Molecular Dynamics Simulation Study on Controlling the Adsorption

802

Langmuir 2007, 23, 802-808

Molecular Dynamics Simulation Study on Controlling the Adsorption Behavior of Polyethylene by Fine Tuning the Surface Nanodecoration of Graphite Xiao-Lin Wang, Zhong-Yuan Lu,* Ze-Sheng Li,* and Chia-Chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, China ReceiVed May 25, 2006. In Final Form: September 19, 2006 Molecular dynamics simulations are applied to study the adsorption of polyethylene with different chain lengths on patterned graphite surfaces that contain nanoscale protrusions. The influence of the nanostructure on the strong attractive interaction inherently in the hydrophobic polyethylene and hydrophobic graphite system is investigated by modifying the top surface area and the height and the shape of the protrusions. The results are analyzed in terms of the chain configuration, the adsorption energy, the global orientational order parameter, and the normalized surfacechain contacting pair number in the first adsorption layer. When the size of the protrusion increases, the adsorption energy, the order parameter, and the normalized surface-chain contacting pair number decrease at a fixed chain length. When the size of the protrusion is fixed, the average adsorption energy per monomer and the order parameter decrease with increasing chain length because of the stronger intramolecular interactions between the monomers. Changing the protrusion shape in a suitable way will effectively reduce the strong surface-chain interaction.

1. Introduction The behavior of liquids on laterally heterogeneous surfaces has attracted much theoretical attention1-4 because of its relevance to the basic properties of liquids that are confined to nanoscale structures.5,6 Related phenomena such as the effect of nanoroughness on wetting7 and the crossover to capillary filling8 have an impact on emerging applications, including nanofluidic devices,9 nanotemplating,10 and surface rheology.11 The problem of wetting (in other words, from the contrary view, hydrophobicity or superhydrophobicity) on the surface has been a hot topic in recent years.12-19 To see how surface structuring can increase the hydrophobicity of a surface, a view to the so-called lotus effect is first useful.20 The water on the surface of a lotus leaf is very unstable, and on rolling off, it takes off any dirt with it. * To whom correspondence should be addressed. (Z.-Y.L.) E-mail: [email protected]. (Z.-S.L.) E-mail: [email protected]. (1) Rascon, C.; Parry, A. O. Nature (London) 2000, 407, 986. (2) Rejmer, K.; Dietrich, S. Napiorkowski, M. Phys. ReV. E 1999, 60, 4027. (3) Milchev, A.; Mu¨ller, M.; Binder, K.; Landau, D. P. Phys. ReV. E 2003, 68, 031601. (4) Lenz, P.; Lipowsky, R. Phys. ReV. Lett. 1998, 80, 1920. (5) Dietrich, S. J. Phys.: Condens. Matter 1998, 10, 11469. (6) Becker, T.; Mugele, F. Phys. ReV. Lett. 2003, 91, 166104. (7) Quere, D. Physica A 2002, 313, 32. (8) Gelb, L. D. Mol. Phys. 2002, 100, 2049. (9) Cao, H.; Yu, Z. N.; Wang, J.; Tegenfeldt, J. O.; Austin, R. H.; Chen, E.; Wu, W.; Chou, S. Y. Appl. Phys. Lett. 2002, 81, 174. (10) Guo, L. J. J. Phys. D 2004, 37, R123. (11) McHale, G.; Shirtcliffe, N. J.; Aqil, S.; Perry, C. C.; Newton, M. I. Phys. ReV. Lett. 2004, 93, 036102. (12) Lipowsky, R. Curr. Opin. Colloid Interface Sci. 2001, 6, 40. (13) Xia, Y. N.; Qin, D.; Yin, Y. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 54. (14) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (15) Song, X. Y.; Zhai, J.; Wang, Y. L.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048. (16) Sun, T. L.; Wang, G. J.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2003, 125, 14996. (17) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857. (18) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X. Langmuir 2005, 21, 1986. (19) Yu¨ce, M. Y.; Demirel, A. L.; Menzel, F. Langmuir 2005, 21, 5073. (20) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1991.

Many plant leaves have this marvelous “skill”. The surface of the lotus leaf is structured on a micrometer length scale. The water drop rests only on the tips of the peaked microstructures so that the contact area between the leaf and droplet is minimized. Some detailed theoretical and experimental studies have already been done on the lotus effect to describe how the hydrophobicity changes as a result of microstructuring.21-24 The problem of drop roll-off from the surface was dealt with by a simple theoretical model of the protrusions on a planar surface by Marmur.21 With the advent of modern techniques, it is possible to prepare different patterned substrates with both chemical and physical methods. For example, lithography techniques can chemically modify a substrate with lyophobic and lyophilic surface domains on the nano- to micrometer scale.25,26 Moreover, thermal oxidation (i.e., ion bombardment and O2 oxidative etching) can be used to control the surface roughness of the basal plane on the nanometer scale.27 These structured surfaces are useful in controlling the wetting of liquid droplets. Among the species used as adsorbates, hydrocarbons, which comprise the simplest family of compounds whose members differ mainly in their lengths, are of considerable interest because of their technological importance. The long-chain hydrocarbons are usually used as components of lubricants and coating materials. Polyethylene (PE) can be taken as a general model of hydrocarbons, and molecular dynamics (MD) simulations have been successfully applied to study its crystallization in vacuum and adsorption on graphite surface.28-33 In these studies, the common (21) Marmur, A. Langmuir 2004, 20, 3517. (22) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (23) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824. (24) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405. (25) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Res. 1998, 28, 153. (26) Volkmuth, W. D.; Austin, R. H. Nature (London) 1992, 358, 600. (27) Tracz, A.; Stabel, A.; Rabe, J. P. Langmuir 2002, 18, 9319. (28) Zhang, X. B.; Li, Z. S.; Lu, Z. Y.; Sun, C. C. J. Chem. Phys. 2001, 115, 10001. (29) Zhang, X. B.; Li, Z. S.; Lu, Z. Y.; Sun, C. C. J. Chem. Phys. 2001, 115, 3916. (30) Fujiwara, S.; Sato, T. J. Chem. Phys. 2001, 114, 6455. (31) Fujiwara, S.; Sato, T. J. Chem. Phys. 1997, 107, 613.

10.1021/la061492h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006

Controlling the Adsorption BehaVior of Polyethylene

PE chain length is between 250 and 2002. Therefore, in this work we also adopt three typical chain lengthss500, 1000, and 3000sboth for clarifying the adsorption differences due to chain length change and saving computation time. Many experiments and simulations have shown that the attractive interaction between graphite and PE is so strong that the crystal-like PE will be two-dimensionally adsorbed to the surface. The motivation of this study is, therefore, to elucidate to what extent the surface nanoroughness influences the intrinsically strong graphite-PE interaction and correspondingly the chain adsorption behavior. A graphite surface is also chosen for of its relative simplicity and rigidity, so it can be treated as a fully rigid body for our purposes. Nanoroughness may arise from imperfections in the planar crystalline surface, resulting in features such as cracks, pits, steps, protrusions, and ledges. More complicated geometries would be possible through a mixture of these basic shapes. Herein we design the protrusions34 of nanometer dimension raised on the planar surface to represent nanoroughness due to its simplicity. This nanoscale surface roughness is amenable to detailed investigations by molecular dynamics simulations.35 In this study, we consider a single PE chain adsorbed on a patterned rigid graphite surface. The adsorption of the chain is investigated through energy minimizations and molecular dynamics simulations. We follow a twostep strategy: first we carry out direct energy minimizations for the PE chain close to the patterned graphite surface with different initial configurations in order to relax the model, and then we use the most stable configuration after minimization to perform MD simulations. With the results of this study, we try to answer the following questions from a microscopic point of view. First, to which extent can the strong attractive interaction between the hydrophobic PE and the hydrophobic graphite surface be reduced by the existence of nanoroughness? Second, what is the size and the shape of the protrusion topography that most influences the adsorption of the PE chain on the hydrophobic surface?

2. Models and Simulation Details The simulation is carried out in a box (X ) Y ) 11.8 nm, Z ) 50.0 nm) with 3D periodic boundary conditions. Three graphite layers attached with protrusions of different sizes and shapes serve as the models of our surface. The schematic representation of the surface is shown in Figure 1. Intuitively, we have considered that, to enhance the effects of nanoroughness on the PE adsorption, the size of the surface protrusion should be comparable to the radius of gyration of PE. If the protrusion size is too small, then PE chain is unable to “feel” the existence of the protrusion. If the protrusion size is too large compared to the PE chain lengths, then PE may feel itself being adsorbed on the basal surface. Therefore, we must choose the surface protrusion size to be comparable to the radius of gyration of PE. We design four nanopatterned surface models by adjusting the top surface area (or the breadth of the protrusion, L), the distance between the protrusions (D), the protrusion height (H), and the protrusion shape.36 This detailed information is listed in Table 1. The length of the simulation box in the Z direction is large enough that the interactions between the adsorbed PE chain and the periodic images of graphite in the top plane can be ignored. In this way, the 3D periodicity inherent in the model is transformed into an (32) Guo, H. X.; Yang, X. Z.; Li, T. Phys. ReV. E 2000, 61, 4185. (33) Liao, Qi; Jin, X. G. J. Chem. Phys. 1999, 110, 8835. (34) Pal, S.; Roccatano, D.; Weiss, H.; Keller, H.; Mu¨ller-Plathe, F. ChemPhysChem 2005, 6, 1641. (35) Pal, S.; Weiss, H.; Keller, H.; Mu¨ller-Plathe, F. Langmuir 2005, 21, 3699. (36) Brault, P.; Moebs, G. Eur. Phys. J.: Appl. Phys. 2004, 28, 43.

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Figure 1. Schematic representations of the patterned graphite surface drawn to scale in the top and side views. H and L denote the height and the breadth of a protrusion, respectively, and D denotes the distance between the protrusions. Table 1. Details of Four Different Nanopatterned Surface Structures pattern

1

2

3

4

L (nm) D (nm) H (nm) shape

1.0 4.9 1.0 diamond

3.0 2.9 1.0 diamond

3.0 2.9 3.0 diamond

3.0 2.5 3.0 square

actual 2D periodicity, thus simulating an infinitely extended surface.37 The PE chain is initially placed above the center of the gap between four protrusions with the basal graphite surface parallel to the XY plane. The Dreiding II force field38 is used in the simulations. The united atom approximation is adopted in order to simplify the calculations.39 The van der Waals interactions are truncated at rc ) 12 Å using a spline function from 11 Å. Before the MD simulations, energy minimizations are performed to relax the local unfavorable structures of the chains. Subsequently, 5 ns MD simulations with a time step 1 fs are performed in NVT ensemble. The equations of motion are integrated using a leapfrog algorithm.40,41 The temperature T ) 300 K is controlled via the Hoover thermostat42 with a relaxation time of 0.1 ps.

3. Results and Discussion We systematically vary the protrusion top surface area (L), the protrusion height (H), the protrusion shape, and the degree of polymerization of PE (N) to investigate the effect of surface roughness on the adsorption of PE on graphite. For PE, both initial random coil (representing Gaussian chain) and orderly folded (prototype for lamellar crystal) configurations are considered; therefore, we have the chance to study the competition between the intrachain and the surface-chain interactions in different situations. All of the simulations are performed until running the program further results in no discernible changes in the structural properties and the total energies beyond natural fluctuations. We then analyze the chain configurations and (37) Wang, X. L.; Lu, Z. Y.; Li, Z. S.; Sun, C. C. J. Phys. Chem. B 2005, 109, 17644. (38) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897. (39) Kavassalis, T. A.; Sundararajan, P. R. Macromolecules 1993, 26, 4144. (40) Allen, M. P.; Tildesly, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, U.K., 1987. (41) Frenkel, D.; Smit, B. Understanding Molecular Simulation; Academic Press: San Diego, CA, 2002. (42) Hoover, W. G. Phys. ReV. A 1985, 31, 1695.

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Table 2. Dependence of the Adsorption Energy and the Global Orientational Order Parameter on the Protrusion Top Surface Area for PE with an Initial Random Coil Configuration protrusion area effect

pattern 1

pattern 2

N

E(kcal/mol)

P2

E(kcal/mol)

P2

500 1000 3000

-2436.06 -3546.75 -5334.17

0.929 0.919 0.890

-2253.86 -2744.97 -3855.43

0.918 0.900 0.884

calculate the adsorption energies, the chain global orientational order parameters, and the normalized surface-chain contacting pair numbers in the first adsorption layer. 3.1. PE Chain with Initial Random Coil Configuration. 3.1.1. Effect of Modifying the Protrusion Top Surface Area. By viewing the snapshots in the simulation with surface pattern 1 (L ) 1 nm), we find that the PE chain changes the configuration from a random coil to a compact form adsorbed on the basal graphite surface. This adsorption behavior is very similar to that of PE adsorbed on a planar graphite surface. After equilibrium, for short chain length N ) 500, the PE configuration is characterized by a 2D single layer, whereas for long chain length N ) 3000 the PE configuration is 3D and possesses a certain height in the direction perpendicular to the surface. The phenomenon for PE with N ) 1000 is between the two abovementioned phenomena. For surface pattern 1, the spaces between the protrusions are enough to put in a short chain and H is small, thus the chain is completely adsorbed onto the basal surface just as PE is being adsorbed on a planar graphite surface. The adsorption behavior of a longer chain on such a surface is also similar to that of PE being adsorbed on a planar graphite surface. Therefore, a small protrusion size may scarcely affect the adsorption of PE on graphite. When the protrusion top surface area is large (pattern 2, where L ) 3 nm), the PE chain, for N ) 500, 1000, or 3000, is first adsorbed onto one protrusion, and then the chain starts to feel the attraction of the basal surface and the top surfaces of the nearby protrusions. Consequently, the chain is adsorbed onto the protrusion and basal surfaces at the same time. Finally, at equlibrium, the PE chain partially sits on the top surfaces of protrusions and partially in the gaps between them, in contact with the basal surface. The adsorption behavior of a chain molecule onto a surface can be quantitatively characterized by the adsorption energy, the chain global orientational order parameter, and the normalized surface-chain contacting pair number in the first adsorption layer. The adsorption energies E, which are listed in Table 2, are calculated via

E ) Etot - (Efrozen + Eplane)

(1)

where Etot is the potential energy of the whole system at equilibrium, Efrozen is the potential energy of the adsorbed chain isolated in vacuum with the geometry unchanged, and Eplane is the potential energy of the surface. The chain global orientational order parameter, also shown in Table 2, is defined as30

P2 ) 〈0.5 × 〈3 cos2(θ) - 1〉bond〉

(2)

where θ is the angle between two adjacent sub-bond vectors i - 1 and i. The sub-bond vector is the vector formed by connecting the centers of two adjacent bonds. 〈...〉bond denotes the average over the sub-bonds in a model. The outermost 〈...〉 means the time average. The parameter P2 takes the value of 1.0 when the PE chain is completely in a planar zigzag form.

Figure 2. Time evolution of the normalized surface-chain contacting pair number in the first adsorption layer of PE with N ) 500 and the initial random coil configuration in a simulation with surface pattern 1.

In Table 2, we compare the adsorption energies and the chain global orientational order parameters at different chain lengths obtained in the simulations with surface patterns 1 (small top area) and 2 (large top area). The adsorption energy decreases with increasing protrusion top surface area at a fixed chain length, which implies that enlarging the protrusion surface to a suitable extent will effectively reduce the strong attractive interaction between the PE chain and the graphite surface. There must be a critical value when increasing the top surface area to reduce the chain-surface interaction because the protrusion top surface area cannot be expanded endlessly and there should be a restrictive relationship between the protrusion top surface area and the chain radius of gyration. One can imagine that if the protrusion top surface area were enlarged as much as the basal surface area or it were so large as to make PE feel itself being adsorbed on the basal surface then the PE adsorption behavior under these conditions would be similar to that on the planar surface. The surface-chain contacting pair number in the first adsorption layer is directly associated with the adsorption energy; therefore, we have calculated the normalized contacting numbers under different conditions during the simulations. The PE chain with N ) 500 is taken as an example. The normalized contacting numbers with surface patterns 1 and 2 are shown in Figures 2 and 3, respectively. The results for PE of other lengths are similar. The large normalized contacting number obviously results in the large adsorption energy. When the protrusion top surface area is fixed, the adsorption energy increases but its average per monomer decreases with increasing N. This is in accord with the limited number of polymer-surface contacting sites in the system. The short chain can be two-dimensionally adsorbed on the surface so that the contacting sites increase proportionally with increasing chain length N. The configuration of long chains after adsorption is still 3D; therefore, the contacting sites per monomer and consequently the adsorption energy per monomer are reduced with increasing N. Such an intuitive explanation can be further checked by comparing the normalized surface-chain contacting pair numbers in the first adsorption layer of PE with different chain lengths as shown in Figure 4, where pattern 1 is used as the surface model. It is apparent that the normalized contacting number decreases with increasing N, which is consistent with the above adsorption energy tendency for a fixed surface pattern. The chain adsorption process can be characterized by the time evolution of the global orientational order parameter, P2. A typical plot for P2 in the case of PE with length N ) 500 and surface pattern 2 is shown in Figure 5. In other cases, the tendency of


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Figure 4. Time evolution of the normalized surface-chain contacting pair numbers in the first adsorption layer of PE with N ) 500, 1000, and 3000 in the simulations with surface pattern 1. The initial chain configuration is random coil.

Figure 5. Time evolution of the global orientational order parameter P2 of PE with N ) 500 in a simulation with the initial random coil configuration and surface pattern 2.

P2 is similar. In the beginning of the simulation, the chain configuration is random coil, for which P2 is roughly 0.60. In a very short time interval, P2 increases to about 0.85, which corresponds to the fast process in which the chain is adsorbed onto the surface. The chain sub-bond vectors contacting the surface are aligned locally according to the sub-bond vectors in

Figure 6. Equilibrium snapshots of a PE chain with N ) 500, 1000, and 3000 being adsorbed on the graphite surface of pattern 3 as shown in a-c, respectively. The dark-gray color denotes three graphite layers in the side view, and the light-gray color denotes the protrusions on the basal surface. The black color represents the PE chain. The simulation box boundary is not shown for clarity. We also adjust the angle of view and scale to ensure that these three snapshots can be put together in one figure.

the first layer of graphite. Afterward, P2 increases slowly, which is manifested in the chain adjusting its configuration to fit the surface sub-bond vectors better. Finally P2 reaches the equilibrium value of 0.918 (indicated by the dotted line in Figure 5), which is the time average taken from the equilibrium states between

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Table 3. Dependence of the Adsorption Energy and the Global Orientational Order Parameter on the Protrusion Height for PE with an Initial Random Coil Configuration protrusion height effect

pattern 2

Table 4. Dependence of the Adsorption Energy and the Global Orientational Order Parameter on the Protrusion Shape for PE with an Initial Random Coil Configuration. protrusion shape effect

pattern 3

pattern 3

pattern 4

N

E(kcal/mol)

P2

E(kcal/mol)

P2

N

E(kcal/mol)

P2

E(kcal/mol)

P2

500 1000 3000

-2253.86 -2744.97 -3855.43

0.918 0.900 0.884

-404.55 -438.06 -501.79

0.883 0.875 0.870

500 1000 3000

-404.55 -438.06 -501.79

0.883 0.875 0.870

-398.66 -416.21 -468.39

0.881 0.868 0.863


4000 and 5000 ps. The average values of P2 for all samples are listed in Table 2. In our simulations, the increase in the chain global orientational order parameter is due to the strong surface adsorption. Therefore, P2 can also be taken as a surface-induced order parameter that shows the strength of the chain-surface interaction. From Table 2, we find that P2 decreases with increasing N in each case of the surface model. Moreover, P2 decreases by increasing the protrusion top surface area at a fixed chain length. All of the above results imply that increasing the protrusion surface to a suitable extent will reduce the attractive interaction between PE and graphite. Such a tendency is chainlength-independent in the range of values selected in this study. 3.1.2. Effect of Modifying the Protrusion Height. The equilibrium snapshots for the simulation with H ) 3 nm are shown in Figure 6a-c for N ) 500, 1000, and 3000, respectively. It is clear that all of the PE chains stay on one protrusion when at equilibrium and the chain itself is folded in an orderly manner. In Table 3, we compare the adsorption energies and the chain global orientational order parameters at different chain lengths for surface patterns 2 and 3. With increasing H, both the adsorption energy and the order parameter decrease at a fixed chain length. As described above, for surface pattern 2, the PE chain is first adsorbed onto one protrusion and then falls down to the basal surface. When H increases, the contacting opportunity between the chain and the surface is reduced. PE chain always stays on one protrusion and will not spread down to the basal surface in the case of surface pattern 3, which is verified by comparing the normalized surface-chain contacting pair numbers between PE/ pattern 2 (Figure 3) and PE/pattern 3 (Figure 7). The value of the latter is much smaller than that of the former, which is attributed to the increase in the protrusion height. Thus, increasing the protrusion height will reduce the attractive interaction between PE and graphite effectively. This tendency can be further verified in the simulations with another nanopatterned surface (pattern 4); see below.

Figure 8. Time evolution of the normalized surface-chain contacting pair number in the first adsorption layer of PE with N ) 500 and the initial random coil configuration in a simulation with surface pattern 4. Table 5. Intrachain Energy per Monomer in the Cases of Two Nanopatterned Surfaces with Different Protrusion Shapes intrachain energy

pattern 3

pattern 4

N

E(kcal/mol)

E(kcal/mol)

500 1000 3000

-1.98 -2.29 -2.43

-2.02 -2.35 -2.50

It should be noted that the above results are true only when the PE chain length is small. As for the longer chain, the phenomenon may be different. For example, if the protrusion top surface is not large enough to sustain the whole chain staying on, then the PE chain will likely spread down to the basal surface. 3.1.3. Effect of Modifying the Protrusion Shape. As shown in Figure 1, the diamond-shaped protrusions are arranged in a diamond pattern on the surface, and the square-shaped protrusions are arranged in the equilateral triangle pattern. By design, the protrusion top surface area of surface pattern 3 is 13.4% smaller than that of pattern 4. Thus, following the above discussion, the chain-surface interaction should be much more reduced in the case of surface pattern 4 instead of pattern 3. In Table 4, we show the adsorption energies and the chain global orientational order parameters for both cases. We find that, at the same chain length, the adsorption energy and the order parameter for surface pattern 4 are smaller than those for pattern 3. This can be further verified by the normalized surface-chain contacting pair numbers, for example, Figure 7 for pattern 3 compared to Figure 8 for pattern 4 with N ) 500. The results indicate that the effect of modifying the protrusion shape on the chain adsorption behavior is ascribed to modifying the protrusion top surface area. The larger the area is, the stronger the ability to reducing the PE-graphite attractive interaction. In addition, modifying the surface into the equilateral triangle pattern can decrease the distance between the protrusions, which reduces the contact opportunity of the chain-basal surface. These two factors of increasing the protrusion top surface area


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Table 6. Adsorption Energy and the Global Orientational Order Parameter for Modifying the Top Surface Area and the Height and Shape of the Protrusions for PE with an Initial Orderly Folded Configuration protrusion type

pattern 1

pattern 2

pattern 3

pattern 4

N

E(kcal/mol)

P2

E(kcal/mol)

P2

E(kcal/mol)

P2

E(kcal/mol)

P2

500 1000 3000

-2420.30 -3355.72 -5672.17

0.930 0.913 0.898

-2202.86 -2618.03 -3966.82

0.917 0.897 0.889

-435.71 -452.77 -489.91

0.885 0.873 0.869

-396.69 -424.70 -446.56

0.879 0.868 0.862

and decreasing D are able to separate the adsorbate-surface complex more easily. Moreover, the tendencies of the adsorption energy and order parameter with increasing N in both surface patterns are the same. The equilibrium snapshots for pattern 4 are also similar to those in Figure 6 for pattern 3. The intrachain energies per monomer at different chain lengths in the simulations with these two patterned surfaces are shown in Table 5. Combining with the results in Table 4, we find a competition between the chain-surface and the intrachain interactions, and such a competition depends on the chain length. The small values of the adsorption energies for pattern 4, as shown in Table 4, imply that such a surface nanomorphology may be the most suitable one for separating this hydrophobichydrophobic PE-graphite system. 3.2. PE Chain with Initial Orderly Folded Configuration. In the above section, we have investigated the influence of the surface nanoroughness on the adsorption of a PE chain with an initial random coil configuration on graphite. Generally, an orderly folded PE chain can be taken as the prototype or the nucleus of its lamellar crystal.28 The adsorption of this compact orderly folded chain on graphite is interesting because it can reveal the competition between strong surface-chain and intrachain interactions. We adopt the same simulation procedure as in the above section but take an orderly folded chain instead of the random coil PE chain in the beginning of the simulations. The results for modifying the top surface area and the height and the shape of the protrusions are listed in Table 6. It is shown that the effect of changing the surface nanomorphology is similar in both simulations with random coil and orderly folded PE initial configurations. By viewing the snapshots, we find that the equilibrium structures with initial orderly folded configuration are very similar to those with an initial random coil configuration. Furthermore, the originally folded PE structures are all destroyed by the strong chain-surface interaction for surface patterns 1 and 2, no matter how many monomers the chain contains. PE chain sub-bond vectors align along the surface sub-bond vectors, and at the basal surface the chain surrounds the protrusions. The folded PE structures with N ) 500, 1000, and 3000 all keep the orderly folded structures but adapt their global orientations to the top surface zigzag of the connecting bonds of graphite in the simulations with surface patterns 3 and 4, whose equilibrium snapshots are similar to those shown in Figure 6 with an initial random coil configuration. The change in the order parameter ∆P2 ) P2,final - P2,initial can be used to describe the effect of the nanopatterned surface on the adsorption of orderly folded PE. We always obtain positive values of ∆P2, which means that because of surface induction the chain global orientational order is better than that of initially folded PE in vacuum. Figure 9 shows the time evolution of P2 of PE with an initial orderly folded configuration and N ) 500 adsorbed on the surface with pattern 3. In a very short time interval, P2 changes from 0.855 to 0.87, which corresponds to the adsorption of the folded chain onto the surface as a whole. Then P2 fluctuates around 0.87 for about 2.5 ns, showing that

Figure 9. Time evolution of the global orientational order parameter P2 of PE with N ) 500 in a simulation with the initial orderly folded configuration and surface pattern 3.

the chain changes its global orientation and the local configuration to fit the surface sub-bond vector. Suddenly P2 jumps from 0.87 to 0.885, which is attributed to a configuration transition of a PE chain. This process occurs comparatively quickly, and in the process, the fold number (the number of the chain re-entering the ordered region) is reduced by one layer, which results in P2 increasing. Then the simulation reaches equilibrium. It is easy to see from Table 6 that the surface of pattern 4 is still the most suitable nanomorphology for reducing the strong interaction in this PE-graphite system. Because the folded chain is anisotropic in space, its interaction with the surface may also be affected by the angle between its axis and the surface normal while placing the folded chain in the beginning of the simulations. We check this effect by keeping the weight center of the folded chain unchanged and rotating it in space. The consequent adsorption behavior is the same for different initial chain global orientations. We have also considered the effect of the initial position of PE being placed on the results. In the above simulations, the PE chain is always placed above the center of the gap between four protrusions. Taking N ) 500 and 1000 as examples, we test the effect by initially placing the ordered PE in the gap between the protrusions in the simulations with surface patterns 3 and 4. The chain is a certain distance from both the protrusions and the basal graphite surface. In this situation, we find different adsorption behavior in contrast to the foregoing results. The PE chain is first adsorbed on the side surfaces of the protrusions and the basal graphite roughly at the same time. Afterward, although most parts of the chain in the center of the folded configuration still keep the original ordered structure, the outermost parts spread onto the basal surface extensively. Because the interaction between PE and the basal graphite surface is stronger than that of the PE-protrusion, the chain is finally adsorbed completely on the basal surface and spreads between the protrusions. That is to say that if the PE chain and the basal graphite surface are close enough then the chain will be always adsorbed onto (43) Pal, S.; Weiss, H.; Keller, H.; Mu¨ller-Plathe, F. Phys. Chem. Chem. Phys. 2005, 7, 3191.

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the basal surface. This is also true for a PE chain with an initial random coil configuration. Therefore, if we try to weaken the strong attractive interaction between PE and the graphite surface, designing a suitable surface nanomorphology that reduces the contact between the chain and the basal surface to the full extent is most important. In the present study, we find, for this purpose, that pattern 4 is the best choice. Consequently, the separation of the PE chain and the graphite surface will take place more easily.

4. Conclusions In this article, MD simulations are used to investigate the adsorption process of PE with different chain lengths on a nanopatterned hydrophobic graphite surface. We find that a wellpatterned surface possessing protrusions with a suitable top surface area, height, and shape is able to reduce the strong attractive chain-surface interaction effectively. Consequently, the separation between graphite and the hydrocarbons that possess a very strong attractive interaction is much easier. Both initial random coil and orderly folded configurations for PE are considered in the simulations. The effects of changing the top surface area, the height, and the shape of the protrusions are similar in both cases. If we mix up these factors or randomize the protrusion shape and type, then it will have an effect on the dewetting ability of the nanostructured surface. However, they cooperate together in a complex way, which may hinder the

Wang et al.

identification of the influence of a specific factor. Moreover, the chain length also has an effect on the adsorption behavior. Therefore, in this study we have not tried to randomize the nanoroughness, and we have studied the influence of specific factors one by one. The above results are obtained in a situation in which PE is initially placed above the center of the gap between the four protrusions. However, if the chain is initially placed between the protrusions, then it will be totally adsorbed onto the basal graphite surface. Although in solution the latter situation is less likely, fine tuning the surface nanomorphology to reduce the likelihood of occurrence is still important in reducing the strong surface-chain interaction. The influence of nanostructuring is generic and independent of the chemistry.43 Therefore, once the protrusions are optimized, they may be transferred to another material having different properties. Even for the system possessing a very strong affinity, the nanopatterned surface also offers an opportunity to control the adsorption and spreading behavior of the adsorbate. Moreover, it is also possible to manipulate the adsorption of the polymer chain on the patterned surface by fine tuning the solvent quality. Acknowledgment. This work is supported by NSFC (20490220, 20404005) and JLSTP (20050562). LA061492H

Molecular Dynamics Simulation Study on Controlling the Adsorption

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