Article pubs.acs.org/JPCC
Upright or Flat Orientations of the Ethanol Molecules on a Surface with Charge Dipoles and the Implication for Wetting Behavior Chunlei Wang,*,† Liang Zhao,†,‡ Donghua Zhang,† Jige Chen,† Guosheng Shi,† and Haiping Fang† †
Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Using molecular dynamics simulations, we have found that the charge dipole length has a large effect on surface−ethanol adsorption behavior, particularly the orientations of the ethanol molecules of the first ethanol layer and subsequently the wetting behavior. At surfaces with large charge dipole length, an ordered ethanol monolayer forms with upright orientations of the ethanol molecules and an ethanol droplet forms on this ordered monolayer, which can be termed as “ordered ethanol monolayer does not completely wet ethanol”. The upright orientations, with the OH groups buried beneath the monolayer, exclude the possibility of forming hydrogen bonds between the ordered ethanol monolayer and ethanol molecules in contact with the monolayer, leading to a phenomenon wherein the ordered ethanol monolayer does not completely wet ethanol. The strong binding mainly occurs between the negative charges of the surface and the OH group of the ethanol molecules while the hydrophobic ethyl tails point away from the surface. When the surface charge dipole length is small, a flat orientation of the first ethanol layer and an ethanol droplet are observed despite the large charge value. This can be attributed to weak surface−ethanol interactions, where the steric exclusion effect prevents the OH group of the ethanol molecules from attaching to the surface charge dipoles. Our work shows the effect of the surface lattice structure on the orientations of the adsorbed ethanol molecules and the subsequent wetting behavior.
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ies.36−42 Unlike the water molecule, the ethanol molecule has a hydrophobic nonpolar tail, as well as a OH polar head that can form hydrogen bonds, which indicates that it should have more complex adsorption behavior on solid surfaces than water. Even though great effort has been devoted to studying the adsorption behavior of ethanol,40−42 it is still not clear how the polar surface with charge dipoles affects the adsorption of ethanol molecules and the wetting behavior. In this article, we show that the charge dipole length has a large effect on the surface−ethanol adsorption behavior, particularly the orientations of the first ethanol layer and subsequently the wetting behavior. At certain surfaces with large charge dipole length, an upright orientation of the ordered ethanol monolayer forms and an ethanol droplet appears on this ordered monolayer, which can be termed as “ordered ethanol monolayer does not completely wet ethanol”. The upright orientations with the OH groups buried beneath the monolayer exclude the possibility of forming hydrogen bonds between the ordered monolayer and the molecules in contact with the monolayer, leading to a droplet on the ordered ethanol monolayer. Remarkably, the strong binding mainly from the
INTRODUCTION The molecular adsorption behavior at the solid/liquid interface is important in a variety of physical, chemical, and biological processes,1−12 such as surface wetting/dewetting behavior,13,14 surface slip,15 chemical catalysis,16−18 and the dynamics of biomolecules.19−21 In recent years, the various adsorption behaviors of liquids with rich hydrogen bonding, such as ethanol and water, at the solid surface have attracted considerable research attention. Usually, it is accepted that liquids with rich hydrogen bonding can easily adsorb on polar surfaces, on which hydrophilic behavior occurs. However, several exceptions have been reported where the polar surface is hydrophobic.2,7,8,22 Our previous work7 showed that there was a critical charge dipole length, below which water molecules could not feel the charge dipoles on the polar surfaces, hence leading to hydrophobic behavior at the solid surface. Even though strong adsorption of water molecules on the polar surface was observed, we found an unexpected water droplet on the ordered water monolayer, which we describe as “ordered water monolayer does not completely wet water”.2,8−10 Recently, a similar phenomenon has been found in experimental23,24 and theoretical studies.25−27 Ethanol is a widely used small organic solvent in the life sciences28,29 and other technological fields30−32 and thus receives considerable attention in both experimental33−35 and theoretical stud© XXXX American Chemical Society
Received: June 23, 2013 Revised: December 27, 2013
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electrostatic energies between the surface negative charges and the OH groups of the ethanol makes the hydrophobic ethyl tails assemble a dense ordered monolayer, which is different from the ordered water monolayer bound by both positive and negative charges on the surface as reported in our previous work.2 When the surface charge dipole length is small, a flat orientation of the first ethanol layer and an ethanol droplet are observed despite the large charge value. This can be attributed to the weak surface−ethanol interactions, where the steric exclusion effect prevents the OH group of the ethanol molecules from attaching to the surface charge dipoles. Our work highlights the lattice match between the ethanol molecules and the solid surface structure and confirms the crucial role of the first molecule layer on the surface phenomenon. More importantly, the dipole length of crystal lattice structures predicted by our simulation falls into the bond length range of many existing materials, such as the BN (0.142 nm) and AgI (0.28 nm), suggesting unrecognized possibilities associated with existing materials.
and the data of the last 2 ns were collected for analysis. For all simulations, the ethanol system was in phase-level coexistence with the vapor at 300 K. A constant temperature and constant volume (NVT ensemble) MD simulation was performed using a time step of 1.0 fs with Gromacs 3.3.1.43 Periodic boundary conditions were applied in all directions. The Berendsen thermostat44 with a time constant of 1.0 ps for coupling was used to maintain the temperature of the ethanol molecules at 300 K. The Lennard−Jones parameters of the solid atoms were εss = 0.048 kJ/mol and σss = 3.4 Å, and parameters for the ethanol molecules were taken from the all-atom OPLS (optimized potentials for liquid simulations) force field as in our previous work.28,29 A cutoff of 10 Å was used for both the particle-mesh Ewald method45 with a real space to model longrange electrostatic interactions and for the van der Waals interactions. The criterion for the formation of a hydrogen bond between ethanol molecules was defined as if the O−O distance was less than 3.5 Å and the angle H−O···O was less than 30°.
SIMULATION METHODS Inspired by the real hexagonal boron nitride surface (h-BN), we have designed a highly polar surface with positive and negative charges of the same magnitude q.7 The charges were assigned to every bonding atom of the hexagonal arrangement, represented by red and yellow balls for the positive and negative charges, respectively, in Figure 1a. We set the charge
RESULTS AND DISCUSSION Our simulation results show that the orientations of the first ethanol layer are quite different on surfaces with different charge dipole lengths. Figure 1b shows a typical side view of the first ethanol layer adsorbed on the solid surface with l = 0.162 nm (upper) and l = 0.282 nm (lower) when q = 1.0 e. When l = 0.282 nm and q = 1.0 e, the ethanol molecules in the monolayer form upright orientations relative to the solid surface. This ordered structures cause the OH group pointing toward the solid surface and the ethyl group pointing away from the surface. Similar orientations were observed when l = 0.242 nm. However, when l = 0.202 or 0.162 nm, the ethanol molecules near the solid surface favor flat orientations with the ethyl group in contact with the surface plane despite the large charge value q = 1.0 e. We calculated the distributions of the tilt angle φ to show the orientations of the first layer of ethanol molecules, where the tilt angle is defined as the angle between the line connecting the O atom of OH and the C atom of CH3, and the z axis. The results are shown in Figure 2a. Clearly, there are quite different profiles depending on the surface charge dipole length. When l = 0.162 nm, a peak appears at φ = 60°, which corresponds to a flat orientation of ethanol molecules, and the first ethanol layer lies almost parallel to the surface plane. This is similar to the result when l = 0.282 nm with q = 0 e as shown in Figure 2a. When the dipole length increases to 0.202 nm, the surface charge dipoles weakly affect the orientations of the ethanol molecules. Some of the ethanol molecules weakly bind to the surface charges with the OH group but most bind to the surface with the hydrophobic ethyl groups. When the dipole length further increases to l = 0.242 and 0.282 nm, the position of the peak shifts left, and there is an obvious peak at φ = 10°. This shift of the peak indicates that the orientations of the ethanol molecules of the first ethanol layer change from flat to upright orientations when the dipole length l increases. Our finding is similar to the self-assembled behavior of amyloid-like peptide related to Alzheimer’s disease on hydrophobic, highly oriented pyrolytic graphite or hydrophilic mica surfaces, where different peptide orientations are observed.3,46 To further show the orientations of ethanol monolayer molecules, we have plotted the probability distributions of the C atom in the CH3 group and the O atom in the OH group on the surfaces versus the z axis for various dipole length values in
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Figure 1. Structure of the model surface and snapshots of ethanol molecules on different surfaces at q = 1.0 e. (a) The red and yellow balls represent the atoms on the solid surface with positive charges and negative charges, respectively. (b) Side view snapshot of flat orientations of the first ethanol layer and an ethanol droplet on the surface with l = 0.162 nm (upper) and the upright orientations of ethanol molecules and an ethanol droplet on an ethanol monolayer on the surface with l = 0.282 nm (lower).
values in the range 0 e ≤ q ≤ 1.0 e. The surface was neutral, and all solid atoms were fixed. The length of the charge dipole was denoted by l. Here, we adjusted the dipole length in the range 0.162 nm ≤ l ≤ 0.282 nm. Initially, when l = 0.162 or 0.202 nm, 500 ethanol molecules (see more detailed discussions in PS 2 of Supporting Information related to the effect of the number of ethanol molecules on the surface wetting behavior) in a cuboid shape with dimensions of 6.3 × 8.1 × 1.0 nm3 were placed on the solid surface composed of 2376 atoms in the simulation box with dimensions 9.148 × 8.642 × 15.000 nm3 or 11.547 × 10.908 × 15.000 nm3. When l = 0.242 or 0.282 nm, 1300 ethanol molecules in a cuboid shape with dimensions of 8.0 × 12.5 × 1.5 nm3 were placed on the solid surface composed of 1664 atoms in the simulation box with dimensions of 10.900 × 11.616 × 15.000 nm3 and 12.702 × 13.538 × 15.000 nm3. Each system was simulated by molecular dynamics (MD) for 10 ns, B
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does not completely wet water”.2,8,10 This is the first report on the coexistence of an ethanol droplet and an ethanol monolayer describe as “ordered ethanol monolayer does not completely wet ethanol”, indicating the hydrophobic-like ethanol monolayer. However, when l decreases to 0.202 or 0.162 nm, only ethanol droplets with contact angles of 39° and 75° appear on the solid surfaces and no ordered ethanol monolayer is observed (see Figure 1b). The different wetting behaviors on surfaces with only different surface charge dipole lengths clearly indicate that ethanol wetting behavior greatly depends on the lattice structure. This is similar to our previous study, where the dipole length of the surface affected the surface wetting behavior of water molecules.7 The charge value of the surface significantly affects the molecular wetting of the ethanol molecules when l is large. When q ≤ 0.6 e with l = 0.242 nm and q ≤ 0.4 e with l = 0.282, an ethanol monolayer does not form and only an ethanol droplet forms on the surface. In contrast, there is almost negligible effect on the wetting behavior when l is small despite the large charge values, and ethanol droplets always form on the solid surfaces. We have also found that the charge quantity significantly affects the molecular upright orientations of the ethanol molecules but has a negligible effect on the flat orientations when q decreases from 1.0 to 0 e. It is found that when q ≤ 0.6 e with l = 0.242 nm and q ≤ 0.4 e with l = 0.282, the upright orientations of ethanol molecules disappear. To account for the phenomenon described above, we have calculated the electrobinding energy between the ethanol molecule and the charge dipoles on the solid surface. Epe and Ene denote the average electrobinding energies between each ethanol molecule in the first ethanol layer (with the vertical distance above the solid surface z ≤ 0.6 nm) and all the surface positive and negative charges, respectively. The total energy Etotal is defined as follows:
Figure 2. Distributions of the ethanol molecules near the solid surface. (a) The tilt angle φ distribution for the ethanol monolayer molecules for various l values. (b) Probability distributions of the CH3 and OH groups versus the z axis for various l values with the typical charge value q = 1.0 e.
Figure 2b. When l = 0.242 or 0.282 nm, a peak at z = 0.25 nm in the O atom of the OH profile and a peak at z = 0.45 nm in the C atom of the CH3 profile are observed. The distance between these two peaks is 0.2 nm, which is close to the distance (0.24 nm) between the O atom of OH and the C atom of CH3. When l = 0.162 or 0.202 nm, a peak at z = 0.3 nm in the C atom of the CH3 profile and a peak at z = 0.45 nm in the O atom of the OH profile are observed. The different locations of the first peaks show the different orientations of the ethanol molecules in the monolayer; that is, when l = 0.242 or 0.282 nm, the OH group is pointing toward the solid surface and when l = 0.162 or 0.202 nm, the ethanol molecules lie parallel to the solid surfaces, which is consistent with the results shown in Figure 2a. In addition, when l = 0.242 or 0.282 nm, the second peak of the C atom of CH3 appears close to the first peak, which indicates that a methyl group face-to-face orientation formed between the ethanol monolayer and ethanol molecules in contact with the monolayer. In contrast, when l = 0.162 or 0.202 nm, the second peak of the O atom of the OH profile is close to the first peak of the O atom profile, which indicates the existence of hydrogen bonds between the first and second layers of ethanol molecules. The wetting behavior is different for the surfaces with different charge dipole lengths. Our simulation results show that when l = 0.242 nm or l = 0.282 nm, an ethanol monolayer almost covers the entire solid surface. To our surprise, an ethanol droplet with the contact angle of 71° forms on the ordered ethanol monolayer (see Figure 1b, and the formation process of the ethanol droplet can be found in PS 1 of Supporting Information) with a height of about 5 Å, which is similar to our previous studies of “ordered water monolayer
Etotal = Epe + Ene
(1)
As shown in Figure 3, when l = 0.242 nm with q = 1.0 e, Epe = 17 kJ/mol and Ene = −90 kJ/mol, and when l = 0.282 nm
Figure 3. Electrostatic interactions between each ethanol molecule in the first ethanol layer and all the surface positive (black), surface negative (red), and total (blue) charges with respect to the dipole length l for q = 1.0 e.
with q = 1.0 e, Epe = −16 kJ/mol and Ene = −85 kJ/mol. Clearly, the electrobinding energy Ene is much larger than the electrobinding energy Epe and the negative charges make the most of the contributions to the binding of the ethanol monolayer. This binding energy analysis indicates that the ethanol molecules in the monolayer are mainly bound to the surface negative charges by the H atom of the OH group of each ethanol molecule. Owing to the strong binding to the C
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negative charges, the hydrophobic ethyl tails assemble a dense monolayer with upright orientations. However, when l = 0.162 nm with q = 1.0 e, the average electrobinding energies between the first ethanol layer molecules and all the surface positive and negative charges are 20.0 kJ/mol and −19.8 kJ/mol, respectively, while the total energy Etotal is ∼0 kJ/mol, even though the large quantity of charge q = 1.0 e. A similar result is found for l = 0.202 nm. Thus, the total electrointeractions between the solid surface and the ethanol molecules in the monolayer are negligible. We have also calculated the minimum distance between the O atom or H atom of the ethanol molecules and the positively or negatively charged surface atoms, denoted by DO+, DH+, DO−, and DH−, respectively. When l = 0.162 nm and q = 1.0 e, DO+ = 0.34 nm, DH+ = 0.33 nm, DO− = 0.34 nm, and DH− = 0.33 nm. These values are almost equal and indicate that the OH groups of ethanol molecules seem to not feel the existence of charges. When the hydroxyl O atoms (with negative partial charge) approach the positive surface atoms, they will also be spatially close to the negative surface atoms and experience electrostatic repulsions. This is also the case when the hydroxyl H atoms come close to the negative charges. This induces the steric exclusion effect, which prevents the hydrogen atoms from staying very close to the negative charge and the oxygen atoms from staying very close to the positive charge when l is sufficiently small. In this case, the ethanol molecules in the first layer bind to the solid surface with the hydrophobic ethyl group and form flat orientations with respect to the surface. In contrast, when l increases to 0.282 nm, DO+ = 0.24 nm, DH+ = 0.23 nm, DO− = 0.23 nm, and DH− = 0.14 nm. DO− is larger than DH−. Clearly, the H atom of the OH group is spatially closer to the negative charge than the O atom, consistent with the previous energy analysis that ethanol molecules in the monolayer are mainly bound to the surface negative charges by the H atom of the OH group of each ethanol molecule. There is a close relationship between the orientations of the ethanol molecules in the first layer and the wetting properties described previously. When l = 0.242 or 0.282 nm, the surface with appropriate charge dipole length l can accommodate the dense ethanol monolayer with upright orientations of the ethanol molecules, which seems to exclude the possibility of forming hydrogen bonds between the ethanol monolayer and ethanol molecules interacting with this monolayer. This is similar to the case of ethanol on the alumina surface proposed by Phan et al.41 We have determined the number of hydrogen bonds formed between the surface and the ethanol monolayer, between the ethanol molecules in the monolayer, and between the molecules in the monolayer and the droplet. The values are 1, 0, and 0, respectively, when l = 0.242 or 0.282 nm. There is no hydrogen bonding between the ethanol molecules in the ethanol monolayer, which differs from the case of ethanol adsorbed on the alumina surface,41 where hydrogen bonds formed between ethanol molecules in the ethanol monolayer. Clearly, all the ethanol molecules in the monolayer form one hydrogen bond with the solid surface, but no hydrogen bond forms between the monolayer and the droplet (see Figure 4). In this case, the face-to-face orientations of CH3 lead to weak interactions (vdW interactions rather than the hydrogen bonds) between the ethanol monolayer and ethanol molecules above the monolayer. Thus, an ethanol droplet forms on the ethanol monolayer, which can be described as “ordered ethanol monolayer does not completely wet ethanol”. This is different from our previous work of “ordered water monolayer does not
Figure 4. Average number of hydrogen bonds formed between ethanol molecules and between ethanol molecules and the surfaces for various l values with q = 1.0 e. Black solid squares, open blue triangles, and red solid circles correspond to hydrogen bonds formed between surface and ethanol molecules in the monolayer, between ethanol molecules in the monolayer and the droplet, and between ethanol molecules in the monolayer, respectively.
completely wet water”,2,8 where the ordered hexagonal hydrogen bonding network was induced by the elaborate binding of surface positive and negative charges. The ethanol monolayer molecules are mainly bound to the surface negative charges and have upright orientations with the OH groups buried beneath the monolayer. However, once l decreases to 0.202 or 0.162 nm with q = 1.0 e, the steric exclusion effect prevents the OH head from attaching to the surface charges because of the very short dipole length. In this case, weak vdW interactions dominate the surface−ethanol interactions because the electrostatic interactions can be ignored (see Figure 3). The ethanol molecules in the first layer exhibit a flat orientation with respect to the surface, with the hydrophobic ethyl tails in contact with the solid surface, and an ethanol droplet can be observed. We have also calculated the distributions of the number of hydrogen bonds when l = 0.162 and 0.202 nm. As shown in Figure 4, when l = 0.162 nm the average numbers of hydrogen bonds formed between the monolayer and the droplet molecules, and between the ethanol molecules in the monolayer, are 1.1 and 0.63, respectively. When l = 0.202 nm, the average numbers of hydrogen bonds are 0.45 and 0.97, respectively. However, no hydrogen bonds formed between the solid surface and the ethanol molecules in both cases. Clearly, the molecules in the first ethanol layer mainly form hydrogen bonds with neighboring ethanol molecules and molecules above this layer. This analysis of the number of hydrogen bonds confirms that no strong bonding occurs between the surface and the ethanol molecules with very short charge dipole lengths. At ambient conditions, water molecules are ubiquitous in the atmospheric environments and always exist at the solid surfaces. Here we have investigated the effect of water molecules on the ethanol orientation and the subsequent effect on the surface wetting behavior. Typically, we mainly focus on a simulation system with a 2.5 nm thickness of mixed liquid film of 3294 water molecules (SPC/E water model was adopted here) and 2253 ethanol molecules on the solid surfaces with l = 0.282 nm and q = 1.0 e. All other parameters, including the ethanol molecule parameters and the solid surface parameters, were the same as in Simulation Methods. The system was performed by MD simulation with 10 ns, and the data of last 2 ns were collected for analysis. As shown in Figure 5a, there is no clear ethanol droplet on the solid surface, different from the case with pure ethanol molecules (see also Figure 2b). We have also analyzed the atom distribution of the water molecules and D
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on the surface wetting behavior. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interests.
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Figure 5. (a) Snapshot of the simulation systems with a mixture of water and ethanol molecules showing that no clear ethanol droplet forms. (b) Probability distributions of the O atom of the water molecules, the C atom of CH3, and the O atom of the OH group of ethanol near the solid surface.
ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (nos. 11290164, 11204341), the National Basic Research Program of China (2012CB932400), the Knowledge Innovation Program of SINAP, the Knowledge Innovation Program of the Chinese Academy of Sciences, the Shanghai Natural Science Foundation of China (no. 13ZR1447900), the Shanghai Supercomputer Center of China, and the Supercomputing Center of the Chinese Academy of Sciences.
ethanol molecules near the solid surfaces. As shown in Figure 5b, the peak of the O atom of the water molecules locates closer to the solid surface (z = 0) than the atoms of the ethanol molecules, which shows that the water molecules are closer to the surface than the ethanol molecules. The existence of water molecules prevents the ethanol molecules from approaching to the solid surfaces; thus, no clear upright ethanol monolayer or ethanol droplets appear. This may account for the lack of experimental studies reporting the phenomenon of “ordered ethanol monolayer does not completely wet ethanol”.
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CONCLUSIONS Using molecular dynamics simulations, we have found that the charge dipole length of the solid surface plays a crucial role on the orientations of the first ethanol layer and the consequent wetting property. When the dipole length is large (l = 0.242 and 0.282 nm), the ordered ethanol monolayer molecules are mainly bound to the surface negative charges with upright orientations and the OH groups are buried beneath the monolayer. This excludes the possibility of the ethanol molecules in the ordered monolayer forming hydrogen bonds with ethanol molecules above the monolayer, leading to the formation of a droplet on the hydrophobic-like ethanol monolayer, which can be described as “ordered ethanol monolayer does not completely wet ethanol”. These results highlight the interfacial molecular layers separating the bulk from the substrate beneath,46 which is the key to understanding surface wetting behavior. However, when the surface charge dipole length is small (l = 0.162 or 0.202 nm), a flat orientation of the first ethanol layer is observed and an ethanol droplet directly forms on the surface despite the large charge value. This can be attributed to the steric exclusion effect, which prevents the OH group of the ethanol molecules from attaching to the surface charge dipoles and leads to the weak surface− ethanol binding energy. Our work shows that the matching/ mismatching between surface lattice structure and adsorbed molecules can affect the surface wettability, which is consistent with our previous results when water molecules adsorb on the surface.2,7 We believe that our results will prompt more experimental works and increase knowledge of adsorption and wetting behavior of ethanol, and even other amphiphilic molecules, on various materials in industrial applications.
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ASSOCIATED CONTENT
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Formation process of the ethanol droplet on the ethanol monolayer and the effect of the number of ethanol molecules E
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