with a wet initial condition, that is, initially the intersolute region was filled up with water, and the other with a dry initial condition, that is, initially the intersolute region was empty. In Figure 2a, we have shown the normalized ensembleaveraged singlet density distribution, F(z)/F0 of water molecules in and around the two-solute system, with a0 ) 5 Å at an intersolute separation of r0 ) 7.2 Å. For the topology and the force field used here, the intersolute region at r0 ) 7.2 Å is geometrically sufficient to accommodate a single layer of water molecules. The density distributions obtained from the simulations with both wet (top panel) and dry (bottom panel) initial conditions reveal significant water accumulations (twice that of the bulk density) on the outside surfaces of the plates. There
J. Phys. Chem. B, Vol. 112, No. 20, 2008 6297 is also a small peak in the middle, representing the presence of water molecule in the intersolute region. The difference in height of the peaks in the middle for two different initial conditions is due to the occurrence of nonperiodic liquid-vapor oscillations for lifetimes of a few nanoseconds, and it will be apparent from the examination of the instantaneous water density in the intersolute region. As the singlet density distribution is an ensemble-averaged quantity, which does not give information about fluctuation in the number of water molecules, the instantaneous number density of water Fc(t) is a better order parameter for monitoring the dewetting transition in the intersolute region. For simulation with the wet initial condition, although the density of water was initially slightly higher than that of the bulk water, after some time, almost all of the water molecules were expelled from the intersolute region (see top panel of Figure 2b). The intersolute region was completely wet for a long time of about 1 ns. Again, as the system entered into an almost dewetted state, it stayed there for a time as long as 4 ns, and after that, it entered the wet state again. As it is found here and also in many recent investigations,10,14,15 the behavior of water confined between two large hydrophobic solutes is subtle, with intermittent wet-dry transitions with a lifetime of a few nanoseconds. In the present case, within the 10 ns time scale explored, the system visited each of the dry and wet states twice. When the simulation was started from the dry initial condition, as shown in the bottom panel of the same figure, the system attained the completely wet state after 1.7 ns. After that, the system returned to the dry state only once in a 10 ns timespan, with the lifetime of the dewetted state around 1 ns. It is important that in both the cases, whenever the system returned to the wet state from its dry precursor, it reached the completely wet state. The difference in density peak heights in the intersolute region as shown in Figure 2a for wet and dry initial conditions arises due to nonperiodic occurrences of the intermittent wet and dry states. The ∼10 ns sampling adopted in the present study is not sufficient and thus results in the difference in peak heights. When the intersolute distance is slightly increased to 7.5 Å, interestingly, the density peak in the middle (see Figure 2c) becomes almost equal for both of the initial conditions, and also, there is significant density enhancement in the middle. The same is corroborated by the instantaneous density plots (see Figure 2d), where wet-dry oscillations beyond a certain time were not observed. Because of more free volume available in this case, the probability of the intersolute space being occupied by water is more as compared to the same at r0 ) 7.2 Å. In summary, for this pair of paraffin plates, when the interplate separation is so narrow that single layer of water molecules can be accommodated, the intersolute state toggles between the wet and the dry states. It is due to the bimodal nature29 of the free energy of occupation. Similar behavior has been observed in many other situations10,14,15,30 with carbon nanotubes, model water channels, and planar solutes. A further increase of the separation between the plates causes the liquid-vapor oscillation to cease, stabilizing a completely wet intersolute state. No permanent dry state is observed at this separation. In summary, the nature of hydration for the model paraffin plate, in which the paraffin molecules are 5 Å apart, is subtle and intriguing. The intersolute state toggles between the wet and the dry states at close separation corresponding to only one intervening water layer. However, at 7.5 Å, no liquid-vapor oscillations have been observed, and thus, the critical distance to observe the liquid-vapor oscillations is below 7.5 Å, corresponding to a surface-to-surface distance of 3.6 Å.
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Figure 3. (a) Same as Figure 2a but for the solutes with an intermolecular spacing of a0 ) 4 Å and (b) the corresponding instantaneous intersolute density. (c) Same as Figure 3a but for the plates that are two atomic layers thick and (d) the corresponding instantaneous intersolute density.
Now, we consider a different model plate which has the intermolecular spacing of a0 ) 4 Å. In this plate, paraffin molecules are slightly more closely packed than in the energyminimized (a0 ) 5 Å) one. In Figure 3a, we shown the normalized single-particle density in and around the two plates at an intersolute distance of r0 ) 7.2 Å. For both wet (top panel) and dry (bottom panel) initial conditions, a dense layer of water within the two plates along with significant accumulations of water at the outside surfaces of the plates has been observed. The density enhancement of water is more in the intersolute region than that in the outside surfaces of the plates. When Fc(t) in the confined region at r0 ) 7.2 is plotted versus time (Figure 3b) for the wet initial condition (top panel), it remains in that state throughout the simulation time. No large fluctuation leading to a dry or a partially dry state has been observed. It is consistent with the singlet density distribution of Figure 3a (top panel). For the dry initial condition (bottom panel of Figure 3b), the initially dry system becomes completely filled up with water within ∼100 ps (see the inset), and thereafter, it remains in the wet state for the entire simulation time, without showing any significant fluctuation. It is important to remember that the potential parameters of the individual atoms of the plate and the water are the same for all of the model plates of this study. The wet hydrophobic interface is a consequence of the cumulative effect of weak attractive van der Waals interactions between the atoms of the solute with those of the solvent. A similar wet interfacehasbeenobservedinmanyotherrecentinvestigations.10,13–15,19,20 The thickness of the model paraffin plate used here is of atomic dimension. In order to verify the effect of the plate thickness, we have also investigated the intersolute region at r0 ) 7.2 Å between two thicker plates. In this case, the plate is made up of two layers of paraffin molecules, and thus, the excluded volume of the plates is more as compared to that of the previous case. We have investigated the intersolute region at r0 ) 7.2 Å between the two thicker plates. In this case, where the excluded volume of the plates is twice that of the previous case, no change in the wet intersolute region is observed (see Figure 3c and d). It is not surprising because the free energy of solvation has a
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Figure 4. Same as Figure 2 but for the solutes with an intermolecular spacing of a0 ) 6 Å with various r0 values as indicated in the figures.
crossover5,8 from the volume to surface area dependence at the nanometer length scale. We have chosen one more model plate with a0 ) 6 Å, which is slightly less dense than the energy-minimized one. In Figure 4a, the normalized singlet density distributions in and around these plates at an intersolute distance of r0 ) 7.2 Å are shown for both wet (top panel) and dry (bottom panel) initial conditions. Surprisingly, in this case, there is almost no density peak in the middle, irrespective of the initial conditions. A clearer picture of the intersolute dewetting emerges from the examination of the instantaneous water density in the intersolute region presented in Figure 4b. For wet initial conditions (see top panel), within ∼400 ps (see the inset of the same), almost all of the water molecules have been expelled from the intersolute region, leading to almost a dewetted state. Likewise, starting from the dry initial condition, the system never reaches (bottom panel, Figure 4b) the wet state within the entire simulation time of around 7 ns. Thus, for this pair of paraffin plates, the intersolute region at r0 ) 7.2 Å is dewetted. Even when r0 is increased to 7.5 Å, the intersolute region is almost dewetted (not shown). The paraffin molecules in this model plate are further apart from the energy-minimized plate configuration, and thus, the van der Waals attractive dispersion interaction of the solute with water and that between two solutes are “diluted” as compared to the two earlier cases. A similar dry solute-solvent interface was observed long ago by Patey and co-workers31 for a simple Lennard-Jones fluid and by many others7–10,14,17,18,24 for water. The present observation is also consistent with the observed difference in the dewetting behavior of fluorocarbon and hydrocarbon plates.32 The consequence5 of such a dewetted intersolute region is to generate a strong attractive hydrophobic interaction between the two hydrophobic solutes, leading to selfassembly. However, further increase of the intersolute distance reveals a phase transition from the dry to wet state through an intermediate “wet-dry” state. At r0 ) 9 Å, the density (see Figure 4c) in the intersolute region shows two small peaks, indicating two partially ordered layers of water molecule parallel to the surfaces. The corresponding Fc(t) plot reveals (see Figure 4d) liquid-vapor oscillation in the intersolute region. Although
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J. Phys. Chem. B, Vol. 112, No. 20, 2008 6299 Concluding Remarks
Figure 5. (a) The F(z)/F0 of water oxygen around the single plate with a0 ) 6 Å. Inset: The F(z)/F0 values of water oxygen at the outside surfaces of the double plate systems with a0 ) 4 Å (black) and a0 ) 6 Å (red) are compared. (b) Instantaneous number density of water, Fhy(t) in the first hydration shell of the above model plate.
not shown here, the liquid-vapor oscillation has also been observed for r0 ) 8 Å, and in this case, only one density peak is observed in the middle. Further increase of r0 to 10 Å leads to a completely wet state in the intersolute region, as is evident from the instantaneous density plot (Figure 4f) and from the two well-defined peaks in the average density plot (Figure 4e). Thus, the vapor cavity in the intersolute region for a0 ) 6 Å plate is stable only up to a certain size corresponding to r0 ) 7.5 Å, above which the system undergoes a transition to the wet state through an intermediate state of liquid-vapor oscillations. The proposed mechanism5,8,33 of hydrophobic assembly depicts the formation of a vapor-liquid interface around a single hydrophobic solute due to breaking of the hydrogen bond network of water as a first step for the dewetting in the intersolute region. Therefore, by this mechanism, if dewetting in the intersolute region is observed, it is expected that a vapor-liquid interface around the single solute will also be formed. To verify this, we have investigated the behavior of water around a single solute plate with a0 ) 6 Å, for which we have observed intersolute dewetting (see Figure 4). The density profile, F(z)/F0, and the instantaneous density of water in the first hydration shell of the plate, Fhy(t), reveal a large first solvation shell peak (Figure 5a) and instantaneous densities (Figure 5b) larger than bulk densities for the entire simulation time and thus do not correspond to any liquid-vapor interface. Also, in the case of the two-solute system with a0 ) 6 Å, the equilibrium density of water at the outer surface of each of the plates (see Figure 4a) has a significant buildup, although the same between the two solutes is negligibly small. The formation of a vapor layer around the single solute is therefore not a prerequisite for the dewetting to be observed in the intersolute region. In order to compare the water penetration into the hydrophobic surface for this plate with the one with a0 ) 4 Å, the density profiles at the outside surface of the plate for the two cases are shown in the inset. As expected, water penetration is more in the case of a0 ) 6 Å. Water penetration into the surface may have two different kinds of effects: it may induce dewetting due to breaking of hydrogen bonds of the water molecule, or on the other hand, it may increase the hydrophilicity of the confined region as the penetrated water further attracts more water near the surface.
The multifaceted nature of hydrophobicity at the nanoscale has been observed through different manifestations of the behavior of water in the intersolute region. Although the effect of the solute dispersion interaction was studied10,14,34 earlier, in the present study, it has been demonstrated that by tunning the intermolecular spacing of the solute plate, keeping individual solute-water interactions unchanged, the manifestation of hydrophobicity can be regulated to achieve any of the wet, dry, and intermittent wet-dry states. The present study thus explains conflicting observations made in earlier experimental and theoretical studies in a single context. The present observation can be used to tailor new materials with required wettability and channels with desired water accessibility and also help to understand the environment near a protein surface. As has been observed in the present study, a small simulation time of a few hundred picoseconds is not sufficient to capture the intersolute hydration behavior of nanoscopic hydrophobic plates. Moreover, it has been demonstrated that the formation of a vapor layer around the single solute need not be the precursor for the dewetting transition in the intersolute region. Acknowledgment. The author is thankful to Prof. B. M. Pettitt for introducing him into the field of hydrophobicity and is also thankful to Dr. S. K. Ghosh and Dr. T. Mukherjee for their interest and encouragements. Thanks are due to the Computer Division, B.A.R.C., Mumbai, for providing Anupam supercomputing facilities and support. References and Notes (1) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley: New York, 1973. (2) Kauzmann, W. AdV. Protein Chem. 1959, 14, 1–63. (3) Ben-Naim, B. Hydrophobic Interactions; Plenum: New York, 1980. (4) (a) Pratt, L. R.; Pohorille, A. Chem. ReV. 2002, 102, 2671–2692. (b) Ashbaugh, H. S.; Pratt, L. R. ReV. Mod. Phys. 2006, 78, 159–178. (5) Chandler, D. Nature 2005, 437, 640–647. (6) Ball, P. Nature 2003, 423, 25–26. (7) Poynor, A. Phys. ReV. Lett. 2006, 97, 266101/1–266101/4. (8) Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570–4577. (9) Wallqvist, A.; Berne, B. J. J. Phys. Chem. 1995, 99, 2893–2899. (10) Choudhury, N.; Pettitt, B. M. J. Am. Chem. Soc. 2005, 127, 3556– 3567. (11) Choudhury, N.; Pettitt, B. M. Modelling Molecular Structure and ReactiVity in Biological Systems; RSC Publishing: London, 2005; Vol. 31, pp 49-57. (12) Huang, X.; Margulis, C. J.; Berne, B. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11953–11958. (13) Ashbaugh, H. S.; Paulaitis, M. E. J. Am. Chem. Soc. 2001, 123, 10721–10728. (14) Choudhury, N.; Pettitt, B. M. J. Am. Chem. Soc. 2007, 129, 4847– 4852. (15) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Nature 2001, 414, 188–190. (16) Sansom, M. S. P.; Biggin, P. C. Nature 2001, 414, 156–159. (17) Steitz, R. Langmuir 2003, 19, 2409–2418. (18) Ge, Z. B.; Cahill, D. G.; Braun, P. V. Phys. ReV. Lett. 2006, 96, 186101/1–186101/4. (19) (a) Doshi, D. A.; Watkins, E. B.; Israelachvili, J. N.; Majewski, J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9458–9462. (b) Takata, Y.; Cho, J.-H. J.; Law, B. M.; Aratono, M. Langmuir 2006, 22, 1715–1721. (20) Seo, Y. S.; Satija, S. Langmuir 2006, 22, 7113–7116. (21) Huang, X.; Zhou, R.; Berne, B. J. J. Phys. Chem. B 2005, 109, 3546–3552. (22) Vaitheeswaran, S.; Yin, H.; Rasaiah, J. C.; Hummer, G. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17002–17005. (23) Zhou, R.; Huang, X.; Margulis, C. J.; Berne, B. J. Science 2004, 305, 1605–1609. (24) (a) Koishi, T.; Yoo, S.; Yasuoka, K.; Zeng, X. C.; Narumi, T.; Sasukita, R.; Kawai, A.; Furusawa, H.; Suenaga, A.; Okimoto, N.; Futatsugi, N.; Ebisuzaki, T. Phys. ReV. Lett. 2004, 93, 185701/1–185701/4. (b) Koishi, T.; Yasuoka, K.; Ebisuzaki, T.; Yoo, S.; Zeng, X. C. J. Chem. Phys. 2005, 123, 204707/1–204707/7.
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