Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen

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Interaction of the Helium, Hydrogen, Air, Argon, Nitrogen Bubbles with Graphite Surface in Water Ruben Bartali, Michal Otyepka, Martin Pykal, Petr Lazar, Victor Micheli, Gloria Gottardi, and Nadhira Laidani ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Interaction of the Helium, Hydrogen, Air, Argon, Nitrogen Bubbles with Graphite Surface in Water Ruben Bartali*,†‡, Michal Otyepka§, Martin Pykal§, Petr Lazar§, Victor Micheli‡, Gloria Gottardi‡, Nadhira Laidani‡ †Department of Physics, University of Trento, Via Sommarive 14 Povo, 38123 Trento, Italy ‡‡Fondazione Bruno Kessler, Center of Materials and Microsystems, Via Sommarive 18, 38123 Trento, Italy §Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Palacký University Olomouc, tř. 17. listopadu 12, 771 46 Olomouc, Czech Republic Keywords: graphite HOPG, graphene, gas-solid interaction, surface tension, bubbles, nanobubbles, molecular dynamics simulation, DFT

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Abstract: The interaction of the confined gas with solid surface immersed in water is a common theme of many important fields such as; self-cleaning surface, gas storage and sensing. For that reason, we investigated the gas-graphite interaction in the water medium. The graphite surface was prepared by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). The surface chemistry and morphology were studied by X-ray photoelectron spectroscopy, profilometry and atomic force microscopy. The surface energy of HOPG was estimated by contact angle measurements using the Owens-Wendt method. The interaction of gasses (Ar, He, H2, N2, and air) with graphite was studied by a captive bubble method, in which the gas bubble was in contact with the exfoliated graphite surface in water media. The experimental data were corroborated by molecular dynamics simulations and density functional theory calculations. The surface energy of HOPG equaled to 52.8 mJ/m2 and more of 95 % of the surface energy was attributed to dispersion interactions. The results on gas–surface interaction indicated that HOPG surface had gasphilic behavior for helium and hydrogen, while gasphobic behavior for argon and nitrogen. The results showed that the variation of the gas contact angle was related to the balance between the gas–surface and gas–gas interaction potentials. For helium and hydrogen the gas– surface interaction was particularly high compared to gas–gas interaction and this promoted the favorable interaction with graphite surface.

Introduction

The liquid-carbon interaction and gas-carbon interaction play a significant role in many fields such as surface physics, electrochemistry, chemical industry, battery technology, nanoconfined liquids, gas sensing, permeation and also in noncovalent functionalization of graphene.1-11 Literature is mainly focused on the study of water-graphite or water-graphene interaction,12-13


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and few papers report the interaction of graphite with other liquids or with gasses.14-18 The interaction of gas bubbles with surface immersed in liquid media is a critical point in nucleation boiling point of liquids19 and is crucial to study the stability of nanobubbles.20-23 Moreover, the interaction of gasses with the matter in liquid media is important in self-cleaning surfaces based on superhydrophobicity phenomena. In the superhydrophobic surfaces, in fact, the nano and micro corrugation of surface promote the trapping of air bubbles beneath the water droplets, and this induces the highly hydrophobic character of surfaces.24-26 Due to the importance of liquidgas-solid interaction in this work, we explore the interactions of bubbles of air, N2, Ar, He, H2 with the surface of exfoliated HOPG immersed in the water media. We observed a variation of bubbles shape in contact with graphite as a function of nature of the gas. To understand better this phenomenon we studied the physicochemical properties and thermodynamic equilibrium of gas and liquid with graphite surface. In the first part of the paper, we report the surface chemistry, the morphology of exfoliated graphite using X–ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). In the second part, we describe the surface tension and surface properties of HOPG using sessile contact angle (gas–liquid-solid system) using different liquids (water, isopropanol, ethanol, glycerol). The surface tension and surface thermodynamics properties were estimated using Owens-Wendt method and work of adhesion.27-29 A comparison of results is discussed taking into account the surface chemistry and morphology of surface. Finally, we report the contact angle of the gasses with HOPG surface immersed in the water media using captive bubble method (liquid-gas-solid system). We compare the experimental results with the results obtained, in nanoscale, with molecular dynamics simulations. Moreover, we analyze the data taking into account the polarizability of the gasses and the potential well depth of gas–gas interaction and gas-solid interactions evaluated by density functional theory.


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The molecular dynamics simulation, that reflected the trends observed experimentally, indicate that the gasses with smaller diameter show a greater adhesion with HOPG surface.

Material and Methods We exfoliated highly oriented graphite (Goodfellow Cambridge, UK) using scotch-tape method to obtain a fresh graphite surface. The thickness of bulk graphite was 2.0 mm, the lateral size was 10 mm × 10 mm and the resistivity was 8·10-5 Ω cm. The structure of exfoliated graphite, studied by X-ray diffraction, is typical of HOPG with a sharp peak at 26.542° (002) with a full width half maximum (FWHM002) of 0.2493° (see Supporting information for details). The morphology was studied by optical microscopy, AFM and profilometry. The sessile water contact angle (WCA) was measured using home–made system, using 2 µl drops. The images were acquired with CMOS camera, and the images were analyzed by Drop-Analysis software.30 For each sample, images of four drops placed at different sites were acquired. The graphite surfaces are prone to a progressive change of the wetting properties in the first minutes due to the capture of airborne contamination then the contact angle tend to stabilize, in agreement with previous studies7, 31 (see Supporting information for details). Therefore, the surface characterizations have been performed 3 minutes after the exfoliation procedure; that is a good compromise to work with a quite stable and clean HOPG surface. It should be noted that the speed of contamination depends on the lab environment and therefore the reasonable time for experiments should be evaluated. We used the Owens-Wendt approach to measure the surface free energy. In the Owens-Wendt approach the total surface energy, γs, is defined as the sum of polar and dispersive components indicated as γsp and γsd, respectively.


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+

(1)

Where the polar components are the sum of hydrogen, polar, inductive and acid-base interactions, and dispersive component are due to London interactions. The surface energy by Owens-Wendt method, which is based on Berthelot hypothesis, can be estimated by the following equation: 1 + cos = 2 + 2

(2)

Where is the contact angle, γl the total surface tension of liquids, γld is the dispersive component of the liquids and γlp is polar component of the liquids. Due to two unknown parameters γsp and γsd in Equation 1, the contact angle has to be measured using two measuring liquids in order to estimate γs of graphite. In this study static contact angle measurements for surface energy estimation were performed with milliQ deionized water and diidiomethan as test liquids. These two liquids have been used because water had a dominant polar component and diiodiomethane had only dispersive component. The total surface energy of liquids and their polar and non-polar components are listed in Table 1. The Young-Dupré equation has been used to estimate the reversible work required to separate liquid–solid and gas–solid: = 1 +

(3)

Where Wa is the work of adhesion that represents the reversible work to separate two phases from interface to infinitive. The work of adhesion was estimated using different liquids probes such as formamide, liquid paraffin, ethanol, 2-propanol, diiodomethane, glycerol, ethylene glycol. In Table 1 are reported the total surface tension γl the dispersive components γld and the polar components γlp of the liquids in mJ/ m2.


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Table 1. Data of surface energy components of the test liquids (mJ/m2).32-36

Liquids

γl

γ ld

γ lp

Water

72.8

21.8

51

Glycerol

64

34

30

Ethylene glycol

48

29

19

Formamide

58

39

19

Diiodomethane

50.8

50.8

≈0

Paraffin liq

31

31

≈0

Ethanol

21.4

18.8

2.6

2-propanol

20.93

12

8.3

Total surface tension γl, dispersive components γld and the polar components γlp of the liquids in mJ/m2. The captive contact angle has been measured using the set up reported in Figure 1. The cell of captive contact angle is composed of a plastic cuvette (1.5 × 1.5 cm) filled with deionized water. The sample is held with bi-adhesive tape on polyethylene raft. Argon, air, nitrogen, hydrogen and helium were inserted into the cell using a micro-syringe.


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Figure 1. Captive bubble method. The set up was used for air, He, H2, Ar and N2 gases. The chemical structure and composition of the HOPG samples were studied by XPS. XPS spectra were recorded with a Scienta-ESCA 200 instrument equipped with a hemispherical analyzer and a monochromatic Al Kα (1486.6 eV) X-ray source. The core lines (C1s, O1s) were acquired at 150 eV pass energy, which leads to an energy resolution of 0.4 eV. After a Shirleytype background subtraction, the spectra were fitted using a non-linear least squares fitting program adopting a Gaussian–Lorentzian peak shape. The morphology and thickness were studied using AFM and mechanical profilometer. The mechanical profilometer was KLA tencor P6. The atomic force microscopy was SIS-AFM. The image has been acquired in contact mode using silicon tip (PPP-CONTR by nanosensor) with a cantilever length of 450 microns and a height of tip of 10 microns and tip radius < 10 nm. Density Functional Theory calculations were performed using the projector-augmented wave method in the Vienna Ab initio Simulation Package (VASP).37-38 We used optimized van der Waals functional optB86b-vdW functional39 in all calculations, which provided balanced description of structural and adsorption properties in our recent studies.15,

40

In addition, we

considered possible effect of many-body dispersion interaction by using the many-body


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dispersion energy method (MBD@rsSCS) introduced by Tkatchenko et al.,41-42 because the many body terms can affect the binding energy of noble gas atoms on graphene.43 The (0001) surface of graphite was modeled using a slab of 3 × 3 elementary cells with a thickness of two layers (36 carbon atoms in total). The two-layer system is denoted as graphite, since it mimics the behaviour of a real graphite substrate due to weal interaction among layers. Previous tests showed that the slab having triple graphene layers gave the adsorption energy very close to those obtained using the double layer system.44 The 6 × 6 × 1 k-point grid was used to sample the Brillouin zone. The periodically repeated slabs were separated by 17 Å of vacuum. The energy cutoff for the plane-wave expansion was set to 400 eV. In accordance with previous studies, the He and Ar atoms were placed into the hollow positions (the center of hexagonal ring). It should be noted that other adsorption positions (top of C atom, bond between two carbon atoms) are almost isoenergetic.44 The H2 and N2 were positioned so that their center of mass lied in the hollow position. The minimum of the energy was found by force-relaxation, i.e., the atomic forces were fully relaxed using conjugate gradient algorithm. For the calculation of dimer potential well depth, the species were placed into a cubic supercell having the length of 22 Å in each direction. The energy cutoff for the plane-wave expansion was increased to 600 eV. We used T-shaped dimer geometry for both H2 and N2 molecule, which is the ground-state geometry of the dimer according to earlier calculations.45-46 The molecular dynamics simulations of noble gasses nanobubbles on graphene were carried out using the GROMACS 4.5. software package.47 Graphene was represented by periodic model with dimensions of 92 × 92 Å. The z-dimension of the box was set to ~100 Å. Carbons in graphene were simulated as uncharged Lennard-Jones spheres with parameters proposed by Cheng and Steele.48 Argon and helium parameters were taken from literature Ref.49, Ref.50,


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respectively. SPC/E water model was used for solvating the system. The initial configurations consisted of 1,567 gas molecules, which were randomly distributed within the simulating box. The equations of motion were integrated with the time step of 2 fs. The electrostatics was treated using the particle mesh Ewald (PME) method. The cutoff distance for vdW interactions and real space cutoff scheme for PME was set to 10 Å. The system was slowly thermalized from 10 to 300 K and equilibrated at the final temperature (300 K) using the NpT ensemble. The Vrescale thermostat was used to maintain the desired temperature. The constant pressure (1 bar) was fixed with the Berendsen barostat. Then the simulation was switched to canonical (NVT) ensemble, whereby last 100 ns of the production run was used for subsequent analysis (the trajectory was recorded every 20 ps. The bonds involving hydrogens (water model) were constrained using LINCS algorithm. Snapshots were rendered with the software PyMOL.51

Results and Discussion The surfaces of exfoliated graphite were in general smooth because their average roughness was lesser than 0.5 nm in the area of 5 µm × 5 µm as measured by AFM, and less than 500 nm on 1 mm of the scan measured by the profilometer. But certain regions were more structured and rough, for instance, the region showed in Figure 2a by an optical image. The surface consisted primarily of two planes I and II, which were separated by consecutive terrace-like structured composed by several layers of HOPG (Figure 2). The step height of the terraces-like structure was 500 nm. Using an optical microscope and AFM, we evaluated the morphology of the HOPG surface in the range of microns and nanometer inside the planes, such as region I and II (Figure 2b). Also inside of the flat region, there were steps and corrugations with nanoscale dimension, the main steps showed the height of 150 nm, but there were also some nano-steps, with the


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thickness in the range of 1-20 nm (Figure 2c).

c)

Figure 2. Optical microscopy of exfoliated HOPG in millimeter range unraveled terraces-like structure (a); optical microscope image in the range of microns (b); and 3D AFM image of a nanostep (c). Under chemical point of view the surface of exfoliated graphite was analyzed by XPS. A survey spectrum (Figure 3a) shows the surface elements; i.e., dominant carbon (98.5 at. %) and minor oxygen (1.5 at. %) were detected.


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Figure 3. Survey XPS spectrum of HOPG revealed dominant presence of carbon (98.5 %) and minor component of oxygen (1.5 at. %) (a). High-resolution C1s line fit with components at 284.70, 285.4, 285.86, 286.48 and 286.90 eV (b). Figure 3b shows the peaks fitting of the carbon C1s core lines. The main band has two components at 284.7 eV and at 285.4 eV. The major component at 284.7 (1) is related to C=C bonds in graphite. The 285.4 eV peak is ascribed to C of hydrocarbons usually due to airborne contamination (2). The components at 285.8 (3), 286.5 (4), 286.9 (5) eV are the components related to the carbon bond with oxygen, the sum of these components is 5.41 at. % of carbon signal. The carbon-carbon bonds represent 87 % of the signal; the airborne contamination amounts to 8 % and the graphite carbon bonds with oxygen and OH to around 5 %. For exfoliated graphite, we tested the wetting behavior and the work of adhesion with different liquids. The liquids contact angles on HOPG surface are reported in Figure 4. Despite the fact that they were acquired on fresh HOPG 3 minutes after exfoliation, the absolute values of contact angles and derived thermodynamics parameters were influenced by the presence of airborne contaminants (cf. Figure S2 and related discussion in Supporting Information). On the other hand, the relative values and comparisons of various liquids remained valid for further


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discussion, because we kept the measurement protocol constant. Water showed a contact angle of 78° in good agreement with a value reported in literature.52 The alcohols such as 2-propanol and ethanol, displayed the lowest contact angle around 5° due to their low surface tension (see Table 1). Also, the diiodomethane showed a contact angle less than 10° even if its surface tension is quite high (50.8 mJ/m2). This indicates that chemical nature of liquids in term of polar and dispersive components is very crucial for wetting of the surface of exfoliated graphite, (see Table 1).

Figure 4. Contact angles of liquids on graphite (a); work of adhesion of the liquids on graphite estimated by the Young-Dupre equation (b). Figure 4b shows the work of adhesion of the liquids with HOPG surface. The isopropanol and ethanol showed the lowest work adhesion, 41 mJ/m2. The formamide and diidomethane, the liquids with the highest dispersive component, showed the highest work of adhesion, > 100 mJ/m2, while water showed a work adhesion of only 87.5 mJ/m2. The contact angles reported in Figure 4a show that many liquids such as water have a quite wide standard deviation of the data. This is probably due to relative wide distribution in roughness of surface that can increase the dispersion of contact angle values in macroscale by Wenzel mechanism.


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Figure 5. Total surface tension dispersive component and polar component of HOPG estimated with Owens-Wendt method. Using water and diidiomethane as probe liquids and Owens-Wendt method, we estimated the total surface energy of graphite. Graphite showed a total surface tension of 52.8 mJ/m2, this value was comparable with the value reported by Good et al. on graphite and coal.20, 53-54 We estimated also the dispersive components (due to London interaction) and the polar component of the surface. The dispersive component was 50.2 mJ/m2 and the polar component 2.2 mJ/m2, Figure 5. This indicates that more of 95 % of graphite interaction is due to London interaction, in accord with theoretical calculations on graphene models,15 and only a small amount of interaction with HOPG is due to the polar component (4.2 %). As indicated by XPS spectra the polar component is due to the presence of the small amount of oxygenation of carbons at the surface (5 %). The surface of graphite showed a high dispersive component and this explains the positive interaction of HOPG with non-polar liquids such as diiodomethane or paraffin oil and the weak interaction with water. The results were corroborated by the high work of adhesion of


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diiodomethane and formamide, that were the liquids with the larger dispersive surface tension. Therefore, chemical nature in terms of polarity and in terms of dispersive interactions plays an important role in the liquid-HOPG interaction. This effect is well reported in literature and some work have also revealed that can be more pronounced in nanoscale in particular in nanofluidics.55 Taking into account the nonpolar character of HOPG we immersed the exfoliated graphite in water and we tested the interaction of gasses with HOPG using the captive bubble method (cf. Figure 1). The surface tension between liquid and gasses can be affected by the nature of the gas and by pressure inside the bubble. To estimate the pressure inside the bubble we used the YoungLaplace equation. The average radius of the bubbles was around 1 mm and therefore, we estimated a bubble internal pressure of ≈1.002 atm. With Massoudi equation, we estimated the effect of the pressure on liquid-gas surface tension for the different of gasses used in the experiment.56 For all gasses we estimated a surface tension with water is around 72.8 mJ/m2 and the expected reduction of surface tension induced by the internal pressure is less than 0.4 %. Therefore, we can consider in first approximation the effect on liquid-gas surface tension due to the pressure and of type of gasses negligible. Taking into account the fact that the water-graphite interaction is identical for all samples, the variation of the bubble contact angle gives an indication of the affinity of the gases to HOPG surface directly. Therefore the contact angles values in Figure 6 are the contact angles between graphite-gas interface and gas-liquid interface instead of the complementary contact angle typically used for the air-bubble contact angle.57 Figure 6a shows a gasphillic behavior of HOPG surface with hydrogen and helium (C.A. less than 90°) meanwhile gasphobic interactions. (C.A. higher than 90°) with the air, nitrogen and argon.


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Even if the surface is relatively fresh, the experimental contact angle can be easily affected by the airborne contaminations or by the roughness of the surface in micro/nanoscale.12, 58-59 The bubble contact angle was less prone to be affected by contamination, contrarily to the sessile contact angle experiments. We observed, in fact, that the bubble contact angle was constant at different time after the exfoliation procedure (see Supporting Information for details). However, even if the bubble contact angle was more stable than sessile contact angle, we cannot exclude the presence of airborne contaminants adsorbed on the surface. To overcome this problem and other issue related to surface roughness on micro/nanoscale, we carried out the MD simulations on an ideal surface, i.e., flat and without contaminations (Figure 6). We have chosen two extreme cases identified by experiments, i.e., the argon and helium gasses. Obtained values of contact angles 73.2º, 80.0º for helium and argon, respectively, match the order obtained from experimental measurements. However, the estimations from MD differ in absolute values of the C.A. It may be explained in part by the: a) size of the studied bubbles (radii amounted to ~40 nm) b) different morphology of surface (no steps) c) effect of airborne contaminants (as shown on simulations of water droplet on graphite60) and d) in part by the fact, that the classical force field methods neglect the polarization effects, that may be important in such cases involving graphene and it would require further analysis. Nevertheless, the result of MD simulations confirms that the variation of the bubble contact angle on HOPG is due to the nature of the gas.


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Figure 6. Contact angles of gas bubbles with graphite (a). Contact angles of helium (b) and argon (c) calculated on the basis of MD; resulting values were averaged over the production run. A snapshot of a typical MD simulation system (d). Since sessile drop analyses demonstrated (cf. Figure 5) that the graphite surface had a nonpolar behavior, we could also expect that the interaction between gas and graphite is mainly due to London interactions. In the dispersive interaction polarizability plays an important role and this is also demonstrated by the correlation of polarizability and contact angle reported in Figure 7. The gasses, in fact, with low polarizability such as helium and hydrogen (1.4-5.4 a.u.) show a


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positive interaction with ex-HOPG while the molecules with high polarizability show a repulsive interaction with HOPG see Table 2. The transition from gasphilic and gasphobic regime in terms of polarizability happens, in these samples, around 7–10 a.u. that is the typical range of polarizability of carbon aromatic bonds reported in the literature (7–10).61 C.A 102

Polarizability a.u of Carbon - Carbon aromatic

100

Ar 98

Air 96

Contact angle [°]

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


N2

94 92 90 88 86

H2

He 84 82 80 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

polarizability a.u

Figure 7. Contact angle as a function of gas polarizability (atomic unit). Therefore, when the polarizability of the gas is higher than the polarizability of surface, the contact angle of the bubble tends to increase and gas-solid interaction tends to decrease (Figure 6). This indicates that under the phenomenological point of view in a system composed of the bubble in contact with a solid surface, there is a sort of dynamic equilibrium between the gas– solid interaction at the interface (bubble–solid) and gas–gas interaction present in “bulk” of the bubble. The higher is the gas–gas interaction the less is the gas adhesion on the surface.


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Usually the gas–solid interaction is stronger than the gas–gas interaction (cf. Table 2). For instance, the N2–HOPG interaction potential is 166 meV meanwhile N2–N2 interaction potential is only 17 meV. If we define χ as the ratio between gas-HOPG and gas-gas potentials (Eq. 4): =

! " # $ ! " # $

(4)

we can observe that N2–graphite interaction is ~7 times higher of N2–N2 interaction. Table 2 shows the values gas-HOPG (gsw) and gas–gas potential well depth (ggw) reported in literature. Anyway, to have a systematic evaluation on flat clean HOPG surface at 300K and a 1atm, gsw and ggw have been calculated by vdW-DF functional. The calculated potential well depths are generally in agreement with other theoretical calculations. For the N2 dimer we obtained the energy of 17 meV and 14 meV using the optB86b-vdW and MBD@rsSCS functional, respectively. Accurate ab-initio calculations at the MP4 and CCSD(T) levels provided the values in the range 7–16 meV42-43, depending on the method and the basis set used. The well depth of argon dimer agrees well to the coupled cluster singles and doubles and perturbative triples – CCSD(T) – value of 11.5 meV62 as well as to the experimental value of 12.4 meV.63 The only slight exception is the well depth of He on graphite. In that case both optB86b-vdW and MBD functional predict the potential well too deep (42 meV and 56 meV, respectively), whereas previous calculations reported the values in the range of 12-20 meV [see Ref.41 and references therein]. The most elaborate study calculated the He–graphene interaction via the method of increments by evaluating two- and three-body dispersion terms at CCSD(T) level64, and obtained 17.7 meV at the distance of 3.2 Å. However, they did not consider other distances, so the minimum of the binding energy lied presumably deeper. The binding of the He dimer was very weak (1 meV using the optB86b-vdW functional and 5 meV using MBD@rsSCS), despite


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overbinding for He on graphite. Comparing optB86b-vdW functional to its many body counterpart MBD@rsSCS, one can observe that the functional yield very similar values, except for the interaction of N2 and Ar with graphite, in which case the MDB functional provided significantly lower values of the well depth. The lower values of the interaction energy were caused by the long-range many-body effects, which brought negative contribution to the well depth (-13 and -11 meV for N2 and Ar, respectively). This finding is analogical to the effect many-body terms observed in previous study by Ambrosetti and Silvestrelli.44 Nevertheless, both our theoretical calculations clearly indicated that the group of gas composed of helium and hydrogen had the lowest contact angle and showed the highest χ value. Meanwhile, the group of gas composed by Ar, Air and N2 that had the highest contact angle, has the lowest χ ratio. This indicates that the equilibrium of interaction in the bubble of gas in contact with a surface immersed in a liquid is tuned by the χ ratio.


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Table 2. Data of static polarizability in atomic units (a.u.) and potential well depth (meV) used in this work.6, 26, 65-67 Gas

Polariz Potential ability well depth1 α0 (meV) (a.u.) Gas– Graphite.

Potential well depth1 (meV) Gas–Gas vdW-DF

χ Ratio Gas– Graphite / Gas– Gas

Potent ial well depth (meV)

vdW-DF

Gas– Graph ite

vdW-DF

Potenti al well depth (meV) Gas– Gas

χ Ratio Gas– Graphite / Gas– Gas

Kinetic Molecul ar Radius (pm)

He

1.38

42 (56)

1 (4)*

~42

16.2

0.88

18.4

260

H2

5.43

65 (79)

5 (9)

13.0

51.7

3.1

16.7

289

Ar

11.07

141 (113)

14 (13)

10.1

96

10.7

8.9

344

N2

11.74

166 (126)

17 (14)

7.2

104

7.89

13.1

346

Air

11.22

172**

20**

8.6

101.2

8.262

12.22

350

1) All values were calculated by optB86b-vdW functional. The values calculated by MBD@rsSCS functional are in parentheses. 2) Estimated as a function of molar fraction of N2 and O2, by Refs.6, 65 * lower than the convergence precision, in literature the value amounts to 0.95 meV68 ** estimated as the weighted average 78 % of N2, 21 % of O2 (potential well depth for gasgraphite is 198 meV and for gas-gas is 10 meV) and 1 % of Ar.

The χ ratio varies in function of the type of gas and as a function of materials surface, due to many physical parameters of the system such as polarizability, first ionization potential, and geometrical factors. We can speculate that some gasses such He and H2 are spread better on HOPG surface due also to the geometrical dimensions of the molecules. This is corroborated by the kinetic molecular radius of the molecules, reported in Table 2. Helium and hydrogen show a


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smaller radius 260 and 285 pm respectively. The typical radius of a ring of graphite is 240 pm. The dimension of He and H2 can help spreading of the gas molecules at the graphite surface that is reflected in the high gas affinity of He and H2 with graphite. We remark that χ clearly indicates that in thermodynamics properties estimation the gas-gas interaction should be explicated. Therefore, some typical equation used to study the interaction of bubbles on solid surface as work of adhesion, needs further investigation to be correctly applied in the gas–surface interaction estimation of carbon-based materials immersed in liquid. Although these results are obtained only on HOPG surface that is perfectly oriented, the findings provide a new insight into interfacial phenomena, which may have an impact on a wide range of fundamental studies and applications in particular in gas storage and on the self-cleaning surfaces.

Conclusion In this work, we studied the bubble gas interaction with HOPG immersed in water media. The bubbles interaction on the surface is important for many applications based on graphite materials such as; gas storage, electrochemistry membrane technology and fuels cells. We observed that the HOPG surface had an attractive interaction with the bubble of H2 and He (gasphilic behavior) and weaker interaction with air, Ar and N2 (gasphobic behavior). The findings of this work show that gasphilic/gasphobic behavior is affected by the ratio between the gas–HOPG and gas–gas potential, the χ ratio. A high χ ratio indicates an high force of interaction at gas-solid interface compared to the force of gas–gas interaction. Helium shows the lowest contact angle and the highest spread on HOPG; these is due to the fact that helium has the highest value of the χ ratio.


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Supporting Information

X-ray diffraction on graphite/exfoliated graphite and water sessile contact angle as a function of the time after exfoliation are reported.

Corresponding Author * Email: [email protected] Acknowledgments Partially supported from the Ministry of Education, Youth and Sports of the Czech Republic, via projects LO1305 and CZ.1.05/2.1.00/19.0377 and from the Czech grant agency via project P208/12/G016, is gratefully acknowledged. M.O. acknowledges support from Neuron fund.

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Table of Contents Graphic


27

Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen

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