Nanowetting of Graphene by Ionic Liquid Droplets - ACS Publications

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Nanowetting of Graphene by Ionic Liquid Droplets Cesar Herrera,† Gregorio García,† Mert Atilhan,‡ and Santiago Aparicio*,† †

Department of Chemistry, University of Burgos, 09001 Burgos, Spain Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

‡

S Supporting Information *

ABSTRACT: The behavior of nanodroplets formed by amino acid based 1-ethyl-3-methylimidazolium glycine ionic liquid on graphene sheets was studied using classic molecular dynamics. Nanodroplets of different sizes were analyzed, and the contact angle was inferred. The ion−graphene interaction energy was calculated as a function of droplets size, together with the ions arrangement at the interface. Likewise, wetting of SiO2 and graphene supported on SiO2 by ionic liquid nanodroplets was also analyzed. Graphene wetting by this ionic liquid is discussed in terms of the changes on fluid structure upon adsorption and the mechanism and strength of ion−graphene interactions.

1. INTRODUCTION The surface coating of materials with graphene sheets have been proposed as a promising alternative for developing advanced applications.1 The possibility of achieving suitable electrical, optical, and wetting properties, through graphene coatings by selecting proper substrates is a remarkable technological possibility. Therefore, applications have been developed in fields such as corrosion inhibition,2 although with long-term problems,3 optics,4 biomedical,5 photocatalysis,6 lubrication,7 or electrochemistry.8 The wetting properties of graphene have been the subject of several studies because of their technical relevance and because of the controversy raised by the results reported by Rafiee et al. showing graphene coatings do not change the wetting behavior of underlying materials, so-called wetting transparency.9 The roles of the hydrophobic character of graphene surface and the background surface have been discussed by several authors showing contradictory results. Shi et al.10 reported roughly 30% transmission of van der Waals interactions between water and substrates through the graphene coating layer, whereas Xu et al.11 and Li et al.12 showed the relevant role of hydrocarbons contamination on the wettability of supported graphene. Raj et al.13 carried out studies using advancing contact angle measurements for multilayer graphene sheets on copper, silica, and glass, showing that coating with two-dimensional (2D) graphene leads to wettability similar to that for threedimensional (3D) graphite. Therefore, understanding the graphene−water interactions with regard to wettability is a pivotal issue for advanced coating applications. For this purpose, Taherian et al.14 carried out molecular dynamics simulations for water droplets contact angles on graphene, showing how the interaction potential energy between water and the graphene-based substrates is the main contribution to © XXXX American Chemical Society

the work of adhesion being independent of the number of graphene layers. Li et al.15 carried out quantum molecular dynamics simulations of water nanodroplets on graphene sheets leading to 87° contact angle showing the induction of positive charges on the contacting graphene surface. The behavior of water droplets on graphite was addressed by Sergi et al.,16 who studied equilibrium contact angles and the transition to the macroscopic limit using molecular dynamics simulations. Kim et al.17 studied the wetting transitions of water on graphite and graphene through quantum chemical calculations developing adsorption potentials for water−graphene interactions, which led to values close to experimental data. The wetting behavior of graphene by water has been widely studied, but still remaining controversial; however, the properties of other liquids, in particular contact angles and nanodroplets interactions, on graphene sheets have been considered in a reduced number of studies. Wang et al.18 measured contact angles of water, formamide, diiodomethane, ethylene glycol, and ethanol on graphene, and also on graphite and graphene oxide, showing that for all of these solvents, graphene is less wettable than graphite. The wetting of graphene sheets by nonwater liquids could open the possibility of new applications beyond aqueous media. Ionic liquids, ILs, are a group of compounds which have attracted great attention in many technological areas,19−22 including those in relationship with graphene.23−25 The most remarkable properties of ILs with regard to graphene wetting applications are (i) their almost null vapor pressure,26 which hinders the evaporation from nanodroplets, being an advantage Received: September 7, 2015 Revised: October 8, 2015

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DOI: 10.1021/acs.jpcc.5b08713 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

angle. Spherical boxes were initially built for each system, using Packmol,41 which were equilibrated in vacuum at 403 K in the NVT ensemble using octahedral periodic boundary conditions, to mimic spherical drops, using simulations during 1 ns. A graphene sheet, with edge in the armchair configuration, was built with 48.7 × 48.8 Å dimensions. Previously equilibrated nanodroplets were placed in the center of the graphene sheet, with 4 Å initial distance, and rectangular periodic boundary conditions were applied with z dimension equal to 120 Å. The dimensions of the simulation boxes are enough to avoid interactions with neighbor cells. All the nanodroplet + graphene systems were simulated in the NVT ensemble at 403 K during 5 ns. Equilibration was assured through the analysis of potential energy. Temperature was controlled using the Nose−Hoover thermostat. Graphene sheets were maintained, fixed along the simulations, to keep the interface planar. Coulombic interactions were handled with the Ewald summation method,42 with cutoff radius of 15 Å. Giovambattista et al.43 reported that the value of the Ewald wave vector cutoff used in the simulations may have effect on the wetting behavior of water nanodroplets. Nevertheless, considering previous literature works for nanowetting of ionic liquids,13,37 and other simulations of the wetting behavior of fluids such as water, in which simulations were carried out for a fixed value of the Ewald cutoff parameter,14 this effect was not analyzed in this work because the main objective was to analyze the effect of the droplet size on the nanowetting properties. Tuckerman−Berne double time step algorithm,44 with long and short time steps of 1 and 0.1 fs, was considered for solving the equations of motion. Lorentz−Berthelot mixing rules were used for Lennard-Jones terms. Force field parametrization for ions and graphene is reported in Table S1. Noncharged graphene was considered in these simulations, and thus only van der Waals contributions are included. Atomic charges for ions were calculated using the ChelpG45 method from previous DFT calculations,39 and thus, anion−cation charge transfer was considered with the total charge of ions being ±0.91. The effects of graphene coatings were studied considering graphene sheets supported on SiO2, and thus, simulations of droplets containing 200 ion pairs on top of SiO2 and on graphene supported on SiO2 were carried out. Castejón et al.37 reported molecular dynamics simulations of ionic liquid nanodroplets on silica using α-quartz slabs with one face terminated in hydroxyl groups (Si−OH), the hydrophilic surface, and the other face with hydrogen atoms (Si−H), the hydrophobic surface. Therefore, following Castejón et al.,37 an α-quartz unit cell (a = b= 4.9133 Å, c = 5.4053 Å; α = β = 90°, γ = 120°) was considered from which a slab 23 × 23 × 1 was built. Surfaces were cleaved along the (0 0 1) plane, and thus, one side of the slab was terminated with hydroxyl groups (SiO2−OH) and the other side with hydrogen atoms (SiO2− H). Likewise, graphene sheets were placed on top of both sides of the silica slab at 3 Å, and thus, two substrates were obtained: (i) grahene on top of SiO2−OH (graphene@SiO2−OH), and (ii) grahene on top of SiO2−H (graphene@SiO2−H). Force field parametrization for SiO2 was obtained from Lopes et al.46 The simulation for droplets on SiO2 or on graphene supported on SiO2 were also carried out in the NVT ensemble for 5 ns at 403 K.

in comparison with water and other organic solvents, and (ii) their tailor-made properties, which can be controlled through suitable combinations of ions, leading to hydrophobic or hydrophilic compounds among the large quantity of available ILs.27 Although surface behavior of ionic liquids on graphene have been studied both using experimental28,29 and computational approaches,30−33 the characterization of liquid−surface interaction through contact angle measurements have so far been the subject of a reduced number of studies. Likewise, ionic liquid droplets on graphene have showed relevant properties such as electricity generation through droplet moving along the sheet.34 Baldelli et al.35 carried out measurements of 1-butyl-3methylimidazolium methylsulfate on graphene, leading to a 58 ± 2° contact angle, with this wetting being justified by the surface−ion interactions through the alkyl and cationic ring, developing π−π interactions with the graphene, sites. Xu et al.36 carried out measurements for 1-butyl-3-methylimidazolium dicyanamide on graphene-coated BaF2, showing a 69 ± 2° contact angle. Considering this absence of information on the wetting behavior of ionic liquids for graphene sheets, theoretical studies using density functional theory (DFT) and classic molecular dynamics simulations may shed light on it, as previous studies have allowed for the analysis of the wetting of other relevant surfaces such as silica.37 Our group reported a recent study on the graphene folding assisted by ionic liquid nanodroplets,38 showing the flexibility of folding behavior, which can be controlled through a suitable selection of involved ions. Likewise, we have also reported a theoretical study using DFT on the interaction of amino acid ionic liquids with graphene sheets,39 leading to strong ion−graphene interaction energies combined with charge transfer. As a continuation of these previous studies, we report in this work a study of the wettability of noncharged graphene sheets by 1-ethyl-3methylimidazolium glycine, [EMIM][GLY] (Figure 1), using

Figure 1. Molecular structure of [EMIM][GLY] ion pair calculated at B3LYP/6-311++g(d,p) theoretical level. Atom color code: gray, carbon; red, oxygen; blue, nitrogen; and light gray, hydrogen.

nanodroplets of different size. The system is analyzed using molecular dynamics simulations. Moreover, the behavior of [EMIM][GLY] nanodroplets on graphene sheet supported on silica in comparison with the properties of these droplets on top of silica was also analyzed to consider the effects of graphene coatings.

2. METHODS Molecular dynamics simulations were carried out using the MDynaMix v.5.2 molecular modeling package.40 [EMIM][GLY] nanodroplets containing 100, 200, 300, 400, and 500 ion pairs were built, to infer the effect of droplet size on contact B


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Figure 2. Top and side views of [EMIM][GLY] nanodroplets on graphene sheet for (a) N = 100 and (b) N = 500, where N stands for the number of ion pairs. Color code: red, [EMIM] cations; green, [GLY] anions.

3. RESULTS AND DISCUSSION The effect on the nanodroplet size on the contact angle, θ, has been discussed in the literature showing that it is remarkably size-dependent.47 In the well-known case of water nanodroplets, the effect of droplet size has been considered on several surfaces such as gold,48 kaolinite,49 or graphite50 substrates, in all cases leading to θ values increasing with increasing droplet size, pointing to the effect of smaller water nanodroplets leading to more compact structures on the considered substrates. This nanosize effect on θ values rises from the fact that in the nanoscale range the Young’s equation has to be modified including a new term due to the line tension of the nanodroplets.51 Therefore, molecular dynamics simulations carried out in this work for [EMIM][GLY] nanodroplets containing N = 100, 200, 300, 400, and 500 ion pairs would allow for interference of the effect of nanosizing on θ values. The calculation of θ was carried out according to the procedure reported by Giovambattista et al.52 The center of mass of the droplet is calculated, and a z-axis, normal to the graphene surface, passing through it is defined. Slabs with 2 Å width perpendicular to the surface are considered, with the density profiles calculated for each slab. The liquid−vapor interface at each slab is defined for 0.2 g × cm−3, and thus, the radii of the droplet at each slab is defined as the distance from the z axis passing through the center of mass and the position of the liquid−vapor interface. Therefore, a profile of droplet radius is obtained, which is fitted to a second degree polynomial. The contact angle was calculated from the fitting polynomial at a height of 3 Å above the graphene surface. The different sizes of the droplets studied in this work raise the question if the simulation time used (5 ns) is enough to reach equilibrium contact angles both for the small and large droplets. Therefore, the time evolution of the contact angle for droplets with N = 100 and N = 500 is reported in Figure S1. The smallest droplet reached equilibrium contact angle for roughly 1 ns, whereas it is reached at roughly 2.5 ns for N = 500. Therefore, all the contact angle data reported in the following

sections were calculated as averages in the 3 to 5 ns simulation time frame. It should be noted that all the simulations reported in this work were carried out at 403 K, and thus, the droplets dynamic properties, including the time required for reaching equilibrium contact angle data, are different close to ambient temperatures. With regard to graphene wetting by [EMIM][GLY], results shown in Figure 2 point to a strong effect of the nanodroplet size, whereas for small droplets (Figure 2a for 100 ion pairs), low contact angles are obtained, with droplets wetting the graphene surface; for the larger studied droplets (400 and 500 ion pairs, Figure 2b), the trend is to develop larger contact angles. Starting from the smaller droplets, N = 100, a strongly adsorbed layer is formed on the graphene sheet, which points to remarkable ion−graphene affinity, and when additional ion pairs are placed on the droplet, N = 200 to 500, these new ions pairs do not tend to be adsorbed on the sheet but interact with other ions to maximize the strong anion−cation interactions through mainly Coulombic forces. Therefore, increasing N does not lead to a remarkable increase on the amount of ions interacting with the graphene but instead to adsorbed droplets with larger heights, Figure 2. The characteristics of the [EMIM][GLY] nanodroplets in the vicinity of the graphene sheet may be analyzed from the density profiles along the coordinate perpendicular to the graphene sheet, z direction, reported in Figure 3. Number density z profiles, Figure 3a, shows strong peaks both for the [EMIM]+ cation and [GLY]− anion, which correspond to the presence of strongly adsorbed layer on the graphene sheets. The peak for [EMIM]+ appears at 3.55 Å, whereas that for [GLY]− is placed at 3.75 Å, showing that although both ions tend to interact with the graphene substrate, the cation interaction is slightly more favorable than the anion one, which is in agreement with the previously calculated interaction energies between ions and graphene using Density Functional Theory, DFT, calculations that showed a planar structure of alkylimidazolium and amino acid based ion pairs allowing a very effective interaction with the graphene sheet, in particular for the imidazolium cyclic moiety.39 Although a second weak and C


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Figure 4. Maximum number density, ρ, as a function of the distance to the droplet center of mass, r, for [EMIM][GLY] nanodroplet with 500 ion pairs.

inferred. Likewise, the number density contour plots reported in Figure 5 show the high density regions for both ions in the vicinity of the graphene sheet but also ion rich regions in the inner regions of the droplets with decreasing intensities on going to the outer limits of the droplets that allow for the liquid−vacuum interface to be defined. In the case of small droplets, N = 100 (Figure 5, panels a and b), the high density region corresponding to the adsorbed layer on graphene determines the structure of the droplet, extending toward the top of the droplets and decreasing toward the liquid-vacuum limits. In the case of large droplets, N = 500 (Figure 5, panels c and d), the effect of the layer adsorbed on graphene also leads to a certain arrangement above this layer, which is shown by the high density spots above the first adsorbed layer, although this effect is weakened with increasing height above the graphene sheet for which ion−ion interactions should dominate over ion−graphene effects. The strongly adsorbed first layer on the graphene sheet leads to a remarkable structuring of the ions in this region, which should be different to that in the inner regions on the droplets for which ion arrangements are controlled by Coulombic interactions between anions and cations. This is quantified through the second order Legendre polynomials, P2cos(θ) = , calculated for the angles, θ, between the vector perpendicular to the graphene surface and selected vectors in both ions reported in Figure 6. Calculated P2 cos(θ) values for [EMIM]+ cation shows that imidazolium rings lie parallel to the graphene surface for the first adsorbed layer, which rises from the optimal interaction in this arrangement between the aromatic rings of the substrate and the imidazolium ring. Likewise, [GLY]− anions also lie parallel to the graphene sheets, thus maximizing the interactions between the substrate and the −NH2 and −COO functional groups, in agreement with previously reported DFT calculations. The results reported in previous paragraphs show remarkable changes in the droplet properties upon adsorption on graphene rising from the affinity of the studied ionic liquid for the graphene sheet. This affinity is quantified from the molecular dynamics results using the average ion−graphene intermolecular energies and from the changes in the anion−cation intermolecular energy upon adsorption. Adsorption of [EMIM][GLY] nanodroplets leads to large structural changes in the droplets internal structure, and thus, the anion−cation interaction should be substantially changed. This is confirmed in results reported in Figure 7a, in which larger anion−cation interaction energies per ion pair are obtained with increasing

Figure 3. (a) Number density, ρ, for [EMIM] and [GLY] ions, and (b) total charge density, ρc, as a function of height on graphene sheet, z, for [EMIM][GLY] nanodroplet with 500 ion pairs. Graphene surface at z = 0.

poorly defined peak in z-density profiles is also obtained for both ions at roughly 7.5 Å, Figure 3a, that points to a second adsorbed layer, this should be remarkably weaker than the first one and thus showing that the structuring effects because of the presence of the graphene sheet only extend a few angstroms in the vicinity of the surface. The effect of the droplet size on the properties of density z profiles are very weak; the maxima of the first peak remains unchanged with decreasing number of ion pairs in the droplet, and only an increase in the values at the maximum are obtained, evolving from 0.0049 atoms × Å−3 for [EMIM]+ in droplets with 500 ion pairs to 0.0069 atoms × Å−3 in droplets with 100 ion pairs, with the same results for [GLY]−. Therefore, the first [EMIM][GLY] adsorbed layer is more intense for small nanodroplets, and it is slightly weakened with increasing nanodroplet size because of the development of a larger number of interactions with ions being placed above this adsorbed layer with increasing droplet size, Figure 2. The slightly outer peaks in z-density for [GLY]− anions in comparison with [EMIM]+ cations in the vicinity of the graphene sheet leads to an asymmetric charge distribution close to the graphene surface, Figure 3b, with a region positively charged, peaking at 2.7 Å, and a negatively charged region, peaking at 3.3 Å. The shape of the studied adsorbed nanodroplets is strongly dependent on the size of the droplet for small droplets. The first peaks reported for density profiles in Figure 3a are followed by minima at roughly 5 Å, and thus, those ions with centers of mass up to 5 Å from the graphene surface, named first adsorbed layer for now, develop the strongest interactions with the surface. For small droplets, N = 100, a remarkable fraction of the ions belongs to the first adsorbed layer (34%), whereas for N = 500, this percentage decreases to only 11%, and thus, an hemispherical droplet is obtained for the larger [EMIM][GLY] nanodroplets because of the development of ionic layers on top of the adsorbed layer on graphene. This may be quantified through the number density as a function of distance to the droplet center of mass reported in Figure 4, from which the adsorbed layer on the graphene sheet is clearly D


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Figure 5. Contour plots of number density as a function of the distance to the droplet center of mass, r, and height on graphene sheet, z. Results for [EMIM][GLY] nanodroplets with (a and b) 100 and (c and d) 500 ion pairs. (a and c) show results for [EMIM] cation and (b and d) for [GLY] anion. Number density in atoms × Å−3.

Figure 7. (a) [EMIM]−[GLY] intermolecular interaction energy per ion pair on graphene sheet, Einter,A−C,gra, and (b) percentage changes on going from droplets in vacuum, Einter,A−C,vac, to droplets on graphene sheet. Values calculated for [EMIM][GLY] droplets with N ion pairs. Intermolecular interaction energies are the sum of Lennard-Jones and Coulombic contributions. Lines are reported for guiding purposes.

Figure 6. Average P2(cos θ) for the angles between the reported vectors and the graphene surface normal (z axis) in [EMIM][GLY] nanodroplet with 500 ion pairs.

droplet size. Likewise, anion−cation interaction energy for the droplets is compared before, in vacuum, and after adsorption on the graphene sheet, Figure 7b. Reported results show large weakening of anion−cation interactions when droplets are adsorbed, making this effect very large for the smallest nanodroplets, 35% weakening for N = 100 and decreasing with increasing droplet size, 14% for N = 500, which is in agreement with the aforementioned fact that in small nanodroplets, a large percentage of ions are involved in interactions with the graphene sheet with this percentage decreasing upon nanodroplet size increase. The effect of N on the weakening of anion−cation interactions is nonlinear, which may be justified considering the subtle effects rising from the changes in structure of the adsorbed layer on graphene and their effect on the structuring of ions in the liquid layers above

this strongly interacting layer in the vicinity of the graphene surface. The kinetics of the adsorption of [EMIM][GLY] nanodroplets on the graphene sheets were analyzed from the time evolution of the ion−graphene interaction energies, Einter(iongraphene), both for cation and anion, Figure 8. The spherical droplets were initially placed at roughly 4 Å of the graphene surface, t = 0, and thus, from this starting point the strongly adsorbed layer begins to be formed. [GLY]−graphene interactions are weaker than [EMIM]−graphene ones. This behavior is in agreement with the fact that [GLY]− anions are placed slightly further from the graphene surface than [EMIM]+ cations, Figure 3, and considering that the planar arrangement of [EMIM]+ cations on the graphene, Figure 6, E


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ions, respectively. This behavior shows that the strongly adsorbed layer on graphene increases when the number of ions in the droplet increases from N = 100 to 200, which will justify the increase in Einter(ion−graphene), but for larger nanodroplets, the additional ions do not lead to increases in the ion−graphene interactions, and most of these ions are placed on top of the adsorbed layer on graphene and increase the anion−cation interactions. Therefore, once a critical number of ions are adsorbed on the graphene sheet, this adsorbed layer is not reinforced by adding additional ions because the large Coulombic interaction between these charged moieties tends to be placed further from the graphene surface, which leads to an increase of the contact angle with increasing droplet size. This is confirmed by the calculated contact angles reported in Figure 10 as a function of N in the nanodroplet, which increases in an almost linear manner with increasing number of ions in the droplet.

Figure 8. Ion−graphene interaction energies normalized per number of ion pairs, Einter(ion−graphene), for [EMIM][GLY] nanodroplet with 500 ion pairs on the graphene sheet as a function of simulation time, t. Values for t = 0 correspond to the droplet initially simulated in vacuum and placed at 4 Å of distance of the graphene sheet. Lines are reported for guiding purposes.

should lead to a more effective interaction with the substrate than in the case of [GLY]−, for which in spite of also developing a parallel arrangement regarding the graphene, a lower number of atoms are involved in the interaction in comparison with the [EMIM]+ cation. The time evolution of Einter(ion−graphene) for N = 500 nanodroplet shows a sudden change in the 0−0.2 ns range, in which Einter(ion-graphene) increases at 19.4 and 14.6 kJ mol−1 ns−1 rates for [EMIM]+ and [GLY]− ions, respectively, and then, once the first adsorbed layer of ions is formed, a further rearrangement of this layer leads to an increase of the ion−graphene interactions at 2.5 and 1.2 kJ mol−1 ns−1 rates for [EMIM]+ and [GLY]− ions, respectively, up to when the final equilibrated structure of the nanodroplet on the layer is reached. The properties of the adsorbed nanodroplets are strongly dependent on the size of the considered droplets, Figure 2, and thus Einter(ion-graphene) should also change with N, Figure 9. Nevertheless, the behavior of Einter(ion-graphene) with regard to N is clearly nonlinear, increasing 24% going from nanodroplets with N = 100 to 200 and then decreasing to 0.06 and 0.04 kJ mol−1 per ion pair for [EMIM]+ and [GLY]−

Figure 10. Contact angle, θ, for [EMIM][GLY] nanodroplets on graphene sheet as a function of number of ion pairs in the nanodroplet, N. Line is reported for guiding purposes.

Considering the relevance of graphene coatings for technological applications, molecular dynamics simulations for nanodroplets on graphene supported on silica, and directly on silica, were also carried out. The topological properties of [EMIM][GLY] nanodroplets with N = 200 ion pairs are reported in Figure 11 for graphene, silica, and graphene-coated silica. As explained in the Methods section, two different silica surfaces were considered: (i) hydrophilic substrate, hydroxyl terminated (silanol groups), SiO2−OH, and (ii) a hydrophobic substrate, hydrogen-terminated (silane groups), SiO2−H. Both types of silica surfaces were coated with graphene, and the behavior of nanodroplets were analyzed on them. The behavior of [EMIM][GLY] nanodroplets on silica surfaces is completely different to that on graphene, for the small nanodroplet reported in Figure 11, good wetting is observed on graphene (θ = 45°, Figure 11a), whereas worse wetting is obtained both for SiO2−OH (θ = 81°, Figure 11b) and SiO2−H (θ = 60°, Figure 11c), showing poor affinity of [EMIM][GLY] for silica surfaces both for hydrophilic and hydrophobic faces. Castejon et al.37 reported contact angles for pyrrolidinium, ammonium, and phosphonium-based ionic liquids on SiO2−OH and SiO2−H surfaces from molecular dynamics showing values in the 37.63 to 56.0° range, with larger angles for the silane surface. This is opposite to the behavior obtained in this work for [EMIM][GLY]. This worse wetting of silica surfaces than for graphene ones, but with θ < 90° showing acceptable wettability in silica

Figure 9. Ion−graphene interaction energies normalized per number of ion pairs, Einter, for [EMIM][GLY] nanodroplets on graphene sheet as a function of number of ions pairs in the nanodroplet, N. Lines are reported for guiding purposes. F


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Figure 12. Ion−substrate interaction energy, Eions−substrate, in relationship with contact angle, θ, for [EMIM][GLY] nanodroplets with 200 ion pairs on graphene sheet, SiO2 terminated with hydroxyl groups, SiO2−OH, SiO2 terminated with hydrogen atoms, SiO2−H, graphene sheet supported on SiO2−OH, and graphene sheet supported on SiO2−H.

Figure 11. Snapshots of [EMIM][GLY] nanodroplets with 200 ion pairs on (a) graphene sheet, (b) SiO2 terminated with hydroxyl groups, SiO2−OH, (c) SiO2 terminated with hydrogen atoms, SiO2− H, (d) graphene sheet supported on SiO2−OH, and (e) graphene sheet supported on SiO2−H. Color code as in Figure 2. Substrates, graphene and SiO2, are not drawn to improve visibility.

only on the graphene but also in the underlying silica substrate, with which it also interacts.

4. CONCLUSIONS The mechanism of graphene wetting by ionic liquid nanodroplets both for pure graphene sheets and for silica coated with graphene was studied in this work using molecular dynamics simulations. The reported results showed a strongly adsorbed layer on the graphene sheets, which is highly structured and in which both ions adopt particular arrangements to maximize the interaction with the graphene aromatic groups. The presence of graphene only leads to a first welldefined layer in the vicinity of the surface, whereas perturbations are weak for larger distances, and thus, for larger nanodroplets the structure in the vicinity of the graphene surface does not change and most of the ions are placed on top of the first adsorbed layer. Contact angles are lower than 90° for the studied nanodroplets range, showing good wettability but increases remarkably from the smallest nanodroplets to the largest ones, in agreement with calculated interaction energies between the ions and the substrate. The effect of coatings on wettability is analyzed considering silica coated with graphene both for hydrophilic and hydrophobic faces, showing that coating leads to remarkable changes in the mechanism of surface wetting, thus exhibiting a certain degree of graphene transparency for these ions. The results reported in this work show that graphene can display some degree of transparency when interacting with ionic species, which can be modulated by the interaction between the ionic species and the material supporting the graphene layer. This behavior can have implications in 2D electronic devices with graphene coating.

substrates, rises from the presence of cyclic imidazolium ring, which leads to a very efficient interaction with graphene aromatic rings and which is lost on going to graphene substrate. Moreover, the better wettability for hydrophobic silica substrates than for hydrophilic ones should rise from the different structure of the first adsorbed layer; as may inferred from the comparison of Figure 11 (panels b and c), the region in the vicinity of silica surface is richer in [EMIM]+ cations than in [GLY]− anions for SiO2−OH, Figure 11b, whereas the opposite effect is obtained for SiO2−H surface. Thus, [GLY]− anions interact efficiently with hydrogen surface atoms on the silane surface. Upon graphene coating of both silica surfaces, the contact angle decreases remarkably for graphene at SiO2− OH (θ = 57°, Figure 11d), whereas for graphene at SiO2−H, it increases to θ = 74° (Figure 11e). Therefore, the contact angle of these nanodroplets on silica coated with graphene is sensitive to the substrate underlying the graphene sheet in direct contact with the nanodroplet, and thus, although graphene may not be considered as a totally transparent sheet, because the contact angles values for coated and clean silica are not the same, it allows a certain degree of interaction between the nanodroplet and the silica substrates that at the same time changes the mechanism of interaction between the nanodroplet and the graphene sheet. The strength of ionic liquid−substrate interactions was quantified through the total ion−substrate interaction energies, Figure 12, which show that this energy decreases on going to any of the silica surfaces considered and when graphene acting as coating is considered. Likewise, an almost linear relationship between the total ion−substrate interaction energy and the contact angle is obtained, the larger the contact angle the lower the energy, showing that this fact controls the wettability of the considered surfaces. Moreover, results in Figure 12 show the relevant role of the whole substrate when graphene coatings are considered, the structure and properties of the nanodroplet adsorbed layer depends not

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08713. Table S1 (force field parametrization) and Figure S1 (time evolution of calculated contact angles) (PDF) G


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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 This work was funded by Ministerio de Economiá y Competitividad (Spain, project CTQ2013-40476-R). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) for providing supercomputing facilities. Gregorio Garciá acknowledges the funding by Junta de Castilla y León, cofunded by European Social Fund, for a postdoctoral contract. The statements made herein are solely the responsibility of the authors.

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Nanowetting of Graphene by Ionic Liquid Droplets - ACS Publications

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