Article pubs.acs.org/JPCC
Wetting Behavior of Nanoscale Thin Films of Selected Organic Compounds and Water on Model Basal Surfaces of Kaolinite Xiao Ni and Phillip Choi* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada ABSTRACT: Molecular dynamics (MD) simulation and density functional theory (DFT) were applied to study the wetting behavior of nanoscale thin films of n-heptane, toluene, pyridine, and water at different thicknesses on the two model basal surfaces of kaolinite at room temperature. Despite the hydrophilicity of the basal surfaces, water exhibited the lowest affinity for them, but was statistically comparable to that of nheptane, as quantified by the corresponding calculated monomolecular layer works of adhesion. The results are consistent with the simulations in which water molecules in monomolecular layer originally sandwiched between a monomolecular layer of aromatic compounds and the two basal surfaces departed from the surfaces, while the aromatic molecules migrated to the surfaces. However, such behavior was not observed in the cases in which water thin films contained multiple molecular layers of water. Interestingly, the corresponding calculated multimolecular layer works of adhesion of water were the highest among the compounds of interest. Analysis of the simulation data on both basal surfaces suggests that such observation is attributed to the water/water long-range charge−charge (69%, averaged over the two basal surfaces) and shortrange hydrogen-bond (31%) interactions. Here, interfacial hydrogen bonds play a relatively minor role in the wetting behavior of water.
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INTRODUCTION Wetting of organic compounds and water on various types of mineral surfaces is always of great interest to scientists working in a variety of disciplines. Obviously, wettability of a liquid on a particular surface depends strongly on the affinity of the liquid for it. Heat of wetting, also known as enthalpy of wetting, is commonly used to quantify such affinity. It is worth noting that three major wetting processes exist: adhesion, spreading, and immersion wetting. Depending on how a wetting experiment is carried out, the measured heats of wetting differ and are referred to as work of adhesion, heat of cohesion, and heat of immersion, respectively. Heat of wetting, regardless of the measurement method, partly signifies the strength of the interaction at the solid/liquid interface. The higher is the heat of wetting, the stronger the interaction is between the liquid and the surface. The concept differs from the concept of heat of adsorption in which vapor molecules are adsorbed on a solid surface. In such a situation, adsorbed molecules may be far apart from each other on the surface and do not necessarily interact among themselves. However, in wetting, interactions between neighboring molecules of the same type in a liquid are not negligible. In other words, the degree of affinity of the liquid for a solid surface depends not only on the interaction between the liquid molecules and the surface atoms but also on the interactions between the liquid molecules. In the case of wetting involving liquid having nanoscale thickness, it is not unreasonable to expect that the wetting behavior (or affinity) of such liquid films would depend on their thickness as the majority of liquid molecules are sandwiched by two nearby © 2012 American Chemical Society
interfaces (i.e., solid/liquid and liquid/gas interfaces). Obviously, such thickness dependence of wetting is difficult to obtain experimentally. Therefore, in this work, we have adopted a molecular simulation approach to study such dependence. In particular, both monomolecular layer and multimolecular layer works of adhesion were calculated to quantify the affinity of four liquids including n-heptane, toluene, pyridine, and water for two model basal surfaces of kaolinite at room temperature. Here, structures and partial atomic charges of the atoms in the basal surfaces will be determined using density functional theory (DFT) calculations, and the wetting process will be studied by molecular dynamics (MD) simulation. The justification for using such an approach was discussed elsewhere.1,2 Various molecular simulation techniques have been used to investigate liquid/solid surfaces. Fouquet et al.3 studied the diffusion motion of benzene molecules in a submonolayer on graphite basal planes using several force fields including COMPASS, the one that will be used in this work; their atomic MD simulation results showed that the COMPASS force field yielded results that were in agreement with experiment. Fleys and Thompson4 simulated water adsorption isotherms on modified zeolite basal surfaces; their results also indicated that results obtained using the COMPASS force field matched experimentally determined isotherms. Work of Received: May 29, 2012 Revised: November 14, 2012 Published: November 25, 2012 26275
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adhesion (referred to as “enthalpy of immersion” in the original reference)5 was calculated by simulating a multimolecular liquid layer (benzene, pyridine, and water) on the kaolinite octahedral surface, and the authors found that water exhibited the highest affinity for the model surface. Their findings will be further discussed in a later section. Grand canonical Monte Carlo simulation was used by Croteau et al.6,7 to study the water adsorption on both basal surfaces (ideal and with trenches) and edges, and they found that water molecules can form more (around four) hydrogen bonds in the monomolecular layer on the octahedral surface. These results are consistent with those of the atomic molecular simulation of monomolecular water layers on the ideal octahedral surface done by Tunega et al.8 Their work also indicated that the water/water hydrogen bonds in the monomolecular layer on the octahedral surface play a major role in the structure of the water molecules. As mentioned, there are three wetting processes. However, literature data on the systems of interest in this work have been obtained from the immersion wetting experiments. This is somewhat expected as heat of immersion experiment involves only immersing solid particles of interest into a selected liquid, and the energy change associated with the process is then measured. Malandrini et al.9 used immersion microcalorimetry to study the interfacial properties of quartz/water and quartz/ selected organic compounds including those of interest in this work (n-heptane, benzene, and pyridine). They also found that water exhibited the highest heat of immersion. Likewise, Zoungrana et al.10 showed that the measured heat of immersion increased in the order of n-heptane, benzene, pyridine, and water on kaolinite surfaces. However, as demonstrated in this work, such a trend could only be reproduced when multiple layers of molecules were used in the calculation of work of adhesion. Interestingly, water exhibited the lowest affinity for the model basal surfaces of kaolinite when a single layer of molecules was used.
this work, liquids of interest were n-heptane, toluene, pyridine, and water, while solid surfaces of interest were the two basal (i.e., octahedral and tetrahedral) surfaces of kaolinite. Details of the models and the strategy used for calculating internal energy of the models will be described in the following sections. Model Clay Surfaces. Our model clay surfaces were cleaved from the kaolinite unit cell from the structure library of Materials Studio version 4.2. When the unit cell was cleaved, it generated both octahedral and tetrahedral surfaces. The cleaving planes were parallel to the (001) face in the bulk kaolinite. The cross-section area of the octahedral and tetrahedral surfaces was determined to be 276 Å2. No substitution or modification of any ions was made to the clay surface model that was made up of five repeating layers (see Figure 1); the total net charge of the clay surface model was set to zero despite the fact that individual atoms in the surface model would carry charges.
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COMPUTATIONAL METHODS AND MODELS From a computational perspective, only the adhesion wetting process can be simulated, as the simulation of the other two wetting processes is prohibitively expensive. The simulation strategy for calculating heat of adhesion involves the calculations of the enthalpy of the liquid film (Hliquid), the enthalpy of the solid surface (Hsurface), and the enthalpy of the entire system when the liquid film and the solid surface are brought in contact (Hliquid+surface), respectively, using molecular dynamics simulation. Please refer to the Molecular Dynamics Simulation section for further details of how the aforementioned enthalpy is calculated. The corresponding enthalpy of adhesion is calculated using the following expression: Figure 1. A model structure containing five repeating units of kaolinite and a nanoscale film of pyridine subjected to three-dimensional periodic boundary conditions (only the periodic image in the positive z direction is shown).
ΔHadhesionwetting = (Hliquid + Hsurface − Hliquid + surface)/A (1)
where A is the corresponding interfacial area. It is worth noting that the enthalpy of adhesion wetting differs from the work of adhesion that includes the entropy term (TΔSadhesion wetting). However, experiments show that the entropy term is much smaller than the enthalpy term.11 Therefore, one can approximate the work of adhesion by the enthalpy of adhesion. In addition, because the pressure−volume term of the heat of adhesion is generally negligible, enthalpy of adhesion can then be approximated by the internal energy of adhesion (ΔUadhesion wetting). As a result, all of the enthalpy terms that appeared in eq 1 can be replaced by internal energy terms. In
Geometric Optimization of the Model Clay Surfaces. Geometric optimization of the model clay surfaces was carried out using the density functional theory (DFT). The CASTEP code, a DFT code, available in the Materials Studio version 4.2 was used. Generalized gradient approximation and the parametrization of Perdue and Wang (PW91) were used for exchange and correlation terms. The energy convergence tolerance between consecutive optimization steps was set at 1.0 × 10−5 Ha. During relaxation, core treatment was conducted to 26276
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all of the electrons; the basis set used was DNP. A kinetic energy cutoff of 340 eV was used, and the k-point sampling mesh separation was set to 0.05 Å. The atoms in both the octahedral and the tetrahedral surface layers were allowed to adjust their positions, while those in the three layers underneath it were fixed during the relaxation. Upon relaxation, the partial atomic charges of the kaolinite atoms were assigned by the Mulliken charges as obtained from the DFT calculations.12 Liquid Models. As mentioned, one aliphatic liquid (nheptane), two aromatic liquids (one is toluene, while the other is pyridine), and water were used in this work. The partial atomic charges of the atoms in these substances were assigned by the COMPASS force field according to the atom type (Figure 2). Partial atomic charges of atoms in the COMPASS force field are derived from ab initio calculation.13,14 Water model incorporated in the COMPASS force field is the simple three-point model (i.e., the SPC model). The partial atomic charges of water are shown in Figure 2. Using the SPC model,
it was demonstrated that the density of water obtained from isothermal−isobaric (i.e., NPT) molecular dynamics (MD) simulation at 293 K and 0.1 MPa is comparable to the experimental data.15 We are aware of the fact that the SPC water model used in the COMPASS force field is not optimized for our systems. However, the same force field was used by Fleys and Thompson4 to study the water adsorption isotherms on silicate and clay surfaces. The authors found that the adsorption properties observed matched with experiment. In addition, Croteau et al.6 found that using SPC and TIP5P models did not yield a significant difference between the structures of water molecules adsorbed on the octahedral surface of kaolinite. Therefore, we feel justified using the SPC model in this work. The Amorphous Cell Tool in Materials Studio version 4.2 was used to construct the initial structures of the bulk liquids. Experimental density data at room temperature were used. In particular, 0.680, 0.862, 0.979, and 0.996 g/mL were used for nheptane,16 toluene,17,18 pyridine,19 and water,20 respectively. Cubic unit cells were used, and they were all subjected to threedimensional periodic boundary conditions. Once the bulk liquid models were generated, each cubic unit cell was elongated in the z-direction to generate a vacuum slab (90 Å) inside the unit cell to ensure that the forces between the liquid molecules of the periodic images were minimized. The liquid inside the unit cell then became a thin film of liquid with a thickness equal to the dimension of the original bulk liquid model in the z direction. Two liquid film thicknesses were used for each compound. One was monomolecular layer thick, while the other contained multiple layers of molecules. The thicknesses of n-heptane, toluene, pyridine, and water with multiple layers of molecules were 16.91, 24.53, 18.55, and 21.76 Å, respectively. All thicknesses used were beyond the interatomic interaction cutoff distance. Multimolecular layer systems were relaxed by the conjugate gradient method using a convergence criterion of 0.4 kJ/mol/Å. However, no energy minimization was performed on the monolayer systems as there were no much strong overlaps between the liquid molecules as compared to the multimolecular layer case. All liquid films models were then assembled with the model basal surfaces to form a structure as shown in Figure 1. Here, the cross-section areas of the liquid films were set to be the same as those of the model surfaces. The Cartesian coordinates of all atoms in the octahedral and tetrahedral model surfaces were then fixed during subsequent molecular dynamics simulations. Molecular Dynamics Simulation. Canonical (i.e., NVT) MD simulations at 298 K were carried out by the Discovery code of the Materials Studio version 4.2. It should be emphasized here that MD simulations were only carried out on systems containing the liquid film, surface layer, and the vacuum slab, not on isolated films and surface layer. This is because nanofilms are mechanically unstable. To calculate Hliquid and Hsurface, we used the following procedure. Once a MD simulation was completed, we calculated the enthalpy of the entire system by averaging the enthalpy of 10 snapshots obtained from the last 100 picoseconds (ps) (equilibrated structures) of the 2000 ps trajectory. We then calculated the enthalpy of the nanofilm of the same 10 snapshots by removing the surface layer (still in the nanofilm form). The same approach was used for the surface as well. In this way, we were able to determine the enthalpy of an isolated nanofilm without doing a separate MD simulation on it. Because nanofilm is not
Figure 2. Molecular structures and partial atomic charges of (a) nheptane, (b) toluene, (c) pyridine, and (d) water. 26277
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mechanically stable, one may want to use the internal energy of an isolated water cluster rather than an isolated water film in eq 1. In this regard, we did an additional MD simulation on a water cluster that contained the same number of water molecules used in the isolated, monolayer water film unit cell adhered to an tetrahedral surface and found that the difference in the internal energy between the two cases was about 1 kJ. This amount of energy is 1 order of magnitude smaller than the uncertainty (∼10 kJ) of calculated work of adhesion of the corresponding system (∼174 kJ) before the value was normalized to the surface area. This means that the reference state of the liquid does not really affect the calculated work of adhesion. As mentioned, the simulation time used for each system was 2000 ps along with a time step of 1 femtosecond (fs). We used the velocity Verlet method to numerically integrate the equations of motion.21 To control the temperature of the systems, we used the Andersen thermostat.22 We did not use the Nose thermostat as our preliminary work using the Nose thermostat showed that molecules in the liquid film adjacent to a model surface tended to leave the liquid film and entered into the vacuum slab above it. The reason was totally unclear. Obviously, this led to a significant variability in the computed work of adhesion. To ensure that there was constant number of molecules in the liquid film, the Andersen thermostat was used. It is worth noting that the Andersen thermostat is able to produce canonical ensemble and proper energy, which is the main interest of the present work. However, it is known to yield slower dynamics due to its stochastic collision procedure used in the heat bath.23 The interatomic cutoff distance for the dispersion forces was set at 9.5 Å. Long-range electrostatic electrostatic interactions were handled with the Ewald method.24
Figure 3. Configuration of various atoms in the top two layers of the octahedral surface of kaolinite with hydroxyl moieties at different positions labeled as 1−4 after DFT relaxation (red, oxygen; white, hydrogen; purple, aluminum; yellow, silicon).
and second layers was expanded by approximately 0.3 Å with respect to each other. The atomic coordinates of the model are summarized in Table 1. The most significant change is the orientations of the surface hydroxyl groups. Figure 3 shows that the hydroxyl moiety at position 2 (OH-2) on the top layer after relaxation adopted a more or less horizontal position upon relaxation. However, hydroxyl moieties at the same position but located at layers below the top layer tend to adopt a vertical position. These results are consistent with the computational results of Benco et al.25,26 This is due to the fact that hydroxyl moieties in the inner layers are stabilized by the hydrogen bonds formed between the octahedral and tetrahedral surfaces of the adjacent layers, while this is not the case for the hydroxyl moieties located on the top layer. It is worth noting that liquid molecules presented on the surface could affect the positions of the surface hydroxyl groups, thereby the work of adhesion. However, our results show that the number of interfacial hydrogen bonds is very low. Therefore, whether we fixed the coordination of the surface hydroxyl groups or not cannot have a significant effect on the results. Obviously, the coordination restriction was used to decrease the computation time. Because the Cartesian coordinates in the z direction and bond angles of the tetrahedral surface did not change significantly, they are not reported here. Table 2 summarizes the calculated works of adhesion of nheptane, toluene, pyridine, and water on the relaxed octahedral and tetrahedral surfaces using thin films containing multimolecular layers and monomolecular layer of molecules at 298 K. In the case of octahedral surface, multimolecular layer nheptane model shows the lowest work of adhesion, while water the highest, and the work of adhesion values of the two aromatic compounds fall between them. Pyridine shows a higher work of adhesion than toluene. This is somewhat expected as the magnitudes of the partial atomic charges on the atoms of the compounds increase from n-heptane to water. The
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RESULTS AND DISCUSSION Relaxation of Model Basal Surfaces of Kaolinite. Following the relaxation strategy described earlier, it was observed that the first layer of the octahedral surface model was elevated slightly in the absence of liquid corresponding to the bottom of the cell. In particular, the distance between the first Table 1. Cartesian Coordinates of Atoms in the z Direction and Bond Angles of the Hydroxyl Moieties of the Octahedral Surface of Kaolinite after DFT Relaxation atom type
partial atomic charge
1
0.227 0.287 0.227 0.306 −0.730 −0.743 −0.730 −0.768 −0.980 −0.983 1.443 1.829
H H2 H3 H4 O1 O2 O3 O4 O5 O6 Al Si bond type Al−O −H Al−O2−H2 Al−O3−H3 Al−O4−H4 1
1
after DFT relaxation 26.97 26.18 26.97 23.94 26.00 26.05 26.00 23.99 23.91 23.90 25.05 22.28 after 118.4 115.5 119.3 111.8 26278
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Table 2. Calculated Multimolecular Layer and Monomolecular Layer Works of Adhesion of n-Heptane, Toluene, Pyridine, and Water on the Model Octahedral and Tetrahedral Surfaces of Kaolinite at 298 K octahedral surface work of adhesion (mJ/m2) n-heptane toluene pyridine water
multimolecular layer 197 216 254 295
± ± ± ±
tetrahedral surface
monomolecular layer
6 8 10 13
175 195 199 154
± ± ± ±
6 3 7 6
multimolecular layer 135 163 202 261
± ± ± ±
2 7 2 11
monomolecular layer 108 123 127 104
± ± ± ±
3 5 5 2
Table 3. Measured Heats of Immersion of n-Heptane, Pyridine, and Water on Kaolinite (Wetting of Basal Surfaces and Edges) and Computed Works of Adhesion, Averaged over Two Basal Surfacesa interface
measured heat of immersion (mJ/m2)
computed work of adhesion (mJ/m2)
our result (mJ/m2)
n-heptane/kaolinite pyridine/kaolinite water/kaolinite
∼92 (301 K) ∼4009 (301 K) ∼48025 (350 K)
N/A 238 ± 45 (300 K) 179 ± 35 (300 K)
104 ± 4 (298 K) 228 ± 10b (298 K) 278 ± 13b (298 K)
9
a
Note that the heat of immersion is expected to increase with decreasing temperature. bAverage of the works of adhesion on the octahedral and tetrahedral surfaces.
Figure 4. Computed (a) dispersion and (b) electrostatic energy of both monomolecular and multimolecular layer thin films of n-heptane, toluene, pyridine, and water on the octahedral surface.
Figure 5. Computed (a) dispersion and (b) electrostatic energy of both monomolecular and multimolecular layer thin films of n-heptane, toluene, pyridine, and water on the tetrahedral surface.
works of adhesion on the octahedral surface are higher than those on the tetrahedral surface. This is because the aluminum atoms near the octahedral surface exhibit much stronger dispersion interactions with the liquid molecules than do the silicon atoms near the tetrahedral surface. Here, it should be noted that we used the default values in the COMPASS force field for the partial atomic charges and Lennard-Jones parameters for describing the dispersion interactions. For clarity, such values are not shown here. However, as can be seen from Table 2, the trend of the work of adhesion for the tetrahedral surface (i.e., n-heptane shows the lowest affinity while water the highest) is basically identical to that of the octahedral surface. In addition, Table 2 clearly shows that the work of adhesion decreases as the thickness of the thin film decreases for all compounds on both types of surfaces with
water exhibiting the largest decrease. In fact, the decrease in the case of water is to the extent that water exhibits the weakest affinity for the model surfaces when there is only one layer of molecules. This observation obviously deserves a closer examination of the data, but let us compare the multimolecular layer results of all liquid models with both experimental and computational data available in the literature first (see Table 3). It is worth pointing out at the outset that one should not expect that the numerical values of the heat of immersion and work of adhesion would agree for the same liquid/solid surface system as the respective measurement procedures differ, and Table 3 compares heat of immersion (experiment) and work of adhesion (simulation) as experimental data on the work of adhesion are not available. However, it is expected that the two 26279
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Figure 7. Snapshots of monomolecular layers of toluene and water on the octahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K.
Figure 6. Snapshots of monomolecular layers of n-heptane and water on the octahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K (red, oxygen; white, hydrogen; purple, aluminum; yellow, silicon; gray, carbon).
repeating layers in their surface model, liquid films with different number of molecules, a short simulation time (200 ps), and differences in the partial atomic charges used. It should also be noted that none of the pure liquid nanofilms “balled up” after 2000 ps of MD simulation, although they were not perfectly smooth at the liquid/vacuum interface (somewhat expected for such nanofilms). As demonstrated, the affinity trend of our multimolecular layer data agrees with that of the heat of immersion measurements, and the remaining question is: why does monomolecular data show a different trend? In particular, water exhibits the lowest affinity for both basal surfaces. To address the above question, we examined the nonbonded energy terms (i.e., intermolecular interactions) corresponding to the wetting of the multimolecular layer and monomolecular layer liquid molecules on both basal surface. The results are depicted in Figures 4 and 5. Each figure indicates that for all eight thin films, the absolute values of electrostatic energies, regardless of their thickness, are higher than the dispersion energies. This suggests that all liquid would wet the surface (negative total energy) and that dispersion forces (positive) act against wetting, while the electrostatic forces (negative) act in favor of wetting. This observation is similar to the finding of Takei and Chikazawa that wetting of silica, a somewhat similar inorganic surface, is dominated by the electrostatic interactions.30 The data also show that by increasing the thickness of
quantities of a given set of solvents would exhibit comparable affinity trends for a given solid surface. Also, only the multimolecular layer works of adhesion are included in the table as such systems mimic more closely to experiment. Unfortunately, we were not able to find experimental data on the toluene/kaolinite system. As can be seen from Table 3, our results (i.e., n-heptane exhibits the lowest affinity while water the highest) agree well with the experimental affinity trend of the liquids for the kaolinite surfaces as quantified by their heats of immersion. Here, the reason that the works of adhesion of pyridine and water are significantly larger than those computed works of adhesion (our own results and those of Hwang et al.5) may be attributed to the ability of the solvents to expand the repeating units made up of the clay particles and to wet their edges, while only the basal surfaces were wetted in the simulation work. In fact, experimentally measured heat of immersion of clay depends not only on the affinity of the liquid for the two basal surfaces and edges, but also depends on the degree of repeating unit expansion.27 Because of these effects, measured heats of immersion are usually higher than the “expected” values.28,29 On the other hand, the difference between our computational results and those of Hwang et al.5 is probably attributed to the fact that the other authors used three 26280
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Figure 8. Snapshots of monomolecular layers of pyridine and water on the octahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K (blue, nitrogen).
the liquid film from a monomolecular layer to a multimolecular layer, the dispersion energy and electrostatic energy do not vary much for all organic compounds, even in the case of pyridine, which is a fairly polar molecule, suggesting that wetting of the organic compounds is mainly controlled by the interactions occurring in the first molecular layer. However, this is not true for water. The substantial decrease in the electrostatic interaction of water obviously originated from the electrostatic interactions due to the additional water molecules present in the multimolecular layer system, as compared to the corresponding monomolecular layer model. The decrease in the electrostatic energy is due to the combined effect of favorable electrostatic interactions between the additional water molecules and the surface as well as the electrostatic (longrange) and hydrogen bond (short-range) interactions among the additional water molecules themselves. Given the fact that both pyridine and water have comparable dipole moments (i.e., 2.2 D for pyridine and 2.3 D for SPC water), they should exhibit a similar screening effect on the electrostatic interactions between the additional molecules in the multimolecular layer models and the surfaces. Because pyridine does not exhibit a significant decrease in the electrostatic energy in the multimolecular layer models while water does, it suggests that the electrostatic interactions from the surfaces are negligible. It is worth noting that according to eq 1, interactions between the water molecules in a liquid film should not make a contribution to the difference in the work of adhesion for systems having different film thicknesses that are significantly larger than nanometre scale. However, in the present work, we basically compare the enthalpy changes of the systems with only one (i.e., monolayer) and several layers (multilayer) of water molecules. The differences in the Hliquid in the monolayer and multilayer water systems are quite different as the ability of the water molecules in the monolayer system to interact with each
Figure 9. Snapshots of monomolecular layers of n-heptane and water on the tetrahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K.
other is much lower than that of the multilayer system. Therefore, we could determine, based upon the approach described in the following, the relative contributions of electrostatic and hydrogen-bond interactions in the multimolecular layer water models. In the COMPASS force field, it does not use a separate energy term to account for hydrogen bonds; rather, such interaction is included in the electrostatic interaction calculation. However, one can define hydrogen bonds geometrically in simulation. Here, following the definition of hydrogen bonds of Bertolini et al.,31 we determined the number of hydrogen bonds in our systems when the hydrogen-bond donor/acceptor distance is below 2.5 Å and the angle is above 90°. In the multimolecular layer water systems, the counting included the hydrogen bonds formed between water molecules and the surface hydroxyl moieties on the octahedral surface, hereafter denoted as interfacial hydrogen bonds. It was found that the number of interfacial hydrogen bonds, averaged over 10 snapshots collected every 10 ps over the last 100 ps of the corresponding MD trajectory, was 10 ± 4, while the number of hydrogen bonds formed among the water molecules was 200 ± 20, suggesting that interfacial hydrogen bonds contribute insignificantly to the wetting. On the other hand, no interfacial hydrogen bonds were observed for the tetrahedral surface, but 26281
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Figure 11. Snapshots of monomolecular layers of pyridine and water on the tetrahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K. Figure 10. Snapshots of monomolecular layers of toluene and water on the tetrahedral surface (a) before and (b) after 2000 ps of MD annealing at 298 K.
surfaces. The organic compound layers consisted of eight molecules of n-heptane, toluene, or pyridine. The initial water layer was assembled on top of the basal surfaces, while the organic compound layer was assembled on the water layer. MD simulation settings were the same as those used in the work of adhesion calculations. Figures 6−8 show the octahedral surface results after 2000 ps of MD annealing at 298 K. As expected (based upon the monomolecular layer of works of adhesion), water molecules left the surface, while toluene and pyridine molecules migrated to the surface. In both cases, the aromatic rings tended to orient parallel to the octahedral surface. However, such migration process on the octahedral surface was not observed for n-heptane as their works of adhesion are statistically comparable. Similar behavior was observed for the tetrahedral surface (see Figures 9−11). In addition, similar simulations were carried out with multiple layers of water molecules (results not shown here), and departure of the water molecules was not observed on both basal surfaces.
the number of hydrogen bonds formed among the water molecules was similar to that of the octahedral surface (i.e., 200 ± 20). Because the number of water molecules used for both surfaces was 100, the average number of hydrogen bonds formed is about 2, which is on the lower end of what was observed in bulk water at room temperature (∼2−3 hydrogen bonds per water molecule).32,33 This is somewhat expected as the majority of water molecules in our system were nearby two interfaces. Given the fact that the difference between the electrostatic energies of the multimolecular layer and monomolecular layer systems were 13.2 × 103 (octahedral) and 13.8 × 103 (tetrahedral) kJ/mol of 100 water molecules (see Figures 3 and 4) and that the typical enthalpy value of hydrogen bonds in bulk water at room temperature is about 21 kJ/mol of hydrogen bonds,33 the energy of the hydrogen bonds in our system was estimated to be 42 kJ/mol of water molecules. This means that water/water hydrogen bonds account for about 31%, averaged over the two basal surfaces, of the total electrostatic interaction. Therefore, the remaining 69% comes mainly from the long-range charge−charge interactions among the water molecules. In other words, the observed thickness dependence of work of adhesion in the case of water is mainly attributed to both short-range and long-range water/water intermolecular interactions. To confirm the thickness dependence of affinity of water, we carried out additional MD simulations in which there were two monomolecular layers of liquids (one molecular layer of water and the other an organic compound) conducted on both basal
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CONCLUSION Works of adhesion of n-heptane (aliphatic), toluene (aromatic), pyridine (aromatic with hydrogen-bond acceptor), and water on model basal surfaces of kaolinite were calculated. The results indicated that the affinity of water for the model surfaces was the weakest as quantified by the monomolecular layer work of adhesion, but was the strongest when multiple molecular layers were used in the simulations. The multimolecular layer results agree well with the affinity behavior observed in the measured heats of immersion of kaolinite in which n-heptane exhibited the lowest affinity while water the highest. Further data analysis on the basal surfaces showed that the calculated works of adhesion depend essentially on the first molecular layer of 26282
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The Journal of Physical Chemistry C
Article
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organic compounds adjacent to the basal surfaces, but this is not the case for water, and electrostatic forces act in favor of wetting, while dispersion forces act against the wetting. Nonetheless, the observed increasing affinity of water when there were multiple layers of molecules in the nanoscale thin film was found to be related to the increasing intermolecular interactions among the water molecules themselves. A rough estimate shows that high work of adhesion, in the case of multimolecular layers of water, originates from the long-range charge−charge (69%, averaged over the two basal surfaces) and short-range hydrogen-bond (31%) interactions. Interfacial hydrogen bonds play a minor role in the water/basal surfaces interaction. Additional molecular dynamics simulations containing two monomolecular layers of water and three aforementioned organic compounds showed that monomolecular layer of water could only wet the model basal surfaces in the case of n-heptane, which exhibited comparable work of adhesion.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Imperial Oil − Alberta Ingenuity Centre for Oil Sands Innovation is gratefully acknowledged. This research has been enabled by the use of WestGrid computing resources, which are funded in part by the Canada Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions. WestGrid equipment is provided by IBM, Hewlett-Packard, and SGI.
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dx.doi.org/10.1021/jp305223w | J. Phys. Chem. C 2012, 116, 26275−26283
