NANO LETTERS
Solvent Effect on Functional Groups Attached to Edges of Carbon Nanotubes
2002 Vol. 2, No. 6 573-575
T. Halicioglu† and R. L. Jaffe* NASA, Ames Research Center, Moffett Field, California 94035-1000 Received March 5, 2002; Revised Manuscript Received April 5, 2002
ABSTRACT Molecular dynamics calculations are carried out to investigate the effect exerted by aqueous solutions on the equilibrium structure and configuration of functional groups attached to carbon nanotubes. In this study, structures of polar and nonpolar functional groups in water are analyzed. Energy changes associated with configurational transformations of functional groups (from folded to extended forms) in water are calculated. Results indicate that polar functional groups are energetically more stable in extended configurations. Nonpolar functional groups, on the other hand, prefer to remain folded.
Introduction. Understanding of the structure and configuration of functional groups attached to carbon nanotubes (CNTs) in solution is becoming an important issue for various applications of CNT sensors.1-4 Although typical functional groups can be quite floppy in the gas-phase, they display varying degrees of rigidity (depending on their polarity and molecular structures) in aqueous solution. The energetically most favorable configuration of a functional group depends strongly on its polarity and the nature of the solvent. In this study, molecular dynamics calculations are carried out to investigate equilibrium configurations of polar and nonpolar functional groups attached to the end of a single-wall carbon nanotube in water. There are three basic types of interaction that control the shape and configuration of an attached functional group in solution. (i) Interactions among the Solvent Molecules. The CNT and the attached functional group in solution reside in a cavity within the solvent. The size and geometry of this cavity are strongly coupled to the nature of the functional group and its configuration. In many cases, folding or unfolding proceses of the functional group go hand in hand with changes in the interfacial surface area associated with the cavity. Changes in cavity characteristics and related energetics are directly linked to the materials properties of the solvent. For aqueous solutions, in general, any change in cavity geometry is an energetic process due to strong forces operational among water molecules. These interactions are denoted by WW. (ii) Interactions between the Solute and Solvent Molecules. This type of interaction is very much dependent on * To whom correspondence should be addressed. † Eloret Corporation, 690 W. Fremont Avenue, Sunnyvale, CA 94087. 10.1021/nl0255449 CCC: $22.00 Published on Web 05/14/2002
© 2002 American Chemical Society
the atomic and/or molecular nature of the solute and solvent molecules. For the case of aqueous solutions, the hydrophobicity or hydrophilicity of the solute plays a very important role. For polar molecules with strong electrostatic interactions, these forces may be strong enough to overcome the WW interactions indicated above. On the other hand, for nonpolar systems solute-solvent interactions arising from dispersion type forces are generally weak. In this paper, interactions between the solute and surrounding solvent (water) molecules are referred by SW. (iii) Interactions Taking Place within the Solute and among Solute Molecules. These include intramolecular interactions, between the functional group and the CNT, as well as interactions within the functional group itself which may play an important role for relatively long functional groups. This part also includes strain energies. Also included, for more concentrated solutions, are interactions between two or more solute molecules leading to aggregation of the solute. Furthermore, this type of interaction depends on the polar or nonpolar nature and structural characteristics of the functional group. Forces involved in this case may vary from weak dispersion and dipole-induced dipole interactions to strong dipole-dipole interactions that affect the folding characteristics of the functional groups involved. These interactions, involving only the solute, are denoted by SS. Energetically speaking, the sum of these interactions determines the final equilibrium configuration of the attached functional groups in solution. In this study, we investigated only the equilibrium energetics and no attempts were made to investigate the barrier heights affecting the rate. This issue, however, may be quite important for some cases and requires a separate investigation.
Calculations and Results. We employed molecular dynamics techniques to investigate the equilibrium energetics of a CNT with a single functional group attached and surrounded by water molecules. The solute, in this case, is composed of a relatively short zigzag CNT (10,0) containing 80 carbon atoms with a polar or nonpolar functional group attached. As a nonpolar functional we considered a propylene (-CHdCH-CH3) group attached to an edge C atom of the CNT. To represent a polar functional, we replaced the -CH3 group of the propylene with a carboxyl group (-CHdCHCOOH). In both cases, the CdC double bond provides two energetically stable structures corresponding to trans and cis forms of the functional groups representing the extended and compact structures, respectively. Among several different possible configurations, the cis form was represented by one of the most compact forms. The functional group in this case is bent and remains close to the cylindrical axis of CNT. The trans structure is obtained by rotating the end of the functional group 180° around its CdC double bond. For the polar case, the -OH group points toward the interior of the CNT, as schematically shown in Figure 1. The CNTs have diameter equal to 8.13 Å and length 7.10 Å. At both ends, dangling bonds are terminated with hydrogen. Figure 1, parts a and b, shows CNTs in the gas-phase with the polar functional group. Furthermore, these figures also display Connally5,6 surfaces representing the outer boundary of the cavity interfacing water. In this case, the probe radius was taken as 3 Å (representing the approximate distance between solute and the closest water molecules). The computational cell contains 600 water molecules surrounding a single solute molecule with 3D periodic boundary conditions imposed to maintain continuity. The cell, here, is a cube of dimension 27.4-30.0 Å depending on the nature of the functional group which ensures large separations between CNTs in neighboring cells. Molecular dynamics simulations were carried out employing the Cerius2 (Version 3.5) package of Molecular Simulations Inc. Investigations were conducted using T ) 300 K and maintaining P ≈ 0. In all cases, time steps were taken equal to 10-15 s. Energy calculations were carried out using the Dreiding force field and the Ewald sum technique to include long-range interactions with partial charges from charge equilibration method.7 For a full equilibration, we performed over 300 000 time steps (for each configuration) and an additional 10 000 steps were conducted to gather equilibrated statistical data. For the isomerization reaction (cis f trans) taking place in an aqueous solution, the total energy change, ∆ET, was calculated as follows: ∆ET ) ∆EWW + ∆ESW + ∆ESS. Here, ∆EWW, ∆ESW, and ∆ESS denote contributions coming from interactions among the water molecules, between the solute and the surrounding water molecules, and solute intramolecular energy, as described above. For the case of the polar functional group the total energy change for the cis f trans process was calculated as -13.7 kcal/mol. This result indicates that for polar groups the extended trans form is energetically more stable than the folded cis form. For the nonpolar group, on the other hand, 574
Figure 1. Schematic representations of polar functional groups attached to edges of CNTs. Part (a) shows the folded cis form, whereas part (b) represents the extended trans form. In both cases, Connally surfaces, indicated by dotted area, represent the interfacial region between the solute and surrounding water molecules.
the energy change was calculated as +3.64 kcal/mol, indicating that in water, nonpolar groups are more likely to Nano Lett., Vol. 2, No. 6, 2002
Table 1. Calculated Energies and the Change in the Interfacial Areas, ∆A, during the cis f trans Process for Polar and Nonpolar Functional Groups. Energies are in Kcal/mol and ∆As are Given in Å2 functional
∆ET
∆ESS
∆EWS
∆EWW
∆A
polar nonpolar
-13.77 3.64
-0.4 0
-17.24 -2.99
3.87 6.63
22.85 39.05
be found in the folded cis form. This outcome, for polar and nonpolar cases, remains unchanged even if we consider RMS deviations in total energies which were found, in general, less than 3.0 kcal/mol. For both polar and nonpolar functional groups we also estimated energy changes arising from the different types of interactions. (See Table 1). The ∆EWW part, as indicated above, arises from the change in surface area of the solute cavity during the isomerization process. Considering Connally surfaces (shown in Figure 1, parts a and b), we calculated the surface area change as 22.85 Å2 for the polar, and as 39.05 Å2 for the nonpolar cases, respectively. These results show that, in both cases, the surface area of the cavity containing the trans structure is larger than that of the cis case. Taking the experimental surface energy of water as 118 ergs/cm2 (this excludes the entropy part), present calculations produced ∆EWW ≈ 3.87 kcal/mol for the polar group and ≈ 6.63 kcal/mol for the nonpolar case.8,9 Both values, as expected, are positive numbers, indicating that during the cis f trans process cis forms are less disruptive of the bulk water structure. Furthermore, an analysis of the calculated surface areas indicates that the cis form of the polar group is more compact than the nonpolar case. The ESS part for the polar case for the cis w trans process was calculated as -0.4 kcal/mol, whereas for the nonpolar case our calculation produced ∆ESS ≈ 0. These results, corresponding to the gas-phase solute energies, include only energy changes coming from intramolecular interactions. In general, they may also include some strain effect caused by surrounding water molecules. Here, these values were estimated (for both cis and trans forms) considering fully equilibrated gas-phase structures at 300 K. Clearly, these ∆ESS values for both polar and nonpolar cases are quite small. This consideration remains valid even if we take into account mean fluctuations in the calculated total energy values which were found to be about (0.5 kcal/mol. Given the values for ∆EWW and ∆ESS as described above, we estimated ∆ESW, which represents energy contributions coming only from solute-water interactions, as ∆ESW ) ∆ET - ∆EWW - ∆ESS. As shown in Table 1, ∆ESW values for polar and nonpolar cases differ considerably. This result,
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however, is not suprising because there are strong dipoledipole interactions between the polar functional group and the surrounding water molecules which are not present in the nonpolar case. In the case of the trans form the functional group is fully extended and completely surrounded with water molecules, whereas for the cis case the functional group is folded and surrounded by fewer water molecules. (See Figure 1, parts a and b). In solution, the most important energy contribution (for the system with polar groups) comes from ∆ESW (i.e., interactions between the -COOH group and the surrounding water molecules). For the nonpolar case, however, the ∆EWW part dominates, resulting in a positive ∆ET value. Conclusions. In this study, solvent effects on the structure and configurations of functional groups attached to the ends of carbon nanotubes, are investigated. MD calculations were carried out for a CNT with typical polar and nonpolar functional groups. For each case, two different configurations of the functionals were considered (corresponding to cis and trans - forms) immersed in water. Energetics associated with the cis f trans process were calculated from the molecular dynamics simulation. We also estimated energy contributions arising from water-water, water-solute and solute-solute interactions separately and analyzed their respective roles in this process. Results obtained from this study indicate that functional groups with dipoles are more likely to be found in extended configurations in aqueous solutions, whereas nonpolar functionals are expected to be in their compact forms. Acknowledgment. This work was supported by NASA Ames Research Center through a prime contract (No. NAS299092) to Eloret. References (1) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (2) Garg, A.; Sinnott, S. B. Chem. Phys. Lett. 1998, 295, 273. (3) Okabe, Y.; Furugori, M.; Tani, Y.; Akiba, U.; Fujihira, M. Ultramicroscopy 2000, 82, 203. (4) Okabe, Y.; Akiba, U.; Fujihira, M. Appl. Surf. Sci. 2000, 157, 398. (5) Connolly, M. L. Science 1983, 221, 709. (6) Mitchell, A. S.; Spackman, M. A. J. Comput. Chem. 2000, 21, 933. (7) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. J. Phys. Chem. 1990, 94, 8897. (8) CRC Handbook of Chemistry and Physics, 78th ed.; CRC Press: Boca Raton, Fl, 1997, p. 6-3. (9) Walther, J. H.; Jaffe, R.; Halicioglu, T.; Koumoutsakos, P. J. Phys. Chem. B 2001, 105, 9980.
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