Wettability of Primer-Treated Al2O3 Surfaces by Bisphenol A

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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials 2

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Wettability of Primer-Treated AlO Surfaces by Bisphenol A Diglycidyl Ether: Determination of the Mechanism from Molecular Dynamics Simulations and Experiments Yoshitake Suganuma, Takuya Mitsuoka, Satoru Yamamoto, Tomoyuki Kinjo, Hiroaki Yoneyama, and Kazuhiko Umemoto J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b00680 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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The Journal of Physical Chemistry

Wettability of Primer-Treated Al2O3 Surfaces by Bisphenol A Diglycidyl Ether: Determination of the Mechanism from Molecular Dynamics Simulations and Experiments Yoshitake Suganuma,*,†Takuya Mitsuoka, †Satoru Yamamoto,‡ǁ Tomoyuki Kinjo, †Hiroaki Yoneyama, † and Kazuhiko Umemoto† †Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan ‡Dassault Systèmes K. K., 2-1-1, Osaki, Shinagawa, Tokyo 141-6020, Japan

ǁ Current address: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

This study aims to develop a molecular dynamics (MD) simulation procedure to investigate the wettability of primer-treated Al2O3 surfaces by bisphenol A diglycidyl ether (BADGE) and to understand the interaction between the surface and the liquid. The MD simulation results were compared with those obtained by contact angle measurements, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM), and were found to be in agreement with the experimental evaluations. The results obtained from both the MD simulations and the experiments suggest that the configuration of the primers on the surface affect its wettability. In other words, silanes lying flat on the surface, such as mercapto silane, make it easy for BADGE to access any polar functional groups of the silane, thereby leading to a strong interaction and good wettability. For amino silane, although the configuration is similar to that of mercapto silane, its amino groups are bound to the surface owing to their high polarity, which results in a reduced accessibility for BADGE and a relatively poor wettability in comparison with mercapto silane. On the contrary, for silanes that stand-up on the surface, including trifluoroalkyl silane, BADGE is hindered from approaching the silanol groups and interacting with them and the surface shows poor wettability.


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Introduction In all applications where a liquid is applied onto a solid surface, such as coating, printing, antifouling treatment, and adhesion, the control of wettability is of key importance.1–10 According to Young’s equation,11 the wettability of a surface by a liquid is determined by the interaction between the solid’s surface and the liquid as well as by their surface tensions. Therefore, several studies have examined how to handle these factors and how they affect wettability.12–22 The interaction between the solid surface and liquid also influences other properties including strength and long-term stability of the interface. Therefore, controlling the interaction between the solid surface and the liquid is needed to achieve not only good wettability but also a good performance in the desired application. Controlling the wettability of the surface and the interactions of the liquid with it require a thorough understanding of the chemical structures formed at the interface as a result of wetting.23,24 Recently, molecular dynamics (MD) simulations have been used to study the chemical structures between a solid surface and a liquid and their contribution to the interfacial interaction and wettability.25–31 Most reports simulated the wetting process of water on carbon materials, such as graphite and carbon nanotubes. However, there are few studies that focused on the wettability between industrially significant materials, such as metals, their oxides, and organic materials. Thus, it is important to control the wettability in systems that have industrial relevance from an understanding of the molecular structures involved in the wetting process. To overcome this limitation, we studied the surface of Al2O3 (0001) as a model for the natural oxide film on aluminum; a lightweight material, which is replacing iron in many industries. For the liquid, we focused on typical organic solvents and used oligomers of bisphenol A diglycidyl



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ether (BADGE) and phenylene sulfide as models for epoxy resin and poly (phenylene sulfide), respectively. In a previous study,32 we reported the results of MD simulations on the wettability of bare Al2O3 (0001) surfaces by these organic materials. The MD simulations were in agreement with the experiments, and we concluded that molecular flexibility affected wettability. Herein, we developed a MD simulation method, which can estimate the wettability by BADGE of Al2O3 surfaces treated with primers. Primer treatment, one of the most common methods used to control the wettability and the interaction between the surface and the liquid, was chosen for this study. Additionally, BADGE was chosen as the liquid because it is used as an epoxy component in a large number of industrial applications, such as an adhesive, a resin for molding, and paint. Based on the results of the MD simulations, we evaluated the chemical structures at the interface that influenced the interaction between the primer-treated Al2O3 surface and BADGE and the wettability of the surface by the liquid. Furthermore, we validated the MD simulation by comparing the results with those obtained by contact angle measurements, time-offlight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). Experiment Materials A single crystal (0001) wafer of α-Al2O3, the surface of which was highly polished to have a surface roughness (Ra) of less than 1 Å, was obtained from Kyocera Co. (Kyoto, Japan). The wafer was washed as follows.33 The wafer was first soaked in 10 mM HNO3 for 30 min. Then, it was cleaned by Milli-Q water and dried in an oven at 350 °C for 30 min. It was then rinsed with Milli-Q water and finally dried in ambient air at room temperature.


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BADGE was used as the probe liquid for contact angle measurements. Four silanes and one phosphonic acid were used as primers to treat the Al2O3 (0001) surface; (3mercaptopropyl)triethoxysilane, (mercapto silane), 3-aminopropyltriethoxysilane (amino silane), trimethoxy(3,3,3-trifluoropropyl)silane (trifluoroalkyl silane), triethoxyphenylsilane (phenyl silane) and phenylphosphonic acid. All of these materials were obtained from Tokyo Chemical Industry Co. (Tokyo, Japan) and were used without further purification. Treatment of the α-Al2O3 (0001) surface For contact angle measurements and TOF-SIMS analyses, the α-Al2O3 (0001) surface was treated with a silane or a phosphonic acid by the following procedure. Figure 1 shows the chemical structures of the primers used for the treatment of the Al2O3 surface. For the silanetreated surfaces, 0.1 g of a silane was dissolved in 100 ml of water/ethanol (50/50, v/v). The Al2O3 substrate was immersed in the solution for 24 h at room temperature. After removal from the solution, the surface was rinsed with ethanol, and finally heated to 120°C in ambient air for 30 min. For treatment with the phosphonic acid, the substrate was soaked in ethanol (100 ml), which contained 0.1 g of the phosphonic acid, for 24 h at room temperature. The substrate was removed from the solution, rinsed with ethanol, and dried in an oven at 60°C for 5 min.



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Figure 1. Chemical structures of the primers used for the surface treatment of the Al2O3 surface.


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Contact Angle Measurements Contact angle measurements were conducted using a Dropmaster DM-500 Hi (Kyowa Interface Science Co., Saitama, Japan) equipped with a heating system for both the probe and stage. The probe liquid, BADGE, was placed on an Al2O3 (0001) surface at room temperature (298 K). These measurements were also performed at probe and stage temperatures of 423 and 498 K. The droplet volume was set to ~1 μL, which is small enough to neglect gravity.34 To determine the contact angles, θ/2 method, which is implemented by the FAMAS software (Kyowa Interface Science Co., Saitama, Japan), was used. TOF-SIMS Analysis Negative ion TOF-SIMS spectra of the Al2O3 surfaces that had been treated with amino and trifluoroalkyl silanes were acquired using a TOF. SIMS 5 (IONTOF GmbH, Münster, Germany) equipped with a liquid metal ion gun filled with bismuth. Pulsed 30 kV Bi+ and Bi3+ sources were selected as the primary ion source and the Bi ions were rastered over a 500 × 500 µm2 area (128 × 128 pixels). The primary ion dose density was always maintained below the static limit of 1012 ions/cm2 and was adjusted to 2.0 × 108 and 8.0 × 107 ions/cm2 for Bi+ and Bi3+, respectively. AFM analysis AFM was used to measure the heights of the amino and trifluoroalkyl silane molecules on the Al2O3 surface. For this purpose, we prepared samples that were different from those used for the contact angle measurements and TOF-SIMS analyses since the surfaces of these samples were covered with a thin film of a primer that made it impossible to measure the heights of the silane molecules. Therefore, for AFM analyses, we prepared primer-treated surfaces where the silane



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molecules were absorbed sparsely over the surface by the following procedure. First, the solutions of amino and trifluoroalkyl silanes, which had been previously used for the contact angle measurements and TOF-SIMS analysis, were diluted 10 times in water/ethanol (50/50, v/v). Second, the washed Al2O3 substrates were dipped in the diluted solutions for 30 min. Finally, after removal from the solutions, the substrates were heated to 120°C in ambient air for 30 min. AFM analyses of these surfaces were performed using a Bruker Dimension Icon AFM (Bruker Nano Inc., Santa Barbara, CA) with ScanAsyst-Air probes (silicon nitride cantilever, spring constant: 0.4 N/m and frequency: 70 kHz). Topographic and adhesion force images were acquired by a peak force quantitative nanomechanics technique. The scan size was set to a square of 2000 × 2000 nm2 with 2014 × 2014 pixels. Then, images with a square of 300 × 300 nm2 were cropped from these images. Simulation Methodology The following calculations were conducted using Materials Studio version 8.0 (Dassault Systèmes BIOVIA, San Diego, CA, 2001–2009). The models were built by constructing an Al2O3 (0001) surface, treating it with a primer, and then placing a droplet of BADGE onto the primer-treated surface. The Al2O3 lattice from the structural database of Materials Studio was used to construct the Al2O3 (0001) surface. Then, the cleaved (0001) surface of Al2O3, the top atoms of which are oxygen,33 was obtained and a vacuum slab was added above the surface so that the lattice size along the z axis was 400 Å. The DMol3 module, which employed the PBE functional and numerical DNP bases set, was used to optimize the surface geometry. By redefining vector A of


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the surface to (210) and constructing a 6 × 10 × 1 supercell, a surface with dimensions of 49.5 × 47.6 × 400 Å3 was built. The structures of the Al2O3 (0001) surfaces treated with primers were constructed by imitating the experimental processes. Since the methoxy or ethoxy groups of the silanes are considered to hydrolyze in the solutions due to the presence of water, they were converted to silanol groups. Next, primer bulks were built using the Amorphous Cell module and were then optimized by MD simulations for 1000 ps in the NPT ensemble. The MD simulations were conducted using the COMPASSII35 force field and the Forcite module. From the bulk constructions, we built rectangular structures with dimensions of 49.5 × 47.6 × 10.0 Å3, which have the same x- and ywidths as the Al2O3 (0001) surface model obtained in the previous part. Then, the thin structures of the primers were placed onto the surface and relaxed by MD simulations for 500 ps in the NVT ensemble. This corresponds to the surface absorption of the primers in the experimental process where the substrates were immersed in the primer solutions. Next, we removed the primer molecules that had no hydrogens less than 2.0 Å from the top oxygens of the Al2O3 surface. This is because these molecules were too far from the Al2O3 surface to have any strong interaction with the surface, and so they were seemingly removed from the surface during the ethanol rinse step. On the contrary, other molecules seemed to experience interactions that were strong enough to remain on the surface during the rinse. Hydrogen atoms that were less than 2.0 Å from the surface were chosen as this distance is close to the aluminum and oxygen separation in Al2O3, 1.8 Å. For the model of the Al2O3 (0001) surface treated with the phosphonic acid, we used the structure obtained at this step as the primer-treated surface. On the contrary, for the models of the silane-treated surface, we performed further procedures to imitate two types of dehydration condensation caused by the heat treatment,36, 37 one of which was among two silanol



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groups of different silane molecules and the other was between a silanol group of a silane and a hydroxyl group on the Al2O3 surface. We mimicked these reactions in the following steps, which can be generally applied to chemical reactions,38,39 instead of using reactive force field like ReaxFF40. To model the condensation reactions among two silanol groups, the lengths between the hydrogen and oxygen atoms of different silanes were measured. Then, when the length was less than 2.0 Å, we manually transformed the two silanol groups to form a siloxane bond (–Si–O– Si–), and then conducted 500 steps of energy minimization to improve the local geometry. For the couplings with a hydroxyl group on the Al2O3 surface, as the Al2O3 model had no hydroxyl groups to condense with silanol groups, we imitated this reaction by fixing the hydrogen atoms of the silane molecules that were less than 2.0 Å from the top oxygens of the Al2O3 surface: these hydrogen atoms seemed to react with the surface owing to their proximity to it. Finally, the surface structures treated with phosphonic acid and silanes were expanded into 4 × 4 × 1 supercells to construct primer-treated surfaces with dimensions of 197.8 × 190.4 × 400 Å3. For the droplet of BADGE, the Amorphous Cell module was used to construct the BADGE bulk, which was optimized by MD simulations for 1000 ps in the NPT ensemble. From the bulk construction, we built a droplet with a radius of 20.0 Å, which is sufficient to measure a contact angle in a simulation.30 Finally, the BADGE droplet was placed onto the primer-treated Al2O3 (0001) surface to produce the initial construction. Figure 2 shows an initial construction for the MD simulation investigating the contact angle of BADGE on the Al2O3 (0001) surface treated with mercapto silane.


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Figure 2. Initial construction for the MD simulation investigating the contact angle of BADGE on the Al2O3 (0001) surface treated with mercapto silane. The initial construction was relaxed by 500 steps of energy minimization. The MD wetting simulations were performed for 1000 ps in the NVT ensemble with a step of 1.0 fs using the Berendsen thermostat,41 which provides reasonable results on the wettability of the Al2O3 (0001) surface by BADGE.32 Additionally, we have confirmed that it gives contact angles which are in good agreement with those obtained by using the Nose- Hoover thermostat.42 The group-based method was used to calculate the van der Waals and Coulomb interactions with a cutoff distance of 20.0 Å. The vibration of the atoms at the Al2O3 surface was so slight that these atoms were fixed during the simulations.



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Results and Discussion Table 1 lists the experimentally obtained contact angles of BADGE on the primer-treated Al2O3 surfaces at 298, 423, and 493 K. The contact angles reported in Table 1 are averages of six to nine measurements from two samples. Of the four silanes, the contact angle for trifluoroalkyl silane had the highest value at every temperature. On the contrary, the substrate treated with mercapto silane recorded the smallest value at every temperature despite amino silane being expected to have a stronger interaction with BADGE and show a better wettability due to its higher polarity. To explain the reason for this counterintuitive result, we conducted MD simulations and validated the simulation models by comparing the simulation results with the experimentally obtained contact angles. The analysis of the interaction between the silane-treated surfaces and BADGE suggested some key factors that influenced the interactions and the surface wettability. Table 1. Experimentally obtained contact angles of BADGE on primer-treated Al2O3 surfaces at 298, 423, and 493 K.

primer

contact angle deg 423 K 493 K 22.2 3.1 8.4 0.6 23.7 3.0 10.6 1.9

mercapto silane phenyl silane

298 K 31.2 1.7 36.0 2.8

amino silane

38.9 1.2

23.5 2.3

16.5 1.6

trifluoroalkyl silane phenyl phosphonic acid

51.7 3.1 27.3 1.5

43.7 2.9 17.9 2.0

39.0 4.2 8.9 3.4

To confirm the validity of the MD simulations, we calculated contact angles based on the simulation results. First, we obtained the profiles of the relative concentration of BADGE molecules, which signified the distributions of the atoms in the droplets, along the x, y, and z


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axes over the last 500 ps of the MD simulation. The values of relative concentration are the concentration of particles relative to a random distribution in any given layer. Hereafter, we analyzed the simulation results using those from the last 500 ps. This is because we have confirmed that, in all calculations, system energy and other thermodynamic properties including radius of gyration of BADGE molecules reach equilibriums within the first 100 ps, and contact angles at 500 ps are almost the same as those at 1000 ps. Therefore, results obtained from the last 500 ps provide properties in equilibrium. Second, we determined the diameter (r) and height (h) of the BADGE droplet from the relative concentration profiles. As an example, Figure 3 shows r and h in the last snapshot of the MD simulation for the surface treated by amino silane at 298 K, and the relative concentration profiles of the BADGE molecules along the x, y, and z axes, from which r and h were obtained. In order to obtain h, Gibbs dividing plane43 should be determined at the interface between the droplet and the primer-treated surface. However, in some calculations, the droplet was so thin that its density profile did not have enough horizontal area to calculate Gibbs dividing plane. Alternatively, we measured the distance between the points at which concentrations were zero in the relative concentration profiles along the z axis, and used it as h (see Figure 3). Finally, we obtained the contact angles from these values using the θ/2 method; the same procedure was used in the contact angle measurement. Figure 4 shows the experimentally determined contact angle as a function of the simulated contact angle. The simulated values have a clear positive correlation with the experimental ones. The simulation results show a correlation coefficient of 0.70, providing good estimates of the wettability by BADGE of the Al2O3 (0001) surfaces treated with primers.



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Figure 3. The diameter (r) and height (h) of the BADGE droplet in the last snapshot of the MD simulation for the surface treated by amino silane at 298 K, and the relative concentration profiles of the BADGE molecules along the x, y, and z axes, from which r and h were obtained.


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Figure 4. Experimentally determined contact angle as a function of the contact angle calculated by MD simulation. To discuss the differences in contact angles for the mercapto, amino, and trifluoroalkyl silane surfaces, as given in Table 1, we evaluated the interactions between BADGE and the silanetreated surfaces, which are one of the factors determining the contact angles according to Young’s equation,11 and their contributions to the contact angles. We calculated the radial distribution functions (RDFs) between the atoms in the BADGE molecules and those in the silane molecules that seemed to interact with each other. For BADGE, we defined the hydrogens that were less than 3.0 Å from atoms in the silane molecules in the snapshot at 510 ps of the MD simulations as interacting hydrogens (Hint). For silanes, we chose the negatively charged atoms in the polar functional groups, namely sulfur atoms for mercapto silane, nitrogen atoms for amino silane, fluorine atoms for trifluoroalkyl silane, and the oxygen atoms in each silane, to be



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interacting with the Hint and to be used in the calculation of the RDFs. Figure 5 shows the RDFs between the Hint and the negatively charged atoms in the silane molecules over the last 500 ps of the MD simulations. Although the RDFs are by far larger than 1 in Figure 5, they have been confirmed to converge to 1 in the range of large r values.


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Figure 5. The RDFs between the Hint and the negatively charged atoms in the silane molecules over the last 500 ps of the MD simulations at (a) 298, (b) 423, and (c) 493 K.



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The RDFs between Hint and the sulfur atoms show the sharpest and highest peaks across all the three temperatures, as shown by orange line in Figure 5. This indicates that the mercapto group has the strongest interaction of the functional groups with the Hint, and therefore the mercapto silane surface exhibits the best wettability by BADGE. For amino silane, the peaks of the RDFs between Hint and the nitrogen atoms are wider than those between Hint and the sulfur atoms for mercapto silane despite the amino group being more polar, which is consistent with the counterintuitive experimental result (see Table 1). For trifluoroalkyl silane, its oxygen atoms are much farther from the Hint than those of the other silanes, as shown by blue dashed line in Figure 5. This suggests that trifluoroalkyl silane has the fewest oxygen atoms that interact with BADGE, and so exhibits the poorest wettability by it. Subsequently, the profile of the relative concentrations of atoms in the silane molecules along the z axis over the last 500 ps of the MD simulation at 298 K provided the configurations of the silane molecules on the Al2O3 surface and their contributions to the interactions between the silane molecules and BADGE. Figure 6 shows these profiles and schematic representations of the silane molecules for each profile. For mercapto silane, the main peaks of the profiles of silicon, sulfur, and oxygen exist at almost the same distance from the Al2O3 surface between 2.5 and 5.0 Å (Figure 6a), indicating that most mercapto silane molecules lie flat on the surface, i.e., their alkyl chains are parallel to the Al2O3 surface. Additionally, the profile for sulfur has its highest peak a little farther from the Al2O3 surface than that of silicon. This suggests that the mercapto groups are exposed at the top of the silane-treated surface, as shown in Figure 6i, enabling BADGE to interact with the mercapto groups easily. Hence, this accessibility and relatively high polarity result in the strongest interaction between the mercapto groups and Hint. For amino silane, the peaks of the profiles for silicon, nitrogen, and oxygen were also observed


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between 2.5 and 5.0 Å, as shown in Figure 6b, which suggests that the amino silane molecules also lie flat on the Al2O3 surface. However, most of the nitrogen atoms are closer or at the same distance from the Al2O3 surface in comparison with the silicon atoms. This implies that amino groups are bound to the Al2O3 surface (Figure 6ii) because they have strong interactions with the Al2O3 surface owing to their high polarity; consequently, BADGE face difficulty while approaching and thereby interacting with these bonded amino groups. Therefore, the interactions of amino groups with Hint are relatively low in spite of their high polarity. For trifluoroalkyl silane, on the contrary to the other silanes, the profile of fluorine has its highest peak at 8.0 Å, as shown in Figure 6c, which is much farther from the surface than for the peaks of silane and oxygen. This suggests that most of the trifluoroalkyl silane molecules stand-up on the Al2O3 surface, i.e., their alkyl chains are perpendicular to the Al2O3 surface. Therefore, the top of the silane-treated Al2O3 surface is mainly covered with trifluoroalkyl groups and the silanol groups are hidden under them, as shown in Figure 6iii. The BADGE molecules cannot access and interact with the silanol groups because of this configuration. Resultantly, the silanol groups of trifluoroalkyl silane have the weakest interaction of the silanes studied here. Therefore, the wettability by BADGE of the Al2O3 surface treated with trifluoroalkyl silane was the poorest.



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Figure 6. The profiles of the relative concentration of atoms in the silane molecules along the z axis over the last 500 ps of the MD simulation for (a) mercapto silane, (b) amino silane, and (c) trifluoroalkyl silane, and schematic representations of the silane molecules for each profile, (i) mercapto silane, (ii) amino silane, and (iii) trifluoroalkyl silane.


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As argued above, the configurations of the silane molecules affect their interactions with BADGE and the wettability of the silane-treated Al2O3 surfaces. Theoretically, the amounts of the primers absorbed on the Al2O3 surface are expected to be different according to their configurations. However, the primer layers formed on the surface by our procedure were so thin that we could not compare the amounts even by surface sensitive analytical method like XPS. Using TOF-SIMS, we then evaluated the Al2O3 surfaces treated with amino and trifluoroalkyl silanes to confirm the validation of the configurations obtained by MD simulations. These two silanes were studied because the difference between the profiles of the relative concentrations of atoms in amino and trifluoroalkyl silanes was the starkest (see Figures 6b and 6c). In other words, the results of the MD simulation indicated that almost all the amino silane molecules lie flat on the Al2O3 surface, whereas most of the trifluoroalkyl silane molecules stand-up on it. Generally, the surface sensitivity of TOF-SIMS varies depending on the species of the primary ion.44 It has been reported that, for Bi+ and Bi3+ ion sources, Bi+ ions yield more surface sensitive spectra. The escape depth of secondary ions, which is related to the surface sensitivity, differs according to the species of the secondary ions. In the case of the secondary ion CH4N+ derived from a protein, for Bi+ and Bi3+, it has been reported to be 1.8 and 2.9 nm, respectively. Using this difference in surface sensitivities between Bi+ and Bi3+, we were able to confirm the MDsimulated configurations of amino and trifluoroalkyl silanes on the Al2O3 surface. To evaluate the configurations, we analyzed the NH− and F− peaks in the negative ion spectra of amino silane and trifluoroalkyl silane respectively, and the Si– peaks in the negative ion spectra for both surfaces. Figure 7 shows the ratios between the peak intensities of NH− and Si− for amino silane and the F− and Si− peak intensities for trifluoroalkyl silane obtained by using the primary ions, Bi+ and Bi3+. The ratios reported in Figure 7 are averages of six measurements from two



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substrate samples. For amino silane, the ratios for Bi+ and Bi3+ are almost the same, as shown in Figure 7a, indicating that the nitrogen and silicon atoms exist at the same depth from the top of silane-treated surface. This is consistent with the simulated configuration, as shown in Figures 6b and 6ii, and implies that most of the amino silane molecules lie flat on the Al2O3 surface. On the contrary, for the trifluoroalkyl silane, the ratio for Bi+ is much higher than that for Bi3+, as shown in Figure 7b. This suggests that the fluorine atoms are at a shallower depth than the silicon atoms, which totally supports the simulated result, as shown in Figures 6c and 6iii, where the trifluoroalkyl silane molecules are in a standing configuration. In addition, aluminum ions were strongly observed in TOF-SIMS analysis even by the primary ion of Bi+, whose escape depth is 1.8 nm. This indicates that the initial constructions for MD simulations, in which the primer layers were around1.0 nm in thickness in Figure 6, are in good agreement with the results obtained by TOF-SIMS in terms of the thickness of the primer layers.

Figure 7. The ratios between the peak intensities of NH− and Si− for (a) amino silane and between F− and Si− for (b) trifluoroalkyl silane obtained by using the primary ions, Bi+ and Bi3+. AFM was used to measure the heights of the amino and trifluoroalkyl silane molecules on the Al2O3 surface to further investigate their configurations. To measure the heights, we obtained both the adhesion force and topographic images as we were not able to recognize a silane molecule solely from topographic images. This is attributed to the fact that the height scale is


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almost the same as the roughness of the Al2O3 surface. Therefore, in the adhesion force images, we identified places where the adhesion force was relatively low to be the site of a silane molecule because it mainly consists of alkyl chains that have a lower polarity and a smaller interaction with the AFM tip compared to the bare Al2O3 surface. Figure 8 shows the adhesion force and topographic images of Al2O3 surfaces treated with amino and trifluoroalkyl silanes. In the adhesion force images, we identified ten silane molecules, as shown by arrows in Figures 8a and 8b. In the topographic images, we measured the heights of the molecules from the crosssection profiles along the lines shown in Figures 8i and 8ii. Figure 9 shows the average heights of ten molecules for each silane. The average height for trifluoroalkyl silane is higher than that for amino silane by 2.5 Å, as shown in Figure 9. Although the experimentally obtained heights of both silanes are slightly larger than the simulated ones due to the roughness of the Al2O3 surface, the results agree with the configurations evaluated by the MD simulation, where the heights of the amino silane molecules that are lying down and the trifluoroalkyl silane molecules that are standing up are around 5.0 and 8.0 Å, respectively, as shown in Figures 6b and 6c.



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Figure 8. Adhesion force images for (a) Al2O3 surface treated with amino silane and (b) Al2O3 surface treated with trifluoroalkyl silane. Topographic images for (i) Al2O3 surface treated with amino silane and (ii) Al2O3 surface treated with trifluoroalkyl silane. Arrows in the adhesion force images signify places recognized as silane molecules. Lines in the topographic images indicate the directions along which the heights of molecules were measured from the crosssection profiles.

Figure 9. The average heights of ten molecules for amino and trifluoroalkyl silane on the Al2O3 surface.


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Conclusion We developed a MD simulation procedure to investigate the wettability of primer-treated Al2O3 (0001) surfaces by BADGE. This simulation gives good estimates for the wettability, which agree with experimentally obtained contact angles. From the results of the MD simulation, we conclude that the configurations of silane molecules on the Al2O3 surface contribute to the interaction between the silane-treated surface and BADGE, and hence the wettability. In other words, easy access by BADGE to the polar functional groups in silane molecules improves the interaction between the surface and BADGE, hence improving the wettability of the surface by BADGE. Furthermore, the configurations of amino and trifluoroalkyl silanes were evaluated by TOF-SIMS and AFM analysis. The results of these evaluations reveal differences in the configurations between amino and trifluoroalkyl silanes on the Al2O3 surface, which fully supports the simulated configurations. Therefore, MD simulation provides a model that is consistent with the experimental results. This MD simulation can aid the understanding of chemical structures at the interface between primertreated Al2O3 surfaces and BADGE, the interactions between them, and the wettability of the surfaces and is therefore a powerful tool to afford control over the wettability of Al2O3 surfaces.

AUTHOR INFORMATION Corresponding Author [email protected]



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Wettability of Primer-Treated Al2O3 Surfaces by Bisphenol A Diglycidyl Ether: Determination of the Mechanism from Molecular Dynamics Simulations and Experiments Yoshitake Suganuma,*,†Takuya Mitsuoka, †Satoru Yamamoto,‡ Tomoyuki Kinjo, †Hiroaki Yoneyama, † and Kazuhiko Umemoto† †Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan ‡Dassault Systèmes K. K., 2-1-1, Osaki, Shinagawa, Tokyo 141-6020, Japa

TOC Graphic



Figure 1. Chemical structures of the primers used for the surface treatment of the Al2O3 surface. 62x190mm (600 x 600 DPI)

ACS Paragon Plus Environment

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Figure 2. Initial construction for the MD simulation investigating the contact angle of BADGE on the Al2O3 (0001) surface treated with mercapto silane. 82x119mm (600 x 600 DPI)



Figure 3. The diameter (r) and height (h) of the BADGE droplet in the last snapshot of the MD simulation for the surface treated by amino silane at 298 K, and the relative concentration profiles of the BADGE molecules along the x, y, and z axes, from which r and h were obtained. 82x174mm (600 x 600 DPI)


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Figure 4. Experimentally determined contact angle as a function of the contact angle calculated by MD simulation. 82x97mm (300 x 300 DPI)



Figure 5. The RDFs between the Hint and the negatively charged atoms in the silane molecules over the last 500 ps of the MD simulations at (a) 298, (b) 423, and (c) 493 K. 82x200mm (600 x 600 DPI)


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Figure 6. The profiles of the relative concentration of atoms in the silane molecules along the z axis over the last 500 ps of the MD simulation for (a) mercapto silane, (b) amino silane, and (c) trifluoroalkyl silane, and schematic representations of the silane molecules for each profile, (i) mercapto silane, (ii) amino silane, and (iii) trifluoroalkyl silane. 82x149mm (600 x 600 DPI)



Figure 7. The ratios between the peak intensities of NH− and Si− for (a) amino silane and between F− and Si− for (b) trifluoroalkyl silane obtained by using the primary ions, Bi+ and Bi3+. 82x32mm (600 x 600 DPI)


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Figure 8. Adhesion force images for (a) Al2O3 surface treated with amino silane and (b) Al2O3 surface treated with trifluoroalkyl silane. Topographic images for (i) Al2O3 surface treated with amino silane and (ii) Al2O3 surface treated with trifluoroalkyl silane. Arrows in the adhesion force images signify places recognized as silane molecules. Lines in the topographic images indicate the directions along which the heights of molecules were measured from the cross-section profiles. 82x85mm (600 x 600 DPI)



Figure 9. The average heights of ten molecules for amino and trifluoroalkyl silane on the Al2O3 surface. 45x33mm (600 x 600 DPI)


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TOC Graphic 85x47mm (600 x 600 DPI)


Wettability of Primer-Treated Al2O3 Surfaces by Bisphenol A

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