Is it Possible to Change Wettability of Hydrophilic Surface by

Letter pubs.acs.org/JPCL

Is it Possible to Change Wettability of Hydrophilic Surface by Changing Its Roughness? Prithvi Raj Pandey and Sudip Roy* Physical Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India S Supporting Information *

ABSTRACT: Wetting behavior of model rough surfaces made of hydrophilic square pillars is investigated. The hydrophilic pillars are equally spaced on hydrophilic surface. The surface roughnesses are altered by varying the pillar width and interpillar spacing. Wetting to dewetting transition is observed for these surfaces. This is one of the first accounts of observation from molecular simulations where hydrophilic surface converts into hydrophobic by changing its roughness. The extent of hydrophilicities are also changed to gain more insightful observations. Energies of the wetting to dewetting transitions are analyzed by calculating the contribution from water−water and water−surface energy components. A correlation between energy and the wetting to dewetting transition has been established, which rationally explains the observed water repellent nature of hydrophilic surface as a function of roughness. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

A

surface with affinity to attract water molecules change its wettability? If so, what are the factors that balance the attractive and repulsive interaction and how roughness is contributing to it? In the present study, we have constructed model hydrophilic surfaces to understand their wetting transitions using all-atom molecular dynamics (MD) simulations. The surfaces are designed with hydrophilic square pillars protruding out of the planar hydrophilic surfaces (Figure 1a). Square and cylindrical pillar arrayed hydrophobic surfaces have been studied in the past to vary surface morphology.1,9,13 Here we have constructed two sets of square pillar surfaces differing in width (W) of the pillars. Set W with pillars width of 1.56 nm and set 2W with pillars width of 3.12 nm (Figure 1a). In the case of set 2W, all four pillars of width 1.56 nm are positioned at the corner of simulation box on the planar surface (Figure 1a). Because of periodic boundary condition (PBC), the effective width of the pillars of set 2W become twice (i.e., 3.12 nm) that of set W. However, in the case of set W, out of four pillars, two are placed at the corner, and the other two are placed in the middle (Figure 1a) of the simulation box. Furthermore, for sets W and 2W, series of surfaces are constructed by varying the interpillar spacing (d) from 0.52 nm (equivalent to one linear array particles of planar surface between the pillars) to 2.86 nm (ten array). All of the surfaces are constructed by multiplying a cubic cell of side 0.26 nm. In the present study, height (h) of the pillars is kept constant at 1.82 nm for all surfaces belonging to both sets.

solid surface can be hydrophobic- or hydrophilicdependent on the property of the material of surface. It is well known that surfaces made of hydrophobic materials repel water molecules. Surface structuring can also have significant effect on surface wetting.1−4 The hydrophobic surface can be converted into more hydrophobic (often referred to as superhydrophobic) by changing it roughness.5 A classical example is the lotus effect.6,7Complex surface structuring on the micro- and nanoscales enhances the hydrophobicity of the surface of lotus leaves, helping them remain dry and clean.7 It remains an unresolved proposition whether the introduction of roughness on hydrophilic surface (i.e., the surface that has higher affinity toward water) can change its characteristics. Some experimental and theoretical evidence is present. For example, Abdelsalam et al reported, that by controlling the surface nanostructuring, hydrophilic gold surface was changed to hydrophobic.8 It shows that favorable interaction between surface and water of hydrophilic surface can be overturned by changing the surface topology. Drelich et al. recently reviewed the superhydrophilic and superhydrophobic materials9 based on surface topography. Yuan et al studied the dynamics of the water droplet on flexible hydrophilic pillars10,11 in the presence of an external force and showed that flexibility of pillars and driving force is needed to wet the surface. Spreading of a liquid on a surface, that is, wetting of the surface, is determined by the electrostatic and van der Waals interactions between the surface and the liquid,12 but it is still not clear whether change in interaction between water and surface or change in water−surface contact area (by changing the surface topology) can tune the wettability of a surface. Observing this, it is intuitive to ask − can structuring of any © 2013 American Chemical Society

Received: September 11, 2013 Accepted: October 16, 2013 Published: October 16, 2013 3692

dx.doi.org/10.1021/jz401946v | J. Phys. Chem. Lett. 2013, 4, 3692−3697

The Journal of Physical Chemistry Letters

Letter

Figure 1. (a) Construction of the surfaces, and snapshots at different time for R = 1.93 of set 2W and R = 1.994 for set W. (b) Plot of surface roughness (R) as a function of d. (c) Plot of LJ potential at different εij values between particles of surface and TIP4P-water oxygen.

interaction parameters, σij = (σiiσjj)1/2 and εij = (εiiεjj)1/2, are defined between TIP4P-water oxygen and surface atoms, as σ and ε for TIP4P-water hydrogen are zero. For each surface belonging to sets W or 2W, the εij is further varied to increase the hydrophilicity. For both sets (W and 2W), three different εij values, 0.74, 1.103, and 1.48 kJ/mol, were considered. The LJ potentials between surface atom and TIP4P-water oxygen for the three εij values are plotted in Figure 1c. As an estimate of the extent of hydrophilicity of the surfaces, the LJ potential between aliphatic alcohol in the OPLS force field15,16 and TIP4P-water oxygen is plotted in the same Figure. εij values considered for the surfaces are higher than εij for OPLS alcohol oxygen (Figure 1c). This indicates higher hydrophilicity of the particles constituting the surfaces with respect to OPLS alcohol oxygen. The particles constituting the surface are not charged. All simulations are performed with GROMACS4.5.5,17 and methods of simulation are provided in ref 18.

Square surface topology, which is a function of different parameters (width, height, interpillar spacing) can be translated to roughness.11 Roughness (R) of the surface containing pillars considered here is defined as R = 1 + 4wh/p2. Here w, h, and p are the width, height, and period of the pillars, respectively (Figure 1a). R = 1 signifies a smooth surface. R as a function of d shows a continuous decrease in roughness with increasing d (Figure 1b) because with increase in d we move closer to smooth surface, and hence roughness decreases. Beyond d = 2.08 nm, the roughness for both sets of surfaces become similar (Figure 1b). Water (TIP4P14) is placed on top of these surfaces such that it completely wets the exposed planar surface and the pillars in both of the sets. Our objective is to recognize whether these wet hydrophilic surfaces remain completely wet and if not how and when the transitions happen. The interaction of water molecules with the surface is modeled via Lennard-Jones (LJ) 12−6 potential. The 3693



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Table 1. Summary of Final States of All the Surfaces along with d and Ra Set 2W d (nm) R εij = 0.74 kJ/mol εij = 1.103 kJ/mol εij = 1.48 kJ/mol

0.52 2.714 dewet dewet wet




d (nm) R εij = 0.74 kJ/mol εij = 1.103 kJ/mol εij = 1.48 kJ/mol




1.3 2.388 dewet wet wet






2.86 1.635 wet




2.34 1.746 wet wet wet



Set W

a

Wet indicates the fully wet (planar surface and pillar walls) state and dewet indicates the dewet interpillar spaces.

Figure 2. Z component of center of mass of water as a function of time at different roughness for both sets.

set W. A 20 ns simulation is performed for set 2W for d = 1.3 and 1.56 nm. For larger d, on average, 40 ns simulations (in some cases 50 and 60 ns) are performed. Choice of simulation times have been made depending on appearance of partially wet states. Dewetting takes place only through the partially wet states. That is why at εij = 1.48 kJ/mol for both sets simulations were not extended beyond 20 ns, as no partially wet states appear and all surfaces remain fully wet, similar to the starting structure. Snapshots for different roughnesses at different εij are provided in the Supporting Information (SI). Studying two different sets of surfaces with varying εij values gives us the chance to understand the wetting-to-dewetting transition as a function of water−surface contact area (different roughnesses) and surface hydrophilicity. Furthermore, to quantify the time at which water molecules completely dewet the interpillar spaces, we plot the z component of the center of mass (CMz) of all water molecules as a function of time in Figure 2 for all surfaces of sets 2W and W at εij = 0.74 kJ/mol. CMz increases initially and then fluctuates near a constant value for all roughnesses for which water moves out of the interpillar spaces (Table 1) to the bulk. But for R = 1.635 of set 2W and R = 1.746, 1.656, and 1.581 of set W, for which water molecules do not move out, CMz

In the case of set 2W, water molecules are observed to be leaving the interpillar spaces as roughness decreases until R = 1.694 (where d = 2.6 nm) at εij = 0.74 kJ/mol. That is, in the final state, only the top surface of the pillars is wet, and the interpillar spaces have undergone dewetting (Figure 1a), but, on increasing the εij to 1.103 kJ/mol, dewetting of interpillar spaces takes place only until R = 2.162 (d = 1.3 nm), and beyond R = 2.162, the surface remains completely wet including the interpillar space in the final state. On further increasing εij to 1.48 kJ/mol there is no dewetting observed, even at R = 2.714 (d = 0.52 nm). All of these observations for both sets of surfaces are summarized in Table 1. In the case of set W, dewetting of interpillar spaces occurs until R = 1.857 (d = 2.08 nm, whereas in set 2W water comes out until R = 1.694 (d = 2.6 nm)) at εij = 0.74 kJ/mol (Table 1). At εij = 1.103 kJ/ mol dewetting occurs until R = 2.68 (up to R = 2.162 for set 2W). And akin to set 2W no dewetting takes place at εij = 1.48 kJ/mol for set W (Table 1). The dewetting of interpillar spaces occurs through a partially wet state (Figure 1a) for both set W and 2W. In the partially wet state, the planar hydrophilic surface and the pillar walls are partially wet as some water molecules already started to move out to the bulk. For d up to 1.56 nm and εij = 0.74 kJ/mol, the systems are simulated up to 10 ns for 3694



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Figure 3. ΔE as a function of R for both sets along with individual components of ΔE as a function of R.

fluctuates near a constant value for whole simulation time. The initial increase in CMz is indicative of the appearance of partially wet states, where some water molecules have moved to the bulk. When the interpillar spaces are completely dewet, that is, no water to move to the bulk, the CMz fluctuates near a constant value. This also justifies our choice of simulation times. If the initial slope was observed, we kept extending the simulation to achieve complete dewetting. Also, it indicated that for the wet cases (Table 1) longer simulations will not make difference. These observations and their explanations can also be translated to CMz plot for εij = 1.103 kJ/mol (where surface remains completely wet after certain R) and 1.48 kJ/ mol (completely wet at all R) illustrated in SI for both 2W and W sets. Now, it is necessary to understand why the water molecules leave the interpillar spaces of the hydrophilic surfaces and move to the bulk. The wetting or dewetting of the surfaces should, in principle, be guided by atomic level water−water and water− surface nonbonded interactions. The interplay of nonbonded interactions between water−surface and water−water should play the decisive role in this observation. Thus, the change in nonbonded potential energy (PE) between final (wet or dewet) and initial (wet) states (ΔE) for all surfaces belonging to sets W and 2W is calculated as ΔE = ΔE LJ + ΔE LJ + ΔECww ws

ww

100 frames and the average PE of the initial 100 frames. Plots of ΔE as a function of R are depicted in Figure 3. The more negative the value of ΔE, the more favorable the final state (wet or dewet). At εij = 0.74 kJ/mol for both sets 2W and W, ΔE moves toward more negative value at higher roughness (low d) but moves toward zero at lower roughness (high d). It indicates, at R = 2.714 and 2.493 for set 2W and R = 3.625 and 3.074 for set W (d = 0.52 and 0.78 nm), that dewetting of the interpillar spaces is energetically favorable, but, as roughness decreases, the PE difference between final and initial states decreases, and eventually it should go to zero. A similar trend can be observed at εij = 1.103 kJ/mol for both cases (Figure 3). PE difference increases with increase in roughness. The difference observed is due to the water molecules, which leave the interpillar spaces. Furthermore, at εij = 1.48 kJ/mol, where there is no dewetting even at R = 2.714 for set 2W and at R = 3.625 for set W (d = 0.52 nm), ΔE does not change much with a decrease in roughness for both sets. The small change in energy is due to the rearrangement of the water molecules. Observing the trends in ΔE, it is imperative to envisage the trends of individual components constituting ΔE. Plots of ΔELJws, ΔELJww, and ΔECww are also depicted in Figure 3. ΔECww is small for all roughnesses at all εij values for both the sets compared with other components. Thus contribution from ΔECww to ΔE is least. So the cohesive interaction due to electrostatic interaction between water molecules does not have much of a role to play in wetting. However, the value of ΔELJws is positive for the roughness values where water dewets the interpillar spaces because in the dewet state only the upper surface of the pillars are wet. This is unfavorable in terms of

(1)

where ΔELJws is the change in water−surface LJ PE, ΔELJww is the change in water−water LJ PE, and ΔECww is the change in water−water electrostatic PE. The individual PE changes are calculated as the difference between the average PE of the last 3695



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addition to this, the shape of the pillars can also be varied. Thus there is plenty of scope to extend the observations gathered here to further investigate and understand wetting of hydrophilic surfaces.

water−surface nonbonded PE, as compared with the fully wet state where surface water contact area is higher, but for roughness values where there is no dewetting (R = 1.635 for set 2W at εij = 0.74 kJ/mol, R = 1.581 to 1.746 for set W at εij = 1.103 kJ/mol, etc.) the value of ΔELJws is smaller (or almost no change). The observed value is due to the rearrangement of water molecules. ΔELJww shows favorable negative values for roughness values where dewetting occurs then slowly moves toward zero as roughness is decreased. It can be observed that the trend in ΔE is primarily guided by ΔELJww. It is evident that there exists a competition between ΔELJww and ΔELJws. With a decrease in roughness, the contact area (not calculated in this work) between water and surface increases, and hence the volume of water filling the interpillar spaces also increases. Now, dewetting of the interpillar spaces implies that water molecules vacate the interpillar spaces and move to the bulk because of favorable interaction, but to influence the water molecules (by favorable energy) to remain in the interpillar spaces, ΔELJws has to overcome ΔELJww. Thus, at εij = 0.74, with decrease in roughness the difference between ΔELJws and ΔELJww (Figure 3) decreases and the surfaces remain wet after certain value of R. At εij = 1.103 kJ/mol, the water−surface interaction is more favorable than that at 0.74 kJ/mol, and thus the roughness below which the surfaces remain wet is also much higher. At εij = 1.48 kJ/mol, the difference between ΔELJws and ΔELJww remains in the range of wet roughness values of 0.74 and 1.103 kJ/mol. Hence, wetting or dewetting is guided by both roughness and εij. There is not much difference in the wetting and dewetting behaviors of surfaces belonging to sets 2W and W with respect to change in roughness at a certain εij. This is possibly because after certain d there is not much difference in roughness of the surfaces. Similar roughness can be achieved from a completely different set of width of the pillars, interpillar distances, and heights (variation in height has not been explored in this work). However it seems that the wetting-todewetting transition very well follows the unified roughness parameter and gives a predictable observation for certain value of εij. Kim et al have studied the drying of the interpillar spaces in hydrophobic surfaces using grand canonical Monte Carlo simulations.5 They have studied the dependency of drying on the shape and size of the hydrophobic pillars. In the present study, we have observed drying of interpillar spaces of hydrophilic surfaces. We have also explored the critical d and εij beyond which the dewetting transition does not occur. We may infer that nanostructuring guides the hydrophilicity of a hydrophilic surface depending on both surface topology (in terms of roughness) and the interaction parameters (εij). To conclude, we have studied wetting and dewetting of hydrophilic pillar surfaces using MD simulations. We have observed that the interpillar spaces undergo dewetting above certain roughness and below certain εij. Competition between LJ PE among water−water and water−surface guides the wetting and dewetting transitions. These, in turn, depend on the volume of water present in the interpillar spaces and hence water−surface contact area. Although in this study we have varied the W and d of the pillars, there is scope of further varying the height of the pillars to understand its effect on wetting. Also, the width of the square pillar can be further varied. Changing both W and d will affect roughness. In

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ASSOCIATED CONTENT

S Supporting Information *

Snapshots for different R and εij are provided. Table containing ΔE values and its components for both sets of surfaces. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91 (020) 25902735. Fax: +91 (020) 25902636. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge CSIR and NCL for financial support. S.R. gratefully acknowledges center for excellence in scientific computing (project code CSC0129, CSIR-NCL) for financial support and computational time.

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REFERENCES

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(16) Kaminski, G. A.; Friesner, R. A.; Tirado-Rives, J.; Jorgensen, W. L. Evaluation and Reparametrization of the OPLS-AA Force Field for Proteins via Comparison with Accurate Quantum Chemical Calculations on Peptides. J. Phys. Chem. B 2001, 105, 6474−6487. (17) Pronk, S.; Páll, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D.; Hess, B.; Lindahl, E. GROMACS 4.5: a high-throughput and highly parallel open source molecular simulation toolkit. Bioinformatics 2013, 29, 845−854. (18) NVT simulations were performed, keeping vacuum on top of water layer in all systems with a time step of 1 fs. Vacuum was considered on top of the water layer so that water molecules on top layer did not interact with the lower part of the planer surface due to PBC. All systems for both sets were energy-minimized using the steepest descent algorithm before generating the MD trajectories. The particles of surface were fixed. Temperature was kept constant at 300 K using a V-rescale thermostat. A cut off of 1 nm was used for both LJ and coulomb interactions. Long-range electrostatic interactions were treated with particle-mesh Ewald (PME) method.

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Is it Possible to Change Wettability of Hydrophilic Surface by

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