Interaction of C60 with Water: First-Principles Modeling and

Article pubs.acs.org/est

Interaction of C60 with Water: First-Principles Modeling and Environmental Implications Ji Il Choi,†,∥ Samuel D. Snow,‡,∥ Jae-Hong Kim,‡,§ and Seung Soon Jang*,† †

Computational NanoBio Technology Laboratory, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ‡ Department of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States § Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States ABSTRACT: The nature of fullerene-water interactions has been the subject of much research and debate. Specifically, the presence of a stabilizing, negative surface potential on colloidal aggregates of C60 in water is unexpected, given the neutral nature of pure carbon, and is not well understood. Previous simulation efforts have focused on the C60water interaction using molecular dynamics simulations that lacked the ability to account for charge transfer and distribution interactions. In this study, first-principles density functional theory was used to analyze the fundamental electronic interactions to elucidate the polarization and charge transfer between water and C60. Simulations show that charge is inductively transferred to the C60 from water molecules, with subsequent polarization of the C60 molecule. In a case with two neighboring C60 molecules, the charge polarization induces a charge onto the second C60. Simulation suggests that this charge transfer and polarization may contribute at least partly to the observed negative surface potential of fullerene aggregates and, combined with hydrogen bonding network formation around C60, provides a fundamental driving force for aggregate formation in water.

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extensive, information available publically online,23 as well as in extensive reviews of the topic as it relates to fullerenes.21,24 The colloidal fullerene formation in water might be explained within the framework of hydrophobic interactions between C60 and water molecules.25−27 The insertion of C60 into water would induce a structural reorganization of the surrounding water molecules to maintain a hydrogen-bonding network, which in turn causes the C60 molecules to aggregate in order to minimize the perturbation of the hydrogen-bonding network.28−31 Unfortunately, it would be an extremely challenging task to experimentally characterize the local changes in a hydrogen bond network of water molecules surrounding C60 in real systems. Thus, quite a few theoretical works employing molecular dynamics (MD) simulation methods have tackled this question, and intriguingly, the results reported from these studies have been somewhat contradictory to the existing understanding of the interaction between nonpolar solutes and water. Walther and co-workers investigated a pair of C60s in water by performing classical MD simulations and found that the main force between C60s is the van der Waals attraction.32 Also, through MD simulations of a system consisting of two C60s in water, Hotta et al. found that the water molecules are

INTRODUCTION Ever since fullerenes (C60) were discovered by Smalley and his co-workers,1 they have attracted a great deal of attention from various fields of science and technology due to their unique physical and chemical properties.2−7 In particular, the solubilities of fullerene with respect to various solvents have been studied extensively;8−11 the fundamental molecular interactions of C60 with solvents play important roles in various applications such as optical, electronic, electrochemical, and biomedical technologies.4,6,7 The interaction of C60 with water is of particular interest, as many applications target biological and ecological systems where water is the dominant solvent. Although the solubility of C60 in water is poor (ca. 7.96 ng/L)12,13 compared to that in nonpolar organic solvents such as benzene (1.7 mg/mL) and toluene (2.9 mg/mL) due to the hydrophobic nature of C60,14 it has been observed that C60 can form colloidal particles, so-called C60 aggregates, or nC60, that are stable in water for over 18 months under dark conditions,14−17 with important implications for environmental fate, transport, and applications,18−20 highlighted by a recent, thorough review.21 The sizes of such C60 aggregates typically range between 50 and 150 nm, depending on the preparation method. Several studies have reported and modeled smaller clusters of C60, in the range of nanometers to several tens of nanometers, that, as building blocks, further aggregate to form the larger, more stable nC60 particles.13−15,22 The topic of hydration in general has received much attention, with © 2015 American Chemical Society

Received: Revised: Accepted: Published: 1529

September 20, 2014 December 19, 2014 January 20, 2015 January 20, 2015 DOI: 10.1021/es504614u Environ. Sci. Technol. 2015, 49, 1529−1536

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Environmental Science & Technology

Figure 1. Energy-minimized structures for various C60-water clusters: (a) a water molecule on the hexagonal site of C60; (b) a water molecule on the pentagonal site of C60; (c) 10 water molecules on C60; (d) 10 water molecules on a C60 dimer; and (e) 20 water molecules on a C60 dimer.

contribution by surface functional groups. Answering this question requires ab initio quantum mechanical (QM) investigation. In this study we performed the first-principles density functional theory (DFT) computations to investigate the electronic structures and interactions of C60 with explicit water molecules, focusing specifically on charge transfer, accompanied changes in charge distribution, and the electronic properties of the hydrated C60 cluster systems. Various configurations, consisting of more than one C60 with varied numbers of explicit water molecules, are studied to examine the fundamental stabilization mechanism of C60 clusters and aggregates in the aqueous phase. Clusters of only several fullerenes are used here since an aggregate of innumerable molecules would be computationally infeasible. Since the polarizable continuum model (PCM) is not considered to be capable of accurately describing the charge transfer and interactions between water and C60, we use explicit water molecules here. For example, the water solvation free energy of C60 was predicted to be −2.9 kJ/mol using the PCM with HF/ 6-31G,54 which significantly deviated from the experimental measurement of −17.4 kJ/mol.55

attracted to C60 due to the attractive dispersion interactions between the C60s and water molecules.33 In recent studies by Choudhury et al.,34,35 which employed isothermal−isobaric (NPT) ensemble MD simulations, it was found that the dynamics of water are slower in the solvation shell of C60 and even slower between two C60s compared to that of bulk water,33,34 an observation that is typical for water surrounding hydrophilic surfaces.36−41 Li et al.42,43 characterized the role of C60-water interactions for the formation of C60 clusters in water using classical MD simulations with Lennard-Jones (LJ) potential and suggested that the C 60 -water dispersion interaction is strong, such that the water actually helps disperse the C60.41,42 It should be noted, however, that the results from MD simulations depend greatly on the choice of the force fields used to calculate the water-C60 potential energy. Further, the fundamental electronic interaction between water and C60, an important component of the theoretical system, is not within the scope of MD simulations. The fundamental interaction between C60 and water is also critical considering the negative surface ζ-potential (−10 ∼ −45 mV) of these fullerene aggregates,16,18,22,44−47 since the negative potential is a primary reason for the aqueous stability. However, the mechanism for the negative surface potential is still not fully understood given the nature of C60. Some have proposed the oxidation on the surface of nC60, introduced during the aggregation process or by ambient O3, which confers a negative charge onto the surface of the nC60 particles via dissociation of the added functional groups.48,49 Accordingly, recent work has suggested that the surface charge of C60 aggregates, altered by the presence of functional groups, may be a key, determining factor of the photoactivity, and induced biological activity, of fullerene aggregates.19,47,50,51 Other studies conjectured that the C60 would be hydrated and stabilized by water molecules through a combination of the localized hydrolysis and the electron donor−acceptor complex formation, based on the experimental evidence that C60s in nC60 retain their pristine state.13,52,53 Questions remain as to whether pristine C60 can still assume any surface charge in the water matrix through charge transfer from surrounding water molecules, regardless of and possibly in addition to charge

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COMPUTATIONAL METHODS

In order to investigate the interaction of C60 with neighboring water molecules, we used hybrid functional, B3LYP,56,57 and hybrid meta-GGA functional M0658 through Jaguar59 with the 6-31G** basis set. We first prepared a C60 monomer and a dimer. The initial C60-C60 distance in the dimer was 9.936 Å based on the face-centered cubic (FCC) crystal structure of C60 with a = b = c = 14.052 Å and α = β = γ = 90°.60 Structures of C60 clusters were then optimized using the C60 force field developed by Goddard and his co-workers61 and subsequently refined using DFT with B3LYP and 6-31G**. Once the C60 clusters were fully optimized, water molecules were added to the C60 clusters to partially solvate the C60s. To determine the positions of the water molecules, we performed geometryoptimization using F3C water FF62 with the fixed C60 cluster, and then performed geometry-optimization using DFT without any constraints. Figure 1 shows the geometry-optimized 1530

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Figure 2. Comparison of energy-minimized single water-C60 geometries.

Table 1. Distances between C60 and Water Molecules and between Water Molecules Which Are Calculated Using DFT with 631G** Basis Set distance (Å) COM (C60) - O (H2O) functional

B3LYP

C60-1H2O (hexagonal) C60-1H2O (pentagonal) C60-10H2O 2C60-10H2O 2C60-20H2O C60-80H2O

5.956 5.889 6.867 ± 0.298

O (H2O) - O (H2O) M06

6.245 5.016 6.425 6.499 6.487 7.334

± ± ± ±

B3LYP

0.178 0.257 0.207 0.854

2.744 ± 0.092

M06

2.794 2.783 2.783 2.795

± ± ± ±

0.090 0.102 0.098 0.098

To evaluate the C60-water interactions, the formation energy (ΔEformation) was calculated as follows:

structures of various C60-water clusters using M06 functional with 6-31G**. The charge transfer and distributions were analyzed using Mulliken population analysis.63,64 The effects of the surrounding water media on the C60-water interaction were investigated by calculating the C60-water binding energy in water phase using the Poisson−Boltzmann solvation method.65

ΔEformation = [E(C60 − nH 2O) − {E(C60) + n × E(H 2O)}]/n

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(1)

where E(C60 − H2O), E(C60), and E(H2O) denote the energies of a C60-water cluster, of a C60, and of a water molecule, respectively, and n is the number of water molecules. The ΔEformation is conceptually equivalent to binding energy but takes into account the interactions between water molecules, except a single water molecule-C60 case where the formation energy is purely the C60-water interaction. Table 1 shows that the calculated values for ΔEformation on the hexagonal site (Figure 2a and b) are −1.277 kcal/mol and −2.983 kcal/mol using B3LYP and M06, respectively, whereas ΔEformation on the pentagonal site (Figure 2c and d) are −1.092 kcal/mol and −2.305 kcal/mol using B3LYP and M06, respectively. The ΔEformation values are all negative, meaning that C60 has attractive interaction with water molecules even though C60 is hydrophobic. In addition, it is noted from both B3LYP and M06 that the hexagonal site of C60 has slightly stronger attraction with water molecules. Hence, we may expect that the oxygen atoms in water molecules bind preferably on the hexagonal sites of C60 and that other water molecules will subsequently be arranged around the C60. Differences in the

RESULTS AND DISCUSSION C60 with a Single Water Molecule (C60-1H2O). The geometry optimization using B3LYP and M06 reveals the preferred interaction sites of water molecules with C60 (Figure 2). Both optimizations present a common structural feature: the water molecule aligned more closely to C60 on the pentagonal site than on the hexagonal site. From the top views, however, we found that B3LYP and M06 generate slightly different geometries. In the B3LYP optimization, the oxygen of water molecule is centered over a carbon of the hexagon (Figure 2a) and of pentagon (Figure 2c), while the oxygen of water molecule in the M06 optimization is positioned above the hollow position of the hexagon (Figure 2b) and the pentagon (Figure 2d). The optimized distance between the oxygen of water and the center of mass of C60 was also measured and summarized in Table 1: For the hexagonal sites, the distances were 5.956 and 6.245 Å using B3LYP and M06, respectively, whereas the distances for the pentagonal sites were 5.889 and 5.016 Å using B3LYP and M06, respectively. 1531

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Environmental Science & Technology binding energy for B3LYP and M06 are also observed: the M06 results in stronger binding energy than B3LYP. This difference is likely due to the incapability of the B3LYP functional in describing the contribution of van der Waals interactions.66,67 Despite the great success of B3LYP being the most popular hybrid functional for accurately predicting chemical properties, it is well-known that B3LYP cannot capture van der Waals interaction for nonbonded, weak interaction between atoms or molecules. Our simulations suggest that the binding energies from M06, a meta-GGA hybrid functional, are 1.3 and 1.8 times larger than those from B3LYP on the hexagonal site and the pentagonal site, respectively, indicating that the contribution of van der Waals force to the overall interaction between C60 and water molecule is not insignificant. The orientation of water molecules onto the C60 surface shown in Figure 2 is particularly noteworthy. It is known that a water molecule forms a hydrogen bonding interaction with the π-electron of a benzene ring by orienting a hydrogen of water toward the molecular plane of benzene perpendicularly.68,69 Accordingly, Rivelino and his co-workers70 hypothesized that the hydrogen bonds would be formed between the hydrogen atoms of water and the hexagons in C60.68,69 The water-C60 interaction observed herein reflects charge transfer complexation which originates from the interaction of the lone pair of electrons of oxygen with π*-orbitals of C6071 rather than a hydrogen bonding interaction as in the case of benzene. As a result, a charge transfer from water molecule to C60 would be expected due to the strong electron affinity of C60.72 From Mulliken population analysis (Table 2), it was found that the Table 2. Charge Transferred from Water Molecules to C60 and Formation Energy of C60-Water Clusters Calculated Using DFT with 6-31G** Basis Set charge of C60 (e) functional C60-1H2O (hexagonal) C60-1H2O (pentagonal) C60-10H2O total 10 H2O C60-H2O 2C60-10H2O Total 10 H2O C60-H2O 2C60-20H2O total 20 H2O C60-H2O

B3LYP

M06

B3LYP

M06

−0.0388 −0.0554 0.0142

−0.0010 −0.0090 −0.0607

−1.277 −0.815 −11.786 −11.143 −0.643

−2.983 −2.305 −12.542 −10.465 −2.077 −12.394 −10.494 −1.900 −12.423 −10.471 −1.952

−0.0481 − −0.1097

Figure 3. Hydrogen bond network of 10 water molecules on a C60 using (a) B3LYP and (b) M06.

formation energy (kcal/mol)

0.092 Å and 2.794 ± 0.090 Å using B3LYP and M06, respectively, in good agreement with reported experimental data, which ranged from 2.73 to 2.88 Å.73−76 The average distances from the center of mass (COM) of C60 to the oxygen atom of each water molecule are 6.867 ± 0.298 Å and 6.425 ± 0.178 Å using B3LYP and M06, respectively (Table 1). Compared to the C60-1H2O pair, the C60-10H2O cluster has greater distances between C60 and water molecules. The hydrogen bonding interactions among water molecules is stronger than the interaction between C60 and water, primarily determining how water molecules arrange around C60 and consequently increasing C60-H2O distance compared to the C60-1H2O case. This explanation is further confirmed by evaluating the hydrogen bonding and C60-H2O interaction energies. First, ΔEformation of the C60-10H2O cluster is calculated to be −11.786 kcal/mol and −12.542 kcal/mol from B3LYP and M06, respectively (Table 2). On the other hand, the ΔEformation calculated only for the 10H2O cluster in the absence of C60, thus representing only the interactions among water molecules, is −11.143 kcal/mol and −10.465 kcal/mol from B3LYP and M06, respectively. This result shows that the ΔEformation for C60-10H2O cluster is mostly attributed to the ΔEformation for the 10H2O cluster (94.5% (B3LYP) and 83.4% (M06)). The C60-H2O interaction is calculated to be −0.643 kcal/mol and −2.077 kcal/mol for B3LYP and M06, respectively, which is 78.9% and 90.1% of ΔEformation for the

charge of C60, Q(C60), has a negative value due to the charge transfer through the hexagonal site Q(C60) = −0.0388e and Q(C60) = −0.0010e using B3LYP and M06, respectively) as well as through the pentagonal site (Q(C60) = −0.0554e and Q(C60) = −0.0090e using B3LYP and M06, respectively). For all the cases, it is observed that the C60 acquires a partial negative charge from a water molecule. Ten Water Molecules Around a C60 (C60-10H2O). The water-C60 interaction is further affected by interactions between water molecules surrounding C60. Simulation results shown in Figure 3a and b were obtained by placing and optimizing a structure of 10 water molecules around C60 to form a hydrogen bonding network. In this network, the average distances between the oxygen atoms of water molecules are 2.744 ± 1532

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C60-C60 interactions on water-induced polarization of C60. In these 2C60 systems, it is observed that the water molecules also form hydrogen bonding networks with O(water)-O(water) distances of 2.783 ± 0.102 Å and 2.783 ± 0.098 Å, respectively (Figures 5a and 6a). The distances were 2.794 ± 0.090 Å for

C60-1H2O cluster, confirming that the surrounding hydrogen bonding network weakens the C60-H2O interaction. Table 2 also presents the charge transfer between C60 and water molecules in the C60-10H2O cluster. B3LYP demonstrates an unexpected charge transfer from C60 to water (Q(C60) = 0.0142e), while M06 shows charge transfer from water to the C60 (Q(C60) = −0.0607e) which is in line with reports in literature.16,22,46,47 This inconsistency in B3LYP simulation is likely due to, as discussed above, the incapability of B3LYP to describe the attractive dispersion interactions between water and C60. Therefore, B3LYP will not be used for further discussions in this study. Based on the charge transfer characterized through Mulliken population analysis using M06, the charge distribution over C60 is presented in Figure 4.

Figure 6. (a) Hydrogen bond network of water molecules and (b) charge distribution for 2C60-20H2O calculated using M06.

the 1C60-10H2O case, suggesting that the presence of another C60 has negligible effect on the hydrogen bonding network formation. The average distances from the COM of C60 to the oxygen atom of each water molecule are also measured to be 6.499 ± 0.257 Å and 6.487 ± 0.207 Å for 2C60-10H2O and 2C60-20H2O, respectively (Table 1). Although these average values obtained from 2C60-water clusters are slightly larger than C60-10H2O, the difference of the values are not significant, given their standard deviations. The cluster formation energies (ΔEformation) are −12.394 kcal/mol and −12.423 kcal/mol for 2C60-10H2O and 2C60-20H2O, respectively. By calculating the hydrogen bonding energy among water molecules (−10.494 kcal/mol and −10.471 kcal/mol for 2C60-10H2O and 2C6020H2O, respectively), we found that 84−85% of ΔEformation of these clusters are derived from the hydrogen bonding interactions. The C60-H2O interaction is calculated to be −1.900 kcal/mol and −1.952 kcal/mol, which are similar to −2.077 kcal/mol for C60-10H2O cluster. We presume that C60s in nC60 (i.e., aggregates of many C60s) would be under a similar solvation environment wherein only a small fraction of C60 surface is exposed to water while the rest is bound to neighboring C60s. The above observations made with 2C6010H2O and 2C60-20H2O systems are, therefore, believed to be a close representation of phenomena involving nC60 in water. The presence of the second C60 does not seem to influence the charge transfer from water molecules to C60. The charges in the C60 dimers (Q(2C60)) are −0.0481e and −0.1097e for 2C60-10H2O and 2C60-20H2O, respectively. However, the second C60 does appear to have an effect on the charge distribution, as presented in Figures 5b and 6b. First, when comparing 2C60-10H2O (Figure 5b) with C60-10H2O (Figure 4), it is observed that the polarization of the first C60 induces the polarization in the second C60. Since induced-polarization occurs without actual charge transfer, it is found, as expected, that the net charge of the second C60 in Figure 5b is slightly positive but nearly zero. This polarization of the two C60s likely

Figure 4. Atomic charge distribution on C60 calculated using M06. C60 is polarized due to the water molecules and the charge transfer. The position 1, the closest portion to the water molecules, is most negatively charged. The position 3 has a positive charge.

Position 1, where the water interacts with C60, has −0.0492e whereas the symmetrically opposite side of C60 (position 3) has 0.0126e. This result indicates that C60 can be polarized depending on the surrounding water environment, which could enhance the solvation of C60 by water. In order to better understand the stability of C60 in the water phase, the polarization and charge transfer must be assessed for systems with more than one C60. C60 Dimer Partially Solvated by Water (2C60-10H2O and 2C60-20H2O). The Results shown in Figures 5 (2C6010H2O) and 6 (2C60-20H2O) take into account the effects of

Figure 5. (a) Hydrogen bond network of water molecules and (b) charge distribution for 2C60-10H2O calculated using M06. 1533

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reason why nC60 forms only at a certain size range, typically beyond tens of nanometers. Environmental Implications. The interactions of fullerenes with water are of utmost importance, because these interactions govern the fate and transport of fullerenes in the aqueous environment. In particular, the simulations and observations herein reveal the importance of charge polarization on fullerene aggregation: C60 molecules are stabilized in water by a net negative charge produced by networks of charge polarization interactions in large aggregates. These findings suggest that C60s or clusters of few C60s would not be stable in the aqueous phase and would tend to aggregate into larger particles, as observed in many studies.47,48,53,77 While aggregation tends to diminish environmental impact (i.e., 1O2 production) of fullerenes, the addition of functional groups to the fullerene cage has been shown to significantly affect nC60’s photochemical and colloidal behaviors.19,45,47,50,51,78,79 These changes were particularly prominent in fullerenes with positively charged functional groups added, likely due to enhanced charge polarization effects. The importance and occurrence of charge polarization demonstrated here suggest that modifications to fullerenes that enhance charge polarization (e.g., endohedral metal inclusions or quaternary ammonium functional groups) would promote smaller, more stable, and potentially more photoactive particles, an important realization for the control and use of fullerenes in aqueous environments.

enhances the strength of the C60-C60 interaction via electrostatic attraction between opposite charges. When 10 more water molecules are placed on opposite sides of C60 dimers, charges are symmetrically distributed with −0.0555e for upper C60 and −0.0551e for lower C60 (Figure 6b). The binding energy between upper C60-10H2O and lower C60-10H2O clusters, calculated based on single point energies shown in Figure 6b is −3.200 kcal/mol, also suggesting enhanced binding of C60 dimers due to charge transfer and subsequent C60 polarization. Implications of C60 Aggregation and Surface Charge. Previous studies on the molecular interactions of water and fullerenes have been limited by the use of tools that do not capture weak van der Waals interactions or the fundamental electronic interactions between water and C60. In this study, we used a quantum mechanical density functional theory approach which addresses both of these pitfalls and found their significance. The results herein present an important step forward in understanding the stabilization of fullerenes in water, which is important particularly for environmental applications and implications. Simulation results suggest that hydrogen bonding networks of water are formed around C60. The hydrogen bonding interaction is more favorable than the C60water interaction, and drives C60s to aggregate in the aqueous phase, providing the primary thermodynamic driving force to form nC60 in the aqueous phase.25−27 More importantly, simulations suggest that charge transfer takes place from the solvating water molecules to the C60, which induces charge polarization in the C60. Through this weak charge transfer complex formation, it is observed that the C60 gains an attractive interaction with individual water molecules (−1.9 ∼ -2.1 kcal/mol). This charge transfer phenomenon is important to notice for two reasons. First, it partly explains how C60 aggregates in water acquire negative surface charge as experimentally observed, although it is difficult to assess exactly how much of the net negative ζpotentials (−10 ∼ −45 mV) can be attributed to charge polarization versus surface functionalization.16,22,46,47 A larger scale simulation, for example, fully solvated, larger C 60 aggregates, would be needed but computation is prohibitively costly. Yet, we believe that 2C60-20H2O simulation in Figure 6b should not be far from nC60 since multiple C60s neighbor each other in the case of aggregated C60 such that water is not likely to fully surround any one C60. Further, aqueous aggregates on the order of tens of nanometers in diameter would contain millions of individual C60s with many that are internal with no interaction with the bulk water phase. In these aggregates it may be expected that charge polarization would induce a net positive charge in the core and presumably more negative charge on the surface of the C60 aggregate in the bulk water phase. Second, this charge transfer induces charge polarization in the C60 that are in direct contact with water molecules. This polarized C60 at the surface of aggregates can effectively induce charge polarization on neighboring C60s. This observation provides a new theoretical framework to understand the stabilization of C60 aggregates in aqueous solutions. When C60s aggregate in the aqueous phase, such polarization will enhance the attractive interaction between C60s within the aggregates. In this framework, fullerenes aggregates that are not stable due to insufficient surface charge will aggregate further until an adequately stable particle has formed, partly explaining the

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-404-385-3356; fax: 404-894-9140; e-mail: [email protected]. Author Contributions ∥

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the National Science Foundation (Grant No. 1439048). We also acknowledge that this research used resources of the Keeneland Computing Facility at the Georgia Institute of Technology, supported by the National Science Foundation under Contract OCI0910735.

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REFERENCES

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