Interaction of Carbon Nanotube with Ethylene GlycolâWater Binary

Article pubs.acs.org/JPCC

Interaction of Carbon Nanotube with Ethylene Glycol−Water Binary Mixture: A Molecular Dynamics and Density Functional Theory Investigation K. Balamurugan,† Prathab Baskar,‡ R. Mahesh Kumar,† Sumitesh Das,‡ and Venkatesan Subramanian*,† †

Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, India Materials Modeling and Product Design, Research Development & Technology, Tata Steel Limited, Jamshedpur 831 001, India

‡

ABSTRACT: Classical molecular dynamics (MD) simulation has been carried out on model systems composed of ethylene glycol (EG) and carbon nanotube (CNT) in water (WAT) medium to gain insight into the interaction between them. The analysis of the MD results reveals that the EG molecules aggregate around CNT expelling water molecules due to the hydrophobic−hydrophobic interaction. Hydrogen-bonding (H-bonding) interaction between two EG molecules increases in the presence of CNT. Further, the presence of CNT decreases the solubility of EG in water. The analysis of the dihedral angle of EG reveals that the CNT induces conformational changes in EG. Specifically, a small fraction of the gauche form of EG is converted into trans. In addition, electronic structure calculations have also been carried out on model systems to quantitatively determine the binding energy (BE). The M05-2X/6-31+G** level calculations on the model systems show that the BE of CNT−WAT and CNT−EG ranges from 11.76 to 17.78 kJ/mol. It is interesting to note from the electronic structure calculations that the BE of trans EG with CNT is more than that of gauche EG with CNT in accordance with the findings from the MD simulation.

1. INTRODUCTION Carbon nanotube (CNT) is one of the exciting materials of the decade. Several applications have been envisaged for CNT in various fields because of its excellent chemical, mechanical, and electrical properties.1−3 The above-mentioned properties advocate CNT as a potential candidate in the field of electronics, material science, and biology. The biological applications of CNT as molecular channels, drug delivery vehicles, biomedical sensors, and artificial muscles have been well proposed.4−7 Further, CNTs and the effect of their curvature on α-helical structure of the protein have been reported.8−10 CNT embedded in polar molecules has received attention due to its application as an industrial coolant and as an energy efficient heat transfer fluid.11 The heat transfer capability of CNT-based nanofluids makes them suitable for their use in cooling of electronic equipments, lasers, fuel cells, car radiators, etc.12 The enhancement in thermal conductivity of CNT-based nanofluids is attributed to two reasons: (i) Thermal conductivity of CNT is very high (∼3000 W/mK), and (ii) CNT has very large aspect ratios.13 In fact, usefulness of nanoparticles to enhance the thermal conductivity of the heat transfer fluid has been demonstrated.14,15 The relative efficiency of thermally conductive of nanofluid depends on the base fluid in which CNT is suspended, but the main limitations are aggregation and entanglement of CNT.16 The dispersion of CNT involves two competitive interactions: (1) van der Waals interaction between the CNTs and (2) the interactions between CNT and dispersion medium.17,18 Dispersion or suspension of nanomaterials of high © 2012 American Chemical Society

thermal conductivities into base fluids enhances the thermal conductivity of the mixtures. Ethylene glycol (EG) has been used as a base fluid. Thermal conductivities of nanofluids containing CNT dispersed in EG and synthetic engine oil improve significantly when compared to pure fluids.19 This shows that CNT−EG suspensions have noticeably higher thermal conductivity than does the pure EG base fluid. Furthermore, it was found that upon adding 1 vol % of CNT to EG the thermal conductivity of the EG increased by 12.5%. EG is one of the simplest polar molecules with internal degrees of freedom, which may be regarded as a water analogue. This alcohol, in principle, can form an inter-/intramolecular H-bonding network exhibiting interesting structural and dynamical properties. As a solvent, EG has many applications especially as a cryoprotectant, which maintains automotive engine from freezing and acts as a coolant to reduce overheating. Thus, aqueous solution of EG is found in the most common antifreeze fluid for standard heating and cooling applications.20 EG has been studied by electron diffraction and spectroscopic techniques like IR, NMR, and Raman to unravel the H-bonding in EG.21 Previous ab initio calculations focused on the energetic of various conformers and vibrational frequencies. Although EG has been investigated thoroughly by using different theoretical and experimental methods, studies on the Received: July 19, 2011 Revised: January 11, 2012 Published: January 14, 2012 4365

dx.doi.org/10.1021/jp206882f | J. Phys. Chem. C 2012, 116, 4365−4373

The Journal of Physical Chemistry C

Article

Table 1. Details of Various Systems Simulated in This Study system name EG−Wat CNT−EG−Wat CNT−EG−Wat(Bin) 4CNT−EG−Wat(Bin) CNT−EG

description

box size (nm)

no. of EG

no. of WAT

no. of CNT

∼mole fraction (EG/water)

a box of EG molecule is created, which in turn is solvated with water molecules CNT is solvated with a box of EG molecule, which in turn is solvated with water molecules CNT is solvated with EG−water binary mixture 4CNT molecules placed at 2.5 nm distance from the centroids of each other is solvated with EG−water binary mixture CNT is solvated with EG molecules

6.0 × 6.0 × 3.212

300

2700

nil

0.1

6.0 × 6.0 × 3.212

284

2565

1

0.1

6.0 × 6.0 × 3.212 6.0 × 6.0 × 3.212

284 238

2565 2152

1 4

0.1 0.1

6.0 × 6.0 × 3.212

1230

nil

1

(iii) How does an increase in the number of CNT affect the interaction between EG and water in the binary mixture? This Article is organized as follows. The next section describes the computational details and construction of various models. The important findings are discussed in the Results and Discussion. The outcome of the MD simulations is summarized in the Conclusion.

aqueous solutions of EG particularly EG−water interaction are of considerable interest. Both MD and Monte Carlo (MC) simulations were employed to study pure liquid EG and its aqueous solution.22−24 Also, Langevin dynamics was applied to probe these systems.25−28 The coolant/antifreezing properties depend on the distribution of the EG and water mixture. In addition, there is an interrelationship found between the concentration of EG in EG−water binary mixture with their mixing state and the freezing point of the binary mixture.29 Recently, a force interaction model for the thermal conductivity calculation has been proposed for liquid EG using MD simulation by Lin et al.30 It is found that the trans/gauche ratio of EG, O−Me−Me−O torsional angle, and the number of inter-/intramolecular H-bonds are important factors influencing the thermal conductivity. The above-mentioned studies have clearly brought out the importance of the conformation of EG and its implications in the thermal properties. Recently, Ramaprabhu et al. have reported the dispersion phenomena and thermal conductivity enhancement in the functionalizedmultiwalled carbon nanotube (f-MWCNT) and EG-based nanofluid system.31 The self-aggregation of CNT limits its applications potential in various systems. There are several reports on the use of surfactants and polymers in the dispersion of CNT’s.32 Recently, Vaisman et al. have reported the dispersion of CNT using water-soluble and water-insoluble polymers.33 In case of the water-soluble polymer, poly ethylene glycol (PEG) is used for the dispersion. It is interesting to probe how a water-soluble polymer like PEG can be used for the separation of the hydrophobic CNTs. As EG molecule is the minimalistic model of PEG, the interaction of EG with CNT will be useful to understand the factors governing the interaction between the two systems. Further, results can be useful to gain insight into the interaction between CNTs and PEG. It is necessary to understand whether the high thermal conductivity and aspect ratio of CNT is alone responsible for the enhancement of heat transferring capability of the EG-based nanofluid or if there will be any other changes in the conformation and aggregation properties of EG upon inclusion of CNTs, which can contribute to the increase in the heat transferring capability of the fluids (as proposed by Lin et al.30). Thus, in this investigation, MD simulation of EG−Wat binary mixture and their interactions with CNT have been investigated to unravel inter-/intramolecular interactions. The objectives of the present study are given below: (i) How does the distribution pattern of EG and water molecules change in binary mixture upon addition of CNT? (ii) Can CNT alter conformation and inter-/intramolecular interactions of EG in the binary mixture?

2. COMPUTATIONAL DETAILS 2.1. Molecular Dynamics Simulation. To understand the interaction of CNT with EG−Wat mixture, a series of MD simulation was carried out on different models. In all of the model systems, the CNT with chirality (6,6) and length ∼3.0 nm was used. CNT was built using the nanotube builder in the VMD package.34 Various models considered in this study are: (i) EG−Wat containing EG solvated with water molecules, (ii) CNT−EG−Wat composed of CNT solvated with EG and then by water molecules, (iii) CNT−EG−Wat(Bin) consisting of CNT solvated with EG−Wat binary mixture, (iv) 4CNT−EG− Wat(Bin) composed of four CNT molecules placed at a distance of 2.5 nm with respect to the centroids of each other and solvated with EG−Wat binary mixture, and (v) CNT−EG comprised of CNT, which is solvated with EG molecules. In this study, the EG molecules were parametrized using OPLSAA force field parameters.28,35,36 The carbon atoms of CNT were modeled as uncharged Lennard-Jones particles using sp2 carbon parameters of the OPLSAA force field.20,37 To simulate an infinite CNT, a segment with length equal to the Lz box dimension was aligned along the z axis with the terminal carbons sharing a chemical bond. In all of the models, the aqueous solution of EG at ∼0.1X (X is the mole fraction) was utilized, and the SPC water model was used.38 Box size and number of molecules of the above-mentioned model systems are compiled in Table 1. All simulations were carried out in the canonical NVT ensemble with periodic boundary conditions. The temperature was retained at 300 K using V-rescale thermostat, respectively.39 A 2 fs time step was used to integrate the equation of motion. Electrostatic interaction was calculated using Particle Mesh Ewald sums with a nonbonded cutoff of 10 Å.40 Bonds between hydrogen and heavy atoms were constrained at their equilibrium length using the LINCS algorithm.41 Potential energy of the system was minimized and equilibrated for 500 ps. Analysis of the energy parameters revealed that the systems were well equilibrated. Subsequently, systems were subjected to a production run of 10 ns with trajectories being saved every 1 ps for further analysis. The MD simulations were carried out using the GROMACS 4.5 package (http://www.gromacs.org), the analysis of the trajectories was made using the GROMACS suite 4366



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Figure 1. The initial and final snapshots of the various models of EG−WAT and CNT−EG−WAT systems. CNT and EG molecules are shown in gray and green, respectively. Water molecules are represented in red and white.

Figure 2. Spatial distribution function (SDF) of the EG (green) and water (red) in the EG−Wat, CNT−EG−Wat, and CNT−EG− Wat(Bin) (from left to right, respectively). For both EG and water, the surface shown corresponds to the isovalue of ∼3.0.

of programs,42−45 and the results were visualized using VMD package and Pymol.34,46 Various parameters derived from the simulations are described in the following section. 2.2. Quantum Mechanics Calculation. 2.2.1. Preparation of Models. The fragment geometry of the curved surface of the nanotube was taken from the fully optimized geometry of armchair (6,6) CNT at HF/6-31G(d) level of theory. Hydrogen atoms were added to the dangling carbon atoms of the fragment of CNT. The hydrogen atoms were relaxed at M05-2X/6-31+G** level of theory, while the positions of carbon atoms were fixed. EG and water molecules were optimized at M05-2X/6-31+G** level of theory.47,48 All of the calculations were carried out using the Gaussian 09 suite of programs.49 The above-mentioned optimized structures were used as the initial geometries for the interaction studies. 2.2.2. Geometry Optimization and Binding Energy Calculation of Intermolecular Complexes. The geometries of various complexes of fragments of CNT with water and EG were optimized by using the M05-2X method employing 6-31+G** basis sets by fixing the coordinates of fragment of CNT. Four model systems were considered for quantum chemistry calculations. They are: (i) CNT−WAT complex in which OH group of water molecule interacts with the π-surface of CNT, (ii) CNT−EG(OH−π) complex in which OH of group of EG interacts with the π-surface of CNT, (iii) CNT−EG(CH−π) complex in which CH of EG interacts with the π-surface of CNT, and (iv) CNT−EG(trans) complex in which the trans conformation of EG interacts with the CNT surface. Binding energy (BE) of all of the systems was calculated using the super molecule approach and corrected for basis set superposition

Figure 3. Trans/gauche ratio of the EG in different model systems.

error (BSSE) using the counterpoise (CP) procedure suggested by Boys and Bernardi.50

BE = − (EComplex − (ECNT + ESol ))

(1)

where EComplex is the energy of the CNT−EG (or CNT−WAT) complex. ECNT and ESol denote energies of the CNT and EG (or WAT) as appropriate for the complexes. 2.2.3. AIM Analysis. “AIM” (Atoms In Molecules) is an elegant approach, which is used to characterize both the bonding and the noncovalent interaction.51,52 The topological descriptors obtained from the AIM theory can be successfully employed to distinguish weak, medium, and strong H-bonds in various molecular systems.53−55 Here, we employed the AIM methodology to characterize the weak interaction existing between the chosen system. The AIM calculation was carried out using the wave function generated from the M05-2X/ 6-31+G** level for the geometries obtained from the same level of theory employing the AIM2000 package.51,52,56 The value of electron density (ρ(rc)) and its Laplacian (∇2ρ(rc)) at the bond critical points were used to characterize the interactions between the two systems. The bond critical points (BCPs) in the bond path between the two systems are designated as 4367



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Figure 4. Molecule-wise RDF of EG and WAT with CNT: (A) CNT−EG−Wat, (B) CNT−EG−Wat(Bin), and (C) 4CNT−EG−Wat(Bin).

Figure 5. Atom-wise RDF of EG (O(EG) and C(EG)), WAT (OW), and CNT (C(CNT)) with each other: (A) EG−Wat, (B) CNT−EG−Wat, (C) CNT−EG−Wat(Bin), (D) 4CNT−EG−Wat(Bin), and (E) CNT−EG.

3. RESULTS AND DISCUSSION The main objective of this study is to gain physical insight into the molecular level interaction between CNT and EG−WAT binary mixture. The initial and final snapshots of various model systems obtained from the MD simulations are shown in Figure 1. In EG−Wat, the EG molecules are completely miscible with water and distributed throughout the box. On the other hand, it is evident from the CNT−EG−Wat, CNT−EG−WAT(Bin), and 4CNT−EG−Wat(Bin) that the surface of CNT is surrounded by a considerable number of EG molecules. The presence of CNT induces the aggregation of EG molecules around its surface. The spatial distribution function between all of the independent atomic pairs can provide a clear picture of the threedimensional neighborhood surrounding a selected molecule. Actually, the usefulness of SDF in delineating the threedimensional distribution of solution structure in methanol, EG, ED, and AE in aqueous solutions has been demonstrated.28,57 To understand the three-dimensional density distribution of EG and water in different systems, the spatial distribution function (SDF) was calculated for models i−iii (EG−Wat, CNT−EG−Wat, CNT−EG−Wat(Bin)). The SDFs calculated

Figure 6. RDF of the intermolecular O(EG) and O(EG) of different systems from the simulation.

weak bond critical points (WBCPs). These parameters were used to characterize the nature of the interaction in CNT−EG and CNT−WAT complexes. 4368



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Figure 7. H-bond fraction of EG molecules within themselves in different model systems.

Figure 8. Coordination number of EG with water in different model systems.

are: (i) local distribution of EG around CNT and (ii) local distribution of water around CNT. The calculated SDFs are displayed in Figure 2, and several important features are evident from the pattern. From the SDF of EG−Wat, it can be seen that the EG molecules do not aggregate, and they are completely distributed thoughout the box. The SDF of CNT−EG−Wat and CNT−EG−Wat(Bin) exhibit two distinct layers surrounding the CNT: (i) EG molecules coat on the surface of CNT, and (ii) the CNT−EG is surrounded by water molecules. Although initial arrangements that EG and water molecules in CNT−EG−Wat and CNT−EG−Wat(Bin) systems are considerably different, aggregation of EG around the surface of CNT is observed. Because of the interaction between the CNT and EG, EG forms a distinct layer around CNT. It is noteworthy to mention that regardless of initial configuration, EG molecules aggregate around the CNT. To understand the structural changes induced by the CNT on EG, the conformational analysis of the EG was carried out. The O−Me−Me−O dihedral angle (φ) present in the EG molecule was calculated for all of the simulated model systems. The torsion angle can be categorized in terms of conformation as (i) gauche conformation 0° < φ ≤ ±120° and (ii) trans conformation ±120° < φ ≤ ±180°. The calculated trans/gauche ratio is plotted in Figure 3. It is observed from the results that in the case of the EG−Wat system, the trans/gauche ratio is very small, indicating that the majority of EG molecules are in gauche form. In the case of CNT−EG−Wat, CNT−EG−Wat(Bin), and 4CNT−EG−Wat(Bin), there is an increase in the trans/gauche ratio, which suggests that some of the EG molecules undergo conformational changes due to their interaction with CNT surface. Close examination of the results for the CNT−EG system reveal that there is a significant increase in the conformational transformation from gauche to trans form due to the presence of four CNTs. Figure 3 exemplifies gauche to trans conformational transformation in the CNT−EG system. However, due to the presence of more EG molecules, the ratio of gauche to trans transformation is less in the CNT−EG system on comparing to that of models ii−iv (CNT−EG−WAT, CNT−EG−Wat(Bin), and 4CNT−EG−Wat(Bin)). To quantify the distribution of EG and WAT molecules around the CNT, molecule-wise RDF of EG and water around the surface of CNT was calculated. The results are presented in

Figure 4. RDF pattern of models ii−iv (CNT−EG−WAT, CNT−EG−Wat(Bin), and 4CNT−EG−Wat(Bin)) shows that the amount of EG molecules present around the CNT surface is very high when compared to the water molecules, and the range of g(r) of EG molecule is several folds higher than that of water. To understand the different interactions present in the multicomponent system, the following RDFs were calculated from the MD trajectory. They are: (i) O of water with another water (OW−OW), (ii) OW with O of EG (OW−O(EG)), (iii) OW with the carbon of EG (OW−C(EG)), (iv) carbon of CNT with O(EG) (C(CNT)−O(EG)), (v) carbon of CNT with C(EG) (C(CNT)−C(EG)), and (vi) carbon of CNT with OW (C(CNT)−OW). The calculated RDFs are plotted in Figure 5. It is evident from the results that EG molecules are found near to the surface of CNT in models ii−iv (CNT−EG−WAT, CNT−EG−Wat(Bin), and 4CNT−EG−Wat(Bin)). Further, RDF corresponding to C(CNT)−OW indicates a limited number of water molecules are observed in this region. Inspection of RDF results reveals that the O(EG) is closer to the CNT followed by the C(EG). The g(r) of C(CNT)−O(EG) and C(CNT)−C(EG) interactions ranges from 2.8−3.4 Å and 3.4−5.0 Å, respectively. To assess the distribution of EG molecules in the various models, the RDF corresponding to the intermolecular O(EG)− O(EG) was calculated. The results are displayed in Figure 6. It is observed that the probability of finding intermolecular O(EG)−O(EG) interaction (the height of the peak at ∼2.8 Å) for EG−WAT, CNT−EG−Wat, CNT−EG−Wat(Bin), and 4CNT−EG−Wat(Bin) appears at ∼1.1, 1.3, 1.3, and 1.6, respectively. Thus, it is clear that the intermolecular O−O interaction of EG increases in the presence of CNT. Further, intermolecular interaction between EG molecules increases with the increase in the number of CNT units as evident from 4CNT− EG−Wat(Bin). All of this evidence shows that aggregation of EG takes place around the surface of CNT. It is noteworthy to mention from the results that the EG molecules form a coating around the surface of each CNT unit, which reduces the tendency of the CNT molecules to self-aggregate. In a larger picture, the EG molecules can be applied as a dispersion agent for the separation of CNTs. It is possible to gain insight about the interaction of PEG with CNT from this study. The solubility of EG in water is higher than that of PEG due to polymerization. 4369



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Figure 9. van der Waals’ energy interaction between the CNT−WAT and CNT−EG with respect to time during the simulation: (A) CNT−EG− Wat, (B) CNT−EG−Wat(Bin), and (C) 4CNT−EG−Wat(Bin).

Table 2. Calculated Binding Energies (BE) of Various Complexes Using M05-2X/6-31+G** Method system

binding energy (kJ/mol)

CNT−WAT CNT−EG(CH−π) CNT−EG(OH−π) CNT−EG(trans)

11.76 12.55 17.20 17.78

PEG aggregates around the hydrophobic surface of CNT. In addition, the chain length of PEG may influence the interaction. PEG with longer chain can wrap/coil around the surface of the CNT to reduce the self-aggregation of the CNT. Thus, PEG can act as a better dispersion agent for CNT. To understand the changes in the hydrogen-bonding pattern between the two EG molecules, the total number of hydrogen bonds present between EG molecules during the simulation was calculated using geometric criteria (H-bond distance ≤3.5 Å and H-bond angle 150° ≤ θ ≤ 180°). The H-bond fraction is

Figure 10. Electrostatic interaction between the EG and WAT in different model systems with respect to time.

In the case of PEG, the number of H-bond donor and acceptors reduces exponentially with the chain length. Consequently, the

Figure 11. Optimized geometries of the (A) CNT−WAT, (B) CNT−EG(CH−π), (C) CNT−EG(OH−π), and (D) CNT−EG(trans) complexes at the M05-2X/6-31+G** level. 4370



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Table 3. Calculated Total Electron Density (ρ(rc)) and Total Laplacian of Electron Density (∇2ρ(rc)) at WBCP for Various Models (values in au) various types of interactions O−H···π models CNT−WAT

C−H···π

ρ(rc)

∇2ρ(rc)

0.0059 0.0064

0.0049 0.0054

CNT−EG(CH−π)

CNT−EG(OH−π) CNT−EG(trans)

0.0096 0.0083

0.0077 0.0066

calculated by using the following equation:

H‐bondfraction no.of H‐bonds totalno.of EGmolecules in thesystem

∇2ρ(rc)

0.0057 0.0042 0.0041 0.0064 0.0064 0.0057 0.0068

0.0045 0.0033 0.0033 0.0052 0.0052 0.0047 0.0053

ρ(rc)

∇2ρ(rc)

0.0050 0.0050

0.0044 0.0045

mixture. The presence of CNT facilitates the aggregation, and hence the coordination number of EG with water decreases. In fact, there is a significant decrease in the coordination number of EG with water in the presence of four CNT molecules, and hence there is a reduction in the solubility of EG in water. Figure 9 illustrates the variation of van der Waals energy contribution with time for the CNT and EG, CNT, and WAT in the respective systems. It can be seen that the van der Waals energy between CNT−EG of CNT−EG−Wat and CNT−EG−Wat(Bin) is higher than that of CNT−WAT. In the case of 4CNT−EG−Wat(Bin), this contribution is further increased due to the presence of four CNTs. On the other hand, the same energy contribution to the CNT−WAT decreases. These findings clearly reveal that EG molecules interact favorably with CNT surface by expelling water molecules. The variation in the electrostatic interaction shown in Figure 10 elicits the similar findings. The electrostatic interaction between the EG and WAT is highest for the EG−Wat. The introduction of the CNT into the binary system decreases the electrostatic interaction. To compare the difference in the binding affinity of CNT with WAT and CNT with EG, electronic structure calculations were carried out on CNT−WAT and CNT−EG models. The optimized geometries of CNT−WAT and CNT−EG complexes are shown in Figure 11. The calculated BEs are reported in Table 2. The calculated BE of the CNT−WAT complex is 11.76 kJ/mol. The calculated BE of CNT−EG(CH−π) and CNT−EG(OH−π) complexes is 12.55 and 17.20 kJ/mol, respectively. The same energy for the CNT−EG(trans) is 17.78 kJ/mol, which is marginally higher than the other complexes. It is evident from the results that the BE(CNT−EG(trans)) > BE(CNT−EG)(OH−π) > (CNT−EG)(CH−π) > BE(CNT− WAT). Hence, EG molecules form a three-dimensional distinct layer surrounding the surface of CNT. The above-mentioned observations are in close agreement with the results obtained from the MD simulations. Furthermore, it can be found that the O−H···π distance is shorter than the C−H···π in accordance with the RDF results. The calculated values of electron density (ρ(rc)) and its Laplacian (∇2ρ(rc)) at WBCPs are listed in Table 3. The molecular graphs of all of the complexes are shown in Figure 12. The red, yellow, and green dots indicate WBCP, ring critical point (RCP), and cage critical point (CCP), respectively. The existence of O−H···π and C−H···π interaction in CNT−WAT and CNT−EG complexes is revealed by the presence of WBCPs in the molecular graphs. The ρ(rc) at the WBCP for all of the clusters ranges from 0.0041 to 0.0096 au. The calculated ∇2ρ(rc) varies from 0.0033 to 0.0077 au. Figure 12 reveals the

Figure 12. AIM topography of the (A) CNT−WAT, (B) CNT− EG(CH−π), (C) CNT−EG(OH−π), and (D) CNT−EG(trans) complexes at the M05-2X/6-31+G** level.

=

lp···π

ρ(rc)

(2)

The calculated H-bond fraction for EG molecules in different model systems is given in Figure 7. Evidence shows that the Hbond fraction within EG molecules increases in the presence of CNT especially in the case of 4CNT−EG−Wat(Bin), where four CNT molecules are present. Similar to the variation in the intermolecular O−O distance in EG, there is an increase in the H-bond fraction from EG−Wat to 4CNT−EG−Wat(Bin). It is understandable from the above-mentioned results that the CNT favors the aggregation of EG molecules around CNT. As a consequence, the intermolecular interaction between EG molecules increases. The coordination number indicates the number of nearest neighbors that are H-bonded to the central molecule. To understand the changes in the solubility of EG in water in the presence of CNT, the coordination number of EG was calculated from the RDF corresponding to OW−O(EG) by integrating the RDF values of the appropriate peak up to its first minima. The results are plotted in Figure 8. It is observed from the results that the coordination number of EG decreases from EG−Wat to 4CNT−EG−Wat(Bin). Results show that the solubility of EG in water is more in the EG−water binary 4371



■

bond path connecting the two molecular species is clearly visible, showing the importance of O−H···π and C−H···π interactions in the complexes. It is noteworthy to mention that in the case of CNT−EG complexes, three WBCPs are found for CNT−EG(CH-π) complex corresponding to two CH···π interactions. The CNT−EG(OH-π) complex exhibits three WBCPs corresponding to one CH···π, one OH···π, and one lone pair···π (lp···π) interactions. In the case of CNT−EG(trans) complex, five WBCPs can be noted from the molecular graphs implying one OH···π, two CH···π, and one lp···π interactions. In fact, CNT−EG(trans) complex has a maximum number of interactions with CNT and hence has a higher stability over other complexes. These findings vividly reveal that CNT induces the conformational transition in EG from gauche to trans.

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4. CONCLUSION In this study, a systematic attempt has been made to understand the interaction between the EG and CNT in aqueous medium using classical MD simulation. In addition, the M052X/6-31+G** method has been used to calculate the BE of CNT with EG. The following important points emerge from this investigation. (i) The EG molecules aggregate on the surface of CNT in aqueous medium due to hydrophobic−hydrophobic interaction. This property of EG can be used to disperse the CNT in the same environment. These findings are in accordance with the increase in the H-bonding interaction between the two EG molecules in the presence of CNT. Evidence from SDFs supports the abovementioned observations. During this aggregation process, the surrounding water molecules are expelled out. (ii) Close scrutiny of MD trajectory reveals that a small fraction of the gauche form EG is converted into trans in the presence of CNT. Furthermore, the intermolecular H-bond fraction of EG increases upon inclusion of CNT in the binary mixture. It has a significant impact on the thermal properties of the overall system. (iii) The calculated BE of the EG−CNT system in the gas phase using M05-2X/6-31+G** varies from 11.76 to 17.78 kJ/mol. It is interesting to note that the BE of trans EG with CNT is higher than that of the gauche form with CNT. (iv) The molecular graphs derived from Bader’s theory of AIM confirm that the CNT−EG(trans) complex has more bond paths than the corresponding CNT−EG(gauche) (CNT−EG(CH−π), CNT−EG(OH−π)) counterpart and hence greater stability.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 44 24411630. Fax: +91 44 24911589. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Tata Steel Limited, Jamshedpur, India and Centre of Excellence in Computational Chemistry (NWP-53), Council of Scientific and Industrial Research (CSIR), New Delhi, India for Financial Support. We also thank Dr. A. B. Mandal, Director CLRI and Dr. T. Ramasami, DST, GOI for their continued support. 4372



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