Elucidating the Properties of Graphene–Deep Eutectic Solvents

Article pubs.acs.org/Langmuir

Elucidating the Properties of Graphene−Deep Eutectic Solvents Interface Mert Atilhan,† Luciano T. Costa,‡ and Santiago Aparicio*,§ †

Department of Chemical Engineering, Texas A&M University at Qatar, Doha, Qatar Instituto de Química - Departamento de Físico-Química, Universidade Federal Fluminense, 24020-141 Niterói, Brazil § Department of Chemistry, University of Burgos, 09001 Burgos, Spain ‡

S Supporting Information *

ABSTRACT: The properties of five deep eutectic solvents prepared based on the selection of choline chloride ionic liquid as hydrogen bond acceptor, which are mixed with several hydrogen bond donors with selected molecular features, were studied theoretically at graphene interfaces via both density functional theory and classical molecular dynamics methods. Molecular structuring at the interfaces, angular orientation, densification, and dynamic properties were analyzed upon adsorption on the graphene surface and when the deep eutectic solvents were confined between two graphene sheets and analyzed in terms of the role of the type of hydrogen bond donor for each solvent. Likewise, the behavior of deep eutectic solvent nanodroplets on graphene was simulated leading to the calculation of contact angles and nanowetting with further studies considering the effect of an external electric field on nanodroplet properties.

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graphite exfoliation,24 graphene functionalization,25 or for the development of graphene-based nanofluids,26 experimentally showing the suitability of DESs for developing applications combined with graphene. Hayyan et al.27 showed the suitable functionalization of graphene by DESs causing useful surface modifications and the use of these DESs-modified graphene in application such as water water treatment, drug delivery or biosensors. Wang et al.28 modified graphene and graphene oxide by DESs for their use supported on silica as solid-phase extractants. Chakrabarti et al.29 showed the suitability of using DESs for production, in gram scale, of high quality graphene. Therefore, the application of DESs in cutting edge technologies related with graphene has been confirmed in the available literature. Nevertheless, the molecular roots of the behavior of DESs at graphene surfaces are not fully understood because of the scarcity of experimental and computational studies. Chen et al.30 analyzed the structure of choline chloride based DES at graphite interface by using atomic force microscopy (AFM), contact angle measurements, and density functional theory (DFT) calculations as a function of the applied potential. Their results showed that the DESs double layer is characterized by the exclusion of the molecular component of the DESs from the interface, with the ions attracted into the Stern layer and the molecular component enriching the layer close to the Stern one. Shen et al.31 carried out molecular dynamics (MD) simulations for analyzing the structure and dynamics of choline

INTRODUCTION Deep eutectic solvents1 (DESs) are a class of fluids formed by the mixing in suitable molar ratios of two high melting point compounds leading to a eutectic mixture with reduced melting point that is close to ambient temperature. The formation of these eutectic mixtures is usually justified by the development of hydrogen bonds between an hydrogen bond acceptor (HBA), usually but not limited a salt, and an hydrogen bond donor (HBD).2 Although research on DESs is still at initial stages, these fluids have attracted great attention in the past few years due to their potential in application for several technologies3−5 such as CO2 capture,6,7 fuel desulfurization,8,9 solvents and catalysts in organic reactions,10,11 biodiesel production,12,13 polymer synthesis,14,15 and in general for green technologies.16 The main advantages of DESs rise from the possibility of developing by selecting HBA:HBD combinations of totally natural origin,5,16 with low toxicity,17,18 high biodegradability,19,20 low cost,16 and fully renewable, which are remarkable advantages regarding other types of solvents such as ionic liquids or traditional organic solvents. Therefore, in spite of some results showing ecotoxicity problems for some types of DESs,21 the large number of HBA: HBD combinations leading to DESs provides a suitable platform for materials development with properties tailored for the required technological applications within a green chemistry framework. One of the most relevant proposed applications of DESs stand on their use in nanotechnology,22 in particular for the development of functional materials.23 Regarding nanomaterials based on carbon, several studies have considered the interaction of DESs with graphite or graphene with the purposes of © 2017 American Chemical Society

Received: March 7, 2017 Revised: May 8, 2017 Published: May 9, 2017 5154

DOI: 10.1021/acs.langmuir.7b00767 Langmuir 2017, 33, 5154−5165

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under EEFs was done to analyze the effect of an electric field on the spreading of DESs on graphene substrate, i.e., to control the liquid−solid contact, which is useful for applications in nanoelectromechanical applications such as film boiling or electrowetting.42,43 The objective of the work is to characterize at the microscopic level the graphene−DESs interfaces as a function of the type of HBD, comparing their properties with bulk liquid phases, and its relationship with the nanowetting behavior of the selected fluids regarding to graphene.

iodide−glycerol DES confined inside graphitic nanopores, showing that all species are present in the surface region close to carbon walls, with the structuring in the adsorbed layer like in bulk liquid DESs. Therefore, the structuring and dynamics of DESs at graphene interfaces deserves additional attention to develop a fully understanding as a function of the types of HBA:HBD considered for DES development. For this purpose, computational chemistry methods are a suitable choice for obtaining the required insights of DESs at graphene interfaces. Theoretical studies using both DFT and MD approaches have been successfully used for the characterization of fluids similar to that of DESs and closely related to DESs, such as ionic liquids (ILs) on graphene surfaces.32−38 Kamath et al.39 showed the ability of bis(trifluoromethylsulfonyl)imide anion−based ILs for graphene bilayer exfoliation using molecular dynamics simulations. Wagle et al.40 used quantum chemical calculation methods to study the interaction between several types of ILs and polyclicic aromatic hydrocarbons as a model of graphene sheets, showing fairly weak interactions between ILs and ordinary model systems with entropy driven ILs desorption processes. Pensado et al.34 studied the interaction of ILs with graphene surface both using quantum chemistry and molecular dynamics methods showing strong adsorption of ILs in the graphene surface. Therefore, a study on the properties of five DESs based on choline chloride (ChCl) as HBA and five different HBDs, urea (1:2 salt:urea molar ratio; ChCl_URE), glycerol (1:2; ChCl_GLY), malonic acid (1:1; ChCl_MAL), levulinic acid (1:2; ChCl_LEV), and phenylacetic acid (1:2; ChCl_PAA), Figure 1, at graphene interfaces is

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METHODS

Molecular dynamics simulations were carried out with MDynaMix v.5.2 molecular modeling package.44 Force field parametrizations for the studied DES were reported and validated against experiments in previous works,45−47 and Lennard−Jones parameters for graphene carbon atoms were also previously reported.33 Initial boxes containing different number of ChCl and HBD depending on the targeted DESs (with approximately the same number of total atoms, see Table S1 in the Supporting Information for more details) were built with the Packmol program,48 and were subjected to several heating and quenching cycles and finally equilibrated (10 ns) at 403 K and 1 bar in the NPT ensemble. These equilibrated DESs boxes were subjected to 10 ns NPT runs at 403 K and 1 bar. A graphene sheet, in the armchair configuration, containing 1500 carbon atoms with dimensions 62 × 62 Å2 was built and placed in the xy plane. On top of this graphene sheet, the previously equilibrated boxes were placed at 3 Å of the sheet, and thus with the interface being placed in the xy plane. Periodic boundary conditions (PBC) were applied in each direction, with the dimension in the z-direction leading to a DES−vacuum interface above the DES liquid layers, Figure 2. These systems were simulated for 20 ns in NVT

Figure 2. ChCl_MAL (1:1) on top of graphene sheet (cyan) used for MD simulations in NVT ensemble at 403 K. Red line shows PBC. Color code: (blue) Ch+ cation, (ref) Cl- anion, and (green) malonic acid. ensemble at 403 K. The selected temperature was high enough to minimize the effects rising from the sluggish character of the studied DESs because of their large viscosity.6 For the study of DESs confined between two parallel graphene sheets, the initial systems used for the simulation of graphene + DESs were considered and an additional graphene layer was placed on top of the system separated by the distances indicated in Table S2 (Supporting Information). NVT simulations at 403 K for 20 ns were carried out with PBC conditions as reported in Figure S1 (Supporting Information). The study of DESs nanodroplets was carried out using droplets with 30 Å radius containing the number of molecules reported in Table S3 (Supporting Information), which were placed on top of a graphene sheet with 140 × 140 Å2 dimensions. Spherical droplets were initially built using Packmol,48 then equilibrated in vacuum using NVT ensemble at 403 K for 20 ns, and then placed on top of the graphene sheet at 3 Å of distance (Figure S2, Supporting Information). Droplets were simulated in the NVT ensemble at 403 K for 20 ns with PBC in the three directions and with x-, y-, and z-dimensions being large enough to avoid interaction with neighbor layers. The behavior of

Figure 1. Ions (HBA) and molecules (HBDs) involved in the DESs considered in this work. For each DES, the molar ratio considered for the DES is reported. Dashed arrows show vector used for defined molecular orientations in following figures.

reported in this work. The behavior of these fluids upon adsorption on single graphene layers and when confined between two parallel graphene layers is studied using MD. Likewise, the interaction of ChCl and of the studied HBDs is also analyzed using DFT. Moreover, the nanowetting of graphene by DESs nanodroplets is also studied using MD and additional simulations are also carried out to infer the behavior of these droplets under external electric fields (EEFs).41 The study of the behavior of DESs nanodroplets 5155

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Figure 3. Structures of Ch+ and HBDs on top of graphene surface calculated at PBE-D2/DZP theoretical level. Visualizations from top and side views are reported in the top and bottom rows.

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DESs nanodroplets on top of graphene sheets were also studied under the effect of a static external electric field (EEF) with intensity E = 0.1 V Å−1. The EEF was applied along the plane of graphene sheet using the DESs nanodroplets previously equilibrated on top of the 140 × 140 Å2 graphene sheet as initial input and then NVT simulations at 403 K were carried out for 20 ns. All the MD simulations carried out in this work were done with the corresponding graphene sheets maintained in fixed positions. The Nose−Hoover method was applied for maintaining temperature in NVT ensembles. Ewald method, with 15 Å cutoff radius, was used for handling Coulombic interactions. The Tuckerman−Berne double time step algorithm49 (1 and 0.1 fs for long and short time steps) was considered for solving the equations of motion. Cross Lennard-Jones terms were calculated according to the Lorentz− Berthelot mixing rules. DFT calculations were carried out using SIESTA 3.2 package.50 Troullier−Martins pseudopotentials51 and numerical double-z polarized (DZP) basis sets were applied together with PBE functional combined with dispersion correction according to Grimme’s method,52 this method is called PBE-D2/DZP along this work. The energy mesh cutoff of 400 Ry and a 8 × 8 × 8 k-point mesh in the Monkhorst−Pack scheme were considered.53 A graphene nanosheet containing 180 atoms with 20 × 20 Å2 dimensions to which PBC conditions were applied in the three space directions was used for all the DFT simulations. PBC in the direction perpendicular to the graphene surface was large enough to avoid interaction with image layers. Single Ch cation, Cl anion, or HBD molecules and HBA:HBD clusters (containing 1 cation:1 anion:2 HBD molecules, for those DES with 1:2 molar ratios or 1 cation/1 anion/1 HBD molecules, for those DESs with 1:1 molar ratio) were placed on top of the graphene sheet and structural relaxations were done using conjugate gradients. The convergence criterion was defined according to forces acting on all atoms being lower than 0.03 eV Å−1. Several initial configurations were checked for each system and the interaction energies, Eint, were calculated as E int = E DES−GRA − (E DES + EGRA )

RESULTS AND DISCUSSION DFT Approach to DES Adsorption on Graphene. The characteristics of the interaction between DESs and graphene were first analyzed using DFT. In a first step, the interaction of the components of the studied DESs were analyzed, and thus, the isolated Ch+, Cl−, and HBDs molecules adsorbed onto graphene were studied, Figure 3. The interaction of Ch+ is characterized by nitrogen atom being placed 3.70 Å above the graphene plane with the hydrogen atom in the hydroxyl group at 2.08 Å, another four hydrogen atoms belonging to methyl and ethyl groups are also placed 2.08 Å above the graphene surface, therefore the interaction between the Ch+ and graphene is dominated by the interaction with alkyl and hydroxyl hydrogen atoms. The interaction energy, Eint, between Ch+ and graphene was calculated as a function of distance to the surface, Figure 4. The curves reported in Figure 4 were built

(1)

Figure 4. Interaction energies, Eint, as a function of distance to the graphene surface, dz (as defined in Figure 3) of Ch+ and HBDs on top of graphene surface calculated at PBE-D2/DZP theoretical level. These curves are obtained from shifting upward or downward the optimized structures reported in Figure 3.

where EDES−GRA stands for the counterpoise corrected energy for the system formed by the DES adsorbed on graphene, EDES for the energy of the isolated DES (not adsorbed on graphene), and EGRA for the energy of clean graphene surface, all of them at the same theoretical level as explained in the previous paragraph. The interaction of the simulated DESs with graphene sheets should lead to charge transfer between both systems, and thus atomic charges were calculated for optimized structures according to the Hirshfeld method.54 Charge variation for i species (i = graphene, Ch+, Cl− or HBDs), Δqi, were calculated as Δqi = qi ,after − qi ,before

starting from the optimized structures reported in Figure 3 (Table 1 for data) and then the distance to the graphene surface was changed but maintained the remaining parameters of the cluster constant, i.e. not optimized scan for a fixed orientation regarding to graphene surface. Results in Figure 4 show a large minimum (−98.5 kJ mol−1) in agreement with strong adsorption, and showing that the interaction between Ch+ and graphene is stronger than any of the HBDs−graphene systems studied in this work, and thus, the interaction of DES

(2)

where qi,after and qi,before stand for the total charge of i after adsorption (graphene + DES cluster) and before adsorption (isolated graphene or isolated DES). 5156

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Table 1. Interaction Energies, Eint, Counterpoise Corrected, Charge Variation, Δqi, for the Studied DES on Graphene Sheet Calculated at PBE-D2/DZP Theoretical Levela Δqi/e DES

Eint/kJ mol−1

graphene

Ch+

Cl−

HBD1

HBD2

ChCl_URE (1:2) ChCl_GLY (1:2) ChCl_MAL (1:1) ChCl_LEV (1:2) ChCl_PAA (1:1)

−65.42 −73.50 −40.04 −75.02 −70.12

0.217 0.249 0.163 0.280 0.210

−0.144 −0.134 −0.134 −0.141 −0.160

−0.012 −0.019 0.013 −0.024 0.007

−0.032 −0.073 −0.042 −0.082 −0.046

−0.029 −0.023 −0.033 −0.011

Model composed by 1 Ch+/1 Cl−/(1 or 2) HBD molecules on top of a graphene layer composed by 180 carbon atoms was used for these calculations. a

Figure 5. Interaction energies, Eint, for Ch+ on top of graphene surface as a function of shifting in x, dx, and y, dy, directions as defined in panel (a) with regard to optimized structure of Ch+ on top of graphene (corresponding to the minimum reported in Figure 4). All values calculated at PBED2/DZP theoretical level. dz was constant for all the reported results (minimum in Figure 4).

with graphene should be dominated by the strong trend of Ch+ to be adsorbed and with a minor role by the HBDs interaction with the surface. Likewise, the Ch+−graphene interaction is characterized by a large charge transfer from the graphene to the cation, Δq = 0.305 e according to Hirshfeld method for the minimum in the curve reported in Figure 4, leading to a cation with +0.695e charge. This charge transfer develops a pivotal role in Ch+ adsorption on graphene as its variation with distance shows (Figure S3, Supporting Information). The dispersion interactions between graphene and Ch+ should also develop a relevant role; these interactions were treated in this work using the Grimme’s52 method and led to a 46.9% of the total Eint. The interaction of Ch+ with graphene was also analyzed as a function of the cation position on the surface for the distance corresponding to the minimum in Figure 4, Figure 5a. The movement of Ch+ along the x-coordinate on top of graphene (dx, Figure 5b) leads to 11.5% decrease (in absolute value) of Eint and moving along y-direction (dy, Figure 5b) leads to an increase (in absolute value) Eint and after to a decrease. Nevertheless, results in Figure 5 show strong adsorption of Ch+ on graphene for all the positions on the surface. The properties of isolated HBDs on top of graphene were also calculated. The optimized structures of HBDs molecules adsorbed on graphene are reported in Figure 3 and the variation of Eint with distance to the surface are showed in Figure 4. The adsorption of the five studied HBDs is characterized by molecules laying almost parallel to the graphene surface, in this way reinforcing van der Waals interactions with the surface. In the case of URE, oxygen atom is at 2.72 Å of the surface whereas carbon (2.88 Å) and nitrogen (3.03 Å) are slightly further with hydrogen atoms at 2.75 Å of the surface. The nonplanarity of −NH2 group in URE on top of graphene may be justified considering the trend of

these hydrogen atoms to interact with the carbon atoms at the surface. The variation of Eint for URE as a function of the distance to the surface shows a minimum of −30.5 kJ mol−1, which is in the range of previously studied organic molecules.55 The URE−graphene interaction is also characterized by a very minor charge transfer from graphene to URE (Δq = 0.019e). Regarding the remaining HBDs, Figure 3, the presence of alkyl backbones in GLY, MAL and LEV, which tend to be almost parallel to the graphene surface determines their mechanism of interaction. The comparison of adsorbed structures shows that GLY, MAL, and LEV are slightly further of the surface than URE, which can be justified by the presence of sp3 carbon atoms bonded to hydrogen atoms in the alkyl backbones tending to interact with the graphene, in contrast with the sp2 carbon atom in URE leading to the almost planar structure reported in Figure 3. Likewise, the hydrogen atoms in hydroxyl groups are skewed to allow their interaction with the surface, especially for GLY. In the case of PAA, the presence of the phenyl ring developing π−π interactions with the graphene surface leads the COOH pointing outward of the surface. The ordering of Eint at the minima reported in Figure 4 is MAL > LEV > PAA > GLY > URE (−57.1, −51.9, −48.4, −40.1, and −30.5 kJ mol−1, respectively). The lower values for URE are justified considering the low number of atoms in this molecule when compared with the other HBDs, which leads to a lower number of van der Waals interactions. The larger Eint values for MAL stands on the almost planar structure on the graphene surface and the presence of two COOH groups increasing van der Waals interactions. Likewise, GLY, MAL, LEV and PAA are characterized by a minor charge transfer from the graphene surface to each HBD in the following order of 0.089 (GLY) > 0.082 (LEV) > 0.069 (PAA) >0.017 (MAL), these values are remarkably lower than those reported in the previous paragraph 5157

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Langmuir for Ch+. Likewise, these charge transfer effects are very minor and have an almost negligible role on Eint considering that MAL leading to the larger Eint shows the lower charge transfer. The dispersion contribution to Eint for the studied HBDs calculated according to Grimme’s method50 are 43.2, 45.2, 49.6, 45.0, and 32% for URE, GLY, MAL, LEV, and PAA, respectively. Therefore, roughly half of the total Eint rise from graphene− HBD interactions. The lower dispersive contribution for PAA in comparison with the other HBDs stands on the adsorption mechanism showed in Figure 3, in which the phenyl ring is placed on top of the surface whereas the remaining atoms (including oxygen and hydroxyl group) are placed out of the surface thus leading to smaller dispersion interactions in comparison with those for the remaining HBDs for which interactions between the carbon atoms of the surface and oxygen, nitrogen or additional hydrogen atoms are developed. The interaction of single Ch+ and HBDs with graphene was analyzed in previous paragraphs to isolate the effects rising from each type of molecule. Nevertheless, the studied DESs are characterized by the presence of hydrogen bonding between the Ch+, Cl− and the corresponding HBDs. Therefore, the interaction of Ch+/Cl−/HBD complexes (according to the stoichiometry reported in Figure 1) was also studied using the same DFT approach as used for isolated cation and HBDs. The structure of these complexes on top of graphene was optimized and the main properties are reported in Table 1. The values of Eint for Ch+/Cl−/HBD complexes are remarkably lower than those obtained from the sum of those reported in previous sections for single Ch+ and HBDs, and even for Ch+/Cl−/MAL complex Eint is lower than the value reported for isolated MAL. These results show that the development of hydrogen bonds in the studied complexes prevail over the development of interactions with the surface, i.e., the structure of Ch+/Cl−/ HBD complexes is only slightly disrupted upon adsorption on graphene. This behavior was quantified by calculating the energy for the Ch+/Cl−/HBD complexes in gas phase and when adsorbed on the graphene surface, all the complexes were destabilized (i.e., lower energies in absolute values) but the difference between both states are not large: 21.2, 22.3, 29.1, 24.6, and 30.1 kJ mol−1 for complexes with URE, GLY, MAL, LEV, and PAA, respectively. The charge transfers from graphene to the molecules forming the Ch+/Cl−/HBD complexes are also reported in Table 1. This charge transfer is lower for Ch+/Cl−/HBD complexes than for single Ch+ and the charge is mainly transferred to Ch+. This is confirmed by the changes in atomic charges reported in Figure S4 (Supporting Information), from which it can be inferred that charge is mainly transferred from graphene carbon atoms placed below the Ch+. Therefore, from DFT viewpoint, the adsorption of the studied Ch+-based DESs is governed by the cation interaction with the surface and the trend to maintain as unperturbed as possible the hydrogen bonding in the Ch+/Cl−/ HBD complexes. The DFT studies in this section were carried out for single complexes on the graphene surface, additional effect rising by the presence of bulk liquid phases on the surface are analyzed in the following section using classical MD approach. MD Approach to DES Adsorption on Graphene. The structural changes of liquid DESs upon adsorption on graphene are studied by classical MD. Number density profiles for the center-of-mass of the involved molecules are reported in Figure 6 showing the development of a first strongly adsorbed layer followed by a less intense second layer. The position of the first

Figure 6. Number density profiles, ρ, for the center-of-mass of ions and molecules in the considered DESs as a function of the distance, z, to the graphene surface. Only data in the vicinity of the graphene surface (z < 15 Å) are reported to improve visibility. Vertical dashed lines are reported for comparison purposes showing boundaries considered for defining the first and second adsorbed layers. Values obtained from molecular dynamics simulations.

density peak for Ch (∼4.5 Å of the surface), Figure 6a, is slightly larger than the values obtained from DFT for the cation (∼4.1 Å), Figure 4, but they are in reasonable agreement. The intensity of this first Ch+ density peak decrease with increasing size of the corresponding involved HBD, the larger the HBD the lower available space on the surface for cations and thus less intense peaks, being this especially remarkable for DES with PAA considering the bulky character of this molecule and its trend to be placed in parallel to the graphene surface, Figure 3. The density peaks corresponding to the first adsorbed layer are also obtained for Cl− and HBDs, showing that the integrity of Ch+/Cl−/HBD complexes is maintained upon adsorption on graphene. The second adsorbed layer is well-defined for Ch+ and the results reported in Figure 6 confirms that the structure of the liquid DES is disrupted by the presence of the graphene surface up to roughly 10 Å from the surface. Therefore, the liquid DES above graphene surface was divided in three regions: first and second layer and pseudobulk, and the number of molecules in each region was calculated for equal volumes (first and second adsorbed layers were defined with 5 Å width, and thus the number of molecules in the pseudo bulk region was calculated for a layer with the same dimensions), Table 2. The first adsorbed layer is characterized by a large number of molecules per volume unit in comparison with the pseudo bulk region for all the studied DESs. It should be clarified that although the first adsorbed layer is defined from z = 0 Å to z = 5 Å (where z = 0 Å stands for the position of the graphene surface), results in Figure 6 show that molecules are concentrated from z = 2.5 Å to z = 5 Å. Nevertheless, results 5158

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Table 2. Number of Ions and Molecules in the First Adsorbed Layer (z < 5 Å), Second Layer (z in the 5−10 Å Range) and Pseudobulk Region (z in the 20−25 Å Range), Where z Stands for the Distance to the Graphene Surface (Figure 5), for the Studied DES on Top of a Graphene Sheet with 62 × 62 Å2 Dimensionsa first layer DES ChCl_URE (1:2) ChCl_GLY (1:2) ChCl_MAL (1:1) ChCl_LEV (1:2) ChCl_PAA (1:1) a

Ch 44.5 31.9 46.5 25.7 10.5

−

+

± ± ± ± ±

Cl 1.9 1.8 1.8 2.2 2.6

27.0 23.9 37.4 17.2 10.4

± ± ± ± ±

second layer HBD

1.9 2.1 1.7 2.3 2.7

101.0 70.0 48.8 53.5 26.8

± ± ± ± ±

Ch 2.1 2.0 1.2 2.0 3.7

39.9 37.2 43.9 36.1 14.4

−

+

± ± ± ± ±

Cl 2.8 3.1 2.3 2.8 3.0

67.9 45.6 55.6 43.6 11.6

± ± ± ± ±

pseudobulk HBD

3.1 3.3 2.1 3.2 2.6

111.2 64.0 46.9 54.6 42.9

± ± ± ± ±

Ch 2.9 2.7 2.3 3.3 3.2

57.2 39.8 58.7 33.3 29.7

Cl−

+

± ± ± ± ±

4.3 3.0 2.2 2.7 2.7

58.3 41.0 55.8 34.1 30.5

± ± ± ± ±

HBD 3.3 3.0 2.5 3.2 2.2

118.0 80.8 56.4 67.3 61.8

± ± ± ± ±

4.0 3.0 2.8 3.2 2.7

Values obtained from molecular dynamics simulations.

charged region close to the upper limit of this layer, rising from the adsorbed Ch+, followed by a peak of negative charge and another one of positive charge close to the surface. The origin of the two charge peaks close to the graphene stands mainly on charges in HBDs (Figure S5, Supporting Information), in agreement with HBDs being placed slightly closer to the surface than Ch+ as reported in Figures 4 and 6. The adsorption of ions and molecules forming DESs leads to a densification for the first adsorbed layer which should lead to changes in molecular orientation for molecules on the surface, for giving stronger interactions with graphene, when compared with random orientation in the bulk liquid phases. Molecular orientation upon adsorption is quantified by the angle formed by the molecular vectors defined in Figure 1 and a vector perpendicular to the graphene surface (φ; i.e., φ = 0° meaning molecular vector perpendicular to the surface and φ = 90° showing a molecular vector parallel to the surface). The vectors reported in Figure 1 were selected to show proper orientation of molecules composing DESs regarding to graphene surfaces. Probability distribution functions for these angles are reported in Figure 8. The orientation of Ch+ was studied defining a molecular vector joining the nitrogen and oxygen atoms in the cation, results reported in Figure 8 show Ch+ orientations almost independent of the considered HBD, two main probability peaks are obtained at roughly 75° (stronger peak) and 135° (weaker peak), which show Ch+ in parallel to the graphene surface but slightly skewed with hydroxyl atoms slightly closer to the surface (75° peaks) and in minor extension Ch+ skewed with hydroxyl group further of the surface (135° peaks). The ratio of probability peaks 75°−135° is in the 4−5 range except for ChCl_PAA for which both peaks are almost equivalent. The orientation of Ch+ as a function of distance to the graphene surface is reported in Figure S6 (Supporting Information) confirming that the preferred parallel orientation regarding the surface almost vanishes in the second layer (5−10 Å distance to the surface) and disappear for distances to the surface larger than 10 Å for all the studied DESs. Regarding the HBDs orientation on the surface, results in Figure 8 confirm the trend to lay parallel to the surface for the five studied DES (φ ∼ 90°), an additional probability peak around 130° is poorly defined for most of the studied HBDs, showing that these HBDs tend to align with the surface in agreement with DFT results reported in Figure 3. Likewise, these preferential HBDs orientation are obtained only for the first adsorbed layer whereas it vanishes for the second layer and the bulk region, Figure S6 (Supporting Information). Only PAA shows a certain preferential parallel orientation with the surface in the second layer. The strength of DESs-graphene interactions was quantified through Eint reported in Figure 9. The comparison of these

for ChCl_PAA in Table 2 show lower number of molecules in the first adsorbed layer because of the planar adsorption of PAA on the graphene surface, improving interaction with the surface, which causes a remarkable change in fluid’s structuring upon adsorption hindering an efficient molecular packing above the graphene surface. This first layer is also characterized by a slight deficit of Cl− atoms in comparison with Cl−, and thus the Ch+/ Cl− ratios in the first layer are larger than 1.0 (except for ChCl_PAA) with values in the 1.2 (ChCl_MAL) to 1.6 (ChCl_URE) range. This anionic deficit in the first adsorbed layer is balanced by a superavit in the second layer (with Ch+/ Cl− ratios in the 0.6, ChCl_URE, to 0.8, all the remaining DES except ChCl_PAA, range). The number of molecules in the second layer shows that this is a transition region between first layer, ions and molecules strongly adsorbed on the surface, and the pseudo bulk liquid region. Chen et al.30 showed experimentally DESs multilayer nanostructuring on graphene, with the extension of layering increasing with the hydrogen bonding ability of DESs molecular components, which is confirmed and quantified from the molecular viewpoint in this work. The adsorption of ions and molecules forming DES on graphene leads to remarkable change in charge distribution near the surface when compared with bulk liquid DESs. Results in Figure 7 shows the charge density profiled as a function of the distance to the graphene surface showing how the structuring in the first adsorbed layer leads to a positively

Figure 7. Total charge density, ρe, in the considered DESs as a function of the distance, z, to the graphene surface. Only data in the vicinity of the graphene surface (z < 15 Å) are reported to improve visibility. Vertical dashed lines are reported for comparison purposes showing boundaries considered for defining the first and second adsorbed layers. Values obtained from molecular dynamics simulations. 5159

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Figure 9. Interaction energy, Eint, between molecules and graphene for DESs on top of a graphene surface. Values obtained from molecular dynamics simulations.

than HBDs−graphene interactions. Nevertheless, it should be considered that the number of HBDs molecules in the first adsorbed layer is twice or even larger than for Ch+ in the same layer (Table 2), which would justify the Eint values in Figure 9. Eint results for Ch+−graphene indicate that DESs with smaller HBDs allow most efficient Ch+−graphene interactions, i.e., larger Eint whereas larger or bulkier HBDs (e.g., LEV and PAA) hinder Ch+−graphene interactions because of steric hindrance. Regarding the HBDs−graphene interactions, Eint reported in Figure 9 show very large values for PAA, which can be justified by the trend of PAA to lay parallel to the graphene surface and the large number of PAA molecules in the first adsorbed layer (Table 2). Regarding the other studied DES. The ordering of Eint is URE ∼ LEV > GLY > MAL, which can be justified considering the large number of URE molecules in the first adsorbed layer, the large number of atoms in LEV (leading to more van der Waals interactions with the surface), and the lower number of GLY and MAL molecules in the first adsorbed layer in comparison with small URE molecules. The main characteristic of DESs in bulk liquid phases is the development of strong HBA-HBD hydrogen bonding, but these hydrogen bonding should suffer changes upon adsorption on graphene considering the strong densification and molecular reordering, reported in Figures 6 and 8. This behavior was quantified by comparing the average number of hydrogen bonds in the first adsorbed layer and in bulk liquid phase, Figure 10. The results in Figure 10 show that the number of all types of hydrogen bonds are reduced upon adsorption on graphene. Therefore, two factors control the behavior of the studied DESs upon adsorption on graphene: (i) the trend to lay parallel to the surface to increase interactions and (ii) the trend to maintain the cation−anion−HBDs interaction by hydrogen bonding. The strong interactions with the surface produce molecular rearrangements upon adsorption, weakening the extension of HBA-HBD hydrogen bonding (but not vanishing these interactions). Therefore, the properties of the studied DES on top of graphene surface are remarkably different to those in bulk liquid phase from energetic and steric viewpoints, although the effect because of the presence of the graphene is almost limited to a region up to 10 Å of the surface. Likewise, these changes in the extension of hydrogen bonding regarding to layering structure show the molecular roots of the layering

Figure 8. Probability distribution plots for the angle, φ, formed between a vector perpendicular to the graphene surface and molecular vectors defined in Figure 1, for DESs on top of a graphene surface. Values reported for molecules in the first adsorbed layer as defined in Figure 6. Color code: (blue) Ch+ and (green) HBDs. Values obtained from molecular dynamics simulations.

results with those obtained from DFT for isolated DESs complexes in Table 1 shows that the presence of neighbor molecules adsorbed on the surface leads to additional complexity. The Eint for Ch+-graphene from MD is largely dependent on the type of considered HBD following the ordering ChCl_MAL > ChCl_URE > ChCl_GLY > ChCl_PAA > ChCl_LEV. Likewise, Eint for Ch+−graphene is lower than Eint for HBDs−graphene, in spite of DFT results reported in Figure 4, which showed stronger Ch+−graphene 5160

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Figure 10. Average number of hydrogen bonds, NH‑bonds, for DESs on top of graphene surface. Values are reported for molecules in the first adsorbed layer, as defined in Figure 6, and those in the bulk regions. The averages are calculated by dividing the total number of hydrogen bonds for each donor−acceptor pair by the total number of Ch+ (for Ch−Cl hydrogen bonds) or by the total number of HBDs (for the remaining hydrogen bonds) in each region (first or bulk layers). Donor−acceptor distance of 3.5 Å and 60° angle were used as cutoffs for hydrogen bond definition.

extension, Figure 6, regarding to the hydrogen bonding changes upon adsorption as reported by Chen et al.30 The confinement of fluids between graphene sheets has been scarcely studied in the literature, some studies for systems closely related to DESs such as ionic liquids have been reported,56,57 with some studies analyzing the effect of graphene−graphene distance on the properties of the confined IL.58 These previous results for ILs showed that adsorbed densified layers are formed on both graphenes confining the IL with a region with bulk properties separating them, which extension is dependent on the graphene−graphene distance and the type of involved ions. The behavior of the studied DESs confined between two graphene sheets can be analyzed from the number and charge density profiles reported in Figures 11 and 12. Number density profiles reported in Figure 11 show that structures for adsorbed layers are equivalent for both sheets, densification appears upon adsorption on both graphenes, and the extension of the first adsorbed layer is equal in both cases. The region of confined fluids between both sheet beyond the 10 Å region above both sheets does not show remarkable features with its properties equal to those in bulk, unconfined, DESs. This is also confirmed by the charge density profiles reported in Figure 12, showing a central confined region with null total charge. DES Nanodroplets on Graphene. The development of adsorbed layers on top of graphene for the studied DESs with their properties remarkably different to those in bulk liquid phases requires the study of additional behavior for DESs on graphene surface. Nanowetting by new solvents is a topic that have gained interest in the recent years due to its possible technological applications.38 For this purpose, previous literature studies have showed the behavior of ILs nanodroplets on graphene,37,38 and thus, the behavior of nanodroplets formed by the selected DESs on top of graphene is reported in this work. Moreover, additional studies have showed the large changes upon application of external electric fields in nanodroplets behavior on solid surfaces, with large changes in droplets shape and contact angles;59 this effect is also analyzed for DESs nanodroplets in this work. It should be remarked that the behavior of ChCl_PAA nanodroplets could not be studied,

Figure 11. Number density profiles, ρ, for the center-of-mass of ions and molecules in the considered DESs confined between two graphene sheets (with sheet−sheet distances as reported in Table S2, Supporting Information). z = 0 stands for the center of the simulation boxes with z coordinate being perpendicular to the graphene sheets. Data are vertically shifted in the figure to improve visibility. Values obtained from molecular dynamics simulations.

when these nanodroplets were placed on top of the graphene they spread over the surface and the droplets were destroyed; therefore, the nanodroplets study was limited to the other four types of DESs. The spreading of ChCl_PAA nanodroplets on graphene may be justified by the trend of PAA to develop parallel arrangements with the graphene layer to improve π−π interactions. 5161

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Table 3. Contact Angle, ϕ, and Long to Short Axis Ratio, re, for DESs Nanodroplets on Graphene in the Absence of EEF, E = 0 V Å−1, and Under EEF with E = 0.1 V Å−1a E = 0 V Å−1 DES ChCl_URE (1:2) ChCl_GLY (1:2) ChCl_MAL (1:1) ChCl_LEV (1:2) ChCl_PAA (1:1)

E = 0.1 V Å−1

ϕ/deg

re

± ± ± ±

1.07 1.10 1.09 1.08

47 28 76 24

3 1 4 2

ϕ/deg

re

± ± ± ±

1.12 1.15 1.15 1.40

38 24 62 45

3 1 3 4

a

Nanodroplets with characteristics reported in Table S3 (Supporting Information). Values obtained from molecular dynamics simulations. Figure 12. Total charge density, ρe, in the considered DESs confined between two graphene sheets (with sheet−sheet distances as reported in Table S2, Supporting Information). z = 0 stands for the center of the simulation boxes with z coordinate being perpendicular to the graphene sheets. Data are vertically shifted in the figure to improve visibility. Values obtained from molecular dynamics simulations.

After placing the spherical nanodroplets on top of the graphene (Figure S2, Supporting Information), they are adsorbed on the surface and develop different forms depending on the considered HBD, Figure 13. The shape of the adsorbed nanodroplets is highly symmetrical in absence of EEFs with the long to short axis ratio, re, being slightly larger than 1, Table 3. The contact angles, ϕ (Table 3), were calculated from the density profiles once the nanodroplets are equilibrated according to a procedure previously reported for ILs.37 The ϕ values reported in Table 3 follow the ordering ChCl_MAL > ChCl_URE > ChCl_GLY > ChCl_LEV, in agreement with the interaction energies calculated for the adsorption of bulk liquid phases, e.g., ChCl_MAL led to the largest interaction energies in Table 1 and to the larger ϕ (lower affinity for the surface). The adsorption of the nanodroplets on the surface for developing the geometries reported in Figure 13 requires around 1 ns (Figure 14) and adsorption is characterized by a stabilization for all the studied DESs in the 5−20 kJ mol−1. The intermolecular interaction energies for all the involved molecules upon nanodroplets adsorption are reported in Table 4. These results show that the main factors from the energetic viewpoint contributing to the adsorption of the nanodroplets are Ch+−graphene and HBDs−graphene interactions, and thus those systems with lower interaction energies (URE and MAL) led to the large contact angles, Table 3.

Figure 14. Variation of potential energy with time with regard to values at t = 0, Epot,t − Epot,t=0, for DESs nanodroplets on top of graphene. Values obtained from molecular dynamics simulations.

The behavior of DESs nanodroplets under EEFs was studied for a static field with E = 0.1 V Å−1. Previous studied on water nanodroplets under EEFs on silicon surface showed several main effects: (i) nanodroplets spreading on the surface upon application of the EEF, and thus leading to a decrease of contact angle, (ii) differences between leading and trailing angles in relation with the EEF but only for E < 0.1 V Å−1, and (iii) development of droplet deformation upon application of EEF.59 Results for the studied DESs under EEF reported in Table 3 show that DESs containing URE, GLY and MAL are spread on graphene when EEF is applied leading to a decrease of the contact angle. Nevertheless, the variation of contact angle for water nanodroplets on silicon surface under EEF (E < 0.1 V

Figure 13. Nanodroplets of considered DESs on top of graphene. Color code: (blue) Ch+, (red) Cl− and (green) HBDs. Values obtained from molecular dynamics simulations. 5162

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Table 4. Intermolecular Interaction Energies, Einter, for DESs Nanodroplets on Graphene in Absence of EEF, E = 0 V Å−1a Einter/kJ mol−1 DES

Ch −Ch

ChCl_URE (1:2) ChCl_GLY (1:2) ChCl_MAL (1:1) ChCl_LEV (1:2)

524.1 323.4 636.4 351.2

+

+

Ch −Cl +

−

−1432.1 −837.8 −1615.4 −906.1

Ch −HBD

Ch −GRA

Cl−−Cl−

Cl−−HBD

Cl−−GRA

HBD−HBD

HBD−GRA

−9.0 −99.3 −125.6 −122.0

−1749.0 −2891.6 −1602.1 −2263.9

577.9 264.8 599.7 256.6

−86.6 −19.8 −40.6 −31.6

−230.3 −436.7 −313.1 −112.6

−20.8 −23.7 −7.6 −10.2

−1610.7 −2934.6 −2094.0 −3027.0

+

+

a Nanodroplets with characteristics reported in Table S3 (Supporting Information). Energies are the sum of coulombic and Lennard−Jones contributions. Values obtained from molecular dynamics simulations.

Å−1) was from 85° to 38°, whereas the largest variation for the studied DESs was 14° for ChCl_MAL. The application of the EEF involves that molecular dipoles in the nanodroplets align with the EEF, which leads to a disruption of the intermolecular interactions. These molecular rearrangements favor the interactions with the graphene, leading to a spreading on its surface, but at the cost of weakening the remaining molecular interactions, Figure 15. The strengths of intermolecular

of the ion−ion and ion−HBD interactions are also reinforced with the EEF application.

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CONCLUSIONS The properties of deep eutectic solvents based in choline chloride as hydrogen bond acceptor and urea, glycerol, malonic acid, levulinic acid, and phenylacetic acid as hydrogen bond donors upon adsorption on graphene surface and the behavior of the corresponding nanodroplets were studied using density functional theory and classical molecular dynamics. Density functional theory studies show strong interactions between the cation and graphene and weaker interactions for the studied hydrogen bond donors. The involved ions and molecules develop a parallel arrangement with the graphene surface confirmed through both theoretical approaches. Likewise, the cation interaction with the surface is characterized by graphene to Ch+ charge transfer and in minor extension to the hydrogen bond donors. Molecular dynamics results show the development of a first adsorbed layer, 5 Å width, characterized by large densification and molecular reorientation to lay parallel to the surface, followed by a transition region (5 Å width) with surface effects vanishing for distance to the surface larger than 10 Å. The behavior of deep eutectic solvent nanodroplets on graphene was also studied in absence and presence of external electric fields. The calculated contact angle is largely dependent on the involved hydrogen bond acceptor, with values lower than 90° for all the studied systems showing affinity for the graphene surface. Likewise, the application of an external electric field leads to droplets spreading on the surface, except for levulinic acid systems, but this effect being lower than for previously studied fluids because of the stronger interactions between the involved ions and molecules.

Figure 15. Variation of intermolecular interaction energies with time with regard to values at t = 0, Epot,t − Epot,t=0, for ChCl_MAL nanodroplets on top of graphene under external electric field with E = 0.1 V Å−1. Values at t = 0 correspond to the behavior of nanodroplets equilibrated on top of graphene in absence of external electric field. Values obtained from molecular dynamics simulations. Energies are the sum of Coulombic and Lennard−Jones contributions.

interactions reported in Table 4 is very large, which makes it difficult for the reorganization to follow the EEF and thus the decrease in contact angle under EEF is modest in comparison with water nanodroplets.59 The application of EEF for URE, GLY, and MAL also leads to a minor droplet deformation as the increase in the values of the long to short axis ratios reported in Table 3 shows; droplets are deformed in the direction of the EEF but this deformation is lower than the one experienced by water nanodroplets.59 Moreover, the behavior of ChCl_LEV nanodroplets under EEF is opposite to the other studied DESs: droplets are largely deformed in the direction of the EEF and instead of spreading an increase in the contact angle is obtained, showing that for this DESs the dipolar alignment with the EEF largely disrupts the nanodroplet structuring decreasing the interactions with the graphene surface. The changes in intermolecular interaction energies in the nanodroplets upon EEF application are reported in Figure 15 for ChCl_MAL. These results show that Ch+−graphene and MAL−graphene interactions are reinforced upon EEF application, which justify the nanodroplet spreading and decrease of contact angle reported in Table 3. Likewise, most

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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.langmuir.7b00767. Systems used for studies on the adsorption on single graphene sheets; plot of ChCl_MAL on top of a single graphene sheet; intersheet distance between graphene sheets used for the study of DESs confinement; plot of ChCl_MAL confined between two graphene sheets; number of ions and molecules forming DES nanodroplets used in the simulations; plot of the initial structure of a ChCl_MAL nanodroplet on top of a graphene sheet; charge transfer upon adsorption of Ch+ on graphene; charge variation upon adsorption on graphene for ChCl_URE; contributions to the total charge for ChCl_URE on top of graphene; probability 5163

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distribution plots of molecular orientations of molecules on graphene surface (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mert Atilhan: 0000-0001-8270-7904 Santiago Aparicio: 0000-0001-9996-2426 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible by Ministerio de Economiá y Competitividad (Spain, project CTQ2013-40476-R) and Junta de Castilla y León (Spain, Project BU324U14). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

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