Enhanced Solubilization of Carbon Nanotubes in Aqueous

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Enhanced Solubilization of Carbon Nanotubes in Aqueous Suspensions of Anionic-nonionic Surfactant Mixtures José Renato Alves da Cunha, Cristiano Fantini, Nadia Ferreira Andrade, Petrus A Alcantara Jr., Gilberto Dantas Saraiva, Antonio Gomes Souza Filho, Mauricio Terrones, and Maria Cristina dos Santos J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Nov 2013 Downloaded from http://pubs.acs.org on November 8, 2013

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The Journal of Physical Chemistry

Enhanced Solubilization of Carbon Nanotubes in Aqueous Suspensions of Anionic-nonionic Surfactant Mixtures J. R. Alves da Cunha a,b, C. Fantini c, N. F. Andrade d, P. Alcantara Jr.a, G. D. Saraivaa,e, A. G. Souza Filho d, M. Terrones b,f, and M. C. dos Santos* b,g a

b

Faculdade de Física, Universidade Federal do Pará, 66075-000, Belém, PA, Brazil.

Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, United States. c

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 30123970, Brazil.

d

Departamento de Física, Universidade Federal do Ceará, P.O.Box 6030 60455-900 Fortaleza, CE, Brazil.

e

Faculdade de Educação Ciências e Letras do Sertão Central, Universidade Estadual do Ceará, 63900-000 Quixadá, CE, Brazil. f

Department of Chemistry and Department of Materials Science and Engineering, The

Pennsylvania State University, University Park, Pennsylvania 16802, United States, and

ACS Paragon Plus Environment

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Research Center for Exotic Nanocarbons (JST), Shinshu University, Wakasato 4-17-1, Nagano 380 8553, Japan g

Instituto de Física, Universidade de São Paulo, 05508-090 São Paulo SP, Brazil.

ABSTRACT: Single-walled carbon nanotubes (SWCNTs) suspensions in aqueous solutions of sodium dodecyl sulfate (SDS) and saturated fatty acids (Cn) are studied. The quality of the dispersions is analyzed by photoluminescence spectroscopy (PL) as a function of the Cn chain length. Resonant Raman Scattering (RRS) measurements and Molecular Dynamics (MD) simulations were also carried out in order to study the effect of the surrounding medium on SWCNTs properties in suspensions. Both PL and RRS data indicate an increased individualization of SWCNTs in the dispersions for Cn's having an alkyl chain longer than SDS. MD simulations showed the formation of mixed Cn-SDS aggregates around a nanotube in water and a Cn binding energy to the nanotube wall that increases linearly with chain length. The enhanced solubilization of SWCNTs is thus interpreted in terms of the reduced electrostatic repulsion within the surfactant aggregates and the increased binding energy to the nanotube wall. Powders prepared by the evaporation of dispersions of Cn's and SWCNT bundles in ethanol were also studied by RRS in the radial breathing mode (RBM) frequency range. All the measured RBM frequencies exhibit a blue-shift (∆ω) with respect to the values obtained for pristine SWCNT powders. Remarkably, nanotubes with diameters smaller than 1.0 nm show ∆ω in the range 2.5 to 4.5 cm-1 while those having diameters larger than 1.0 nm exhibit ∆ω in the range 6.5 to 8.0 cm-1. MD simulations showed that large diameter nanotubes encapsulate Cn's into their cores thus justifying the increased hardening of the RBM mode.


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KEYWORDS: Carbon nanotubes suspensions; photoluminescence; Raman spectroscopy; molecular dynamics.

1. INTRODUCTION The successful preparation of stable single-walled carbon nanotube (SWCNT) dispersions represents an important step toward the effective use of these tubular systems in technological applications. As synthesized SWCNT samples consist of aggregates (bundles) containing nanotubes of different diameters and chiralities, lengths and electronic properties. These bundles are insoluble in water and in common organic solvents due to the enhanced polarizability induced by the high-aspect-ratio cylindrical shapes and hence the strong van der Waals interactions among the nanotubes. This property limits the material processability. In the past 10 years

various techniques have been developed to suspend individual SWCNTs in several

solvents by the so-called non-covalent functionalization.1 These techniques employ a wrapping agent, typically a surfactant2 or an organic polymer,3 that is dissolved in water or some organic solvent.

The

nanotubes

are

then

dispersed

under

strong

sonication

followed

by

ultracentrifugation. Some nanotube bundles are broken, permitting the insertion of the dissolved molecules in between the nanotubes and the subsequent wrapping around individual nanotubes or small bundles. Post-processing of these dispersions, for instance by density gradient ultracentrifugation4 or gel chromatography,5 have been used to separate nanotube populations by diameter, chirality, electronic structure, etc.6 Individualized nanotubes have been experimentally observed by several spectroscopic and imaging techniques allowing the identification of the chiral indices (n,m) that characterize them.7,8 RRS7 and PL8 have been intensively used for this purpose. While PL probes


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individualized or lightly bundled semi-conducting nanotubes, RRS can be used to identify populations of nanotubes in a sample regardless of state of aggregation or electronic property. These techniques rely on groups of optically allowed electronic transitions characteristic of a given (n,m) nanotube and consequently the wavelength of the probing light needs to be varied in order to detect a large population of chiralities of a given sample. The RRS and PL spectral features of SWCNTs depend on intrinsic and extrinsic factors such as nanotube diameter and chirality,9 chemical environment,10,11 aggregation state,12,13 length distribution14 and defect concentration within the nanotubes.15 Many ionic and nonionic surfactants are known to suspend nanotubes with variable performances.13,16,17 Mixtures of anionic surfactants have been used in density gradient ultracentrifugation in order to promote the separation of nanotubes by electronic type through manipulation of the relative concentrations of the surfactants.4 The ability of mixtures involving anionic and nonionic surfactants to suspend nanotubes is not known. According to reports on the behavior of this type of mixture at the solid/liquid interface,18 the adsorption of one of the surfactants is often enhanced by the addition of the other such that the density of the adsorbed mixed layer is increased with respect to the layers formed by each of the surfactants. If the same behavior is reproduced at the nanotube/water interface, one would expect a better coverage of the nanotube wall and an enhanced individualization of nanotubes. We report here on the successful preparation of stable suspensions of SWCNT in aqueous solutions of saturated fatty acids and sodium dodecyl sulfate (SDS). These dispersions were characterized by PL and RRS techniques, and the supramolecular structures formed by these surfactants adsorbed on SWCNTs in water were studied by Molecular Dynamics simulations (MD). Our results are consistent with an enhanced individualization of SWCNTs when the


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suspensions are prepared with fatty acids having more than 12 carbon atoms. Short chain fatty acids display the opposite effect: when added to a dispersion of SWCNT in aqueous solution of SDS, the concentration of individualized nanotubes drastically decreases. These results are rationalized in terms of the formation of fatty acid aggregates in water and the incorporation of SDS in these clusters. Their subsequent adsorption on SWCNT wall is favored when the fatty acid has a binding energy to the nanotube surface that is higher than SDS binding energy. In order to further investigate the interaction of fatty acids with SWCNTs, mixtures of SWCNT bundles and fatty acids in ethanol were prepared. After evaporation of the solvent, the resulting solids were characterized by RRS. MD simulations of these systems were carried out in the liquid. The measured RBM frequencies are blue-shifted with respect to the values observed for pristine nanotube powders, indicating a strong interaction between nanotubes and the fatty acid layer. The blue-shifts exhibit an unexpected dependence on nanotube diameter: there is a sharp increase of blue-shifts for nanotube diameters larger than 1.0 nm, suggesting the emergence of a second mechanism by which fatty acids interact with nanotubes. MD results on open-ended nanotubes demonstrate that nanotubes are filled with ethanol molecules and, for nanotubes whose diameters are larger than 1.0 nm, fatty acids also enter the nanotube cavity.

2. METHODS 2.1. Experimental. Highly purified HiPCO SWCNT bundles19 and saturated fatty acids (purity ≥ 98%) were obtained from Unidym and Sigma Aldrich, respectively. SDS (purity ≥ 98%) was purchased from Merck. All materials were used as received. The fatty acids listed below were used in two different sets of experiments: (i) First, as co-surfactants in SWCNTs dispersions with SDS in aqueous solution; (ii) Second, as dispersants of HiPCO SWCNTs in


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ethanol solutions. The saturated fatty acids used in this work, labeled as Cn, are: butyric acid (C4) (C3H7COOH), caproic acid (C6) (C5H11COOH), caprylic acid (C8) (C7H15COOH), capric acid (C10) (C9H19COOH), lauric acid (C12) (C11H25COOH), mirystic acid (C14) (C13H27COOH), palmitic acid (C16) (C15H31COOH), and stearic acid (C18) (C17H35COOH). Dispersions in water. A main dispersion was prepared by adding 0.03 mg/mL of SWCNTs to an aqueous solution of SDS at 1wt% (≈ 35 mM). The solution was sonicated for 100 min in five cycles of 20 min to avoid over-heating, using an ultrasonic tip processor operating at 40W of power. Nine samples of 10 mL were prepared from the main dispersion, eight of which received 110 mg of the fatty acids. These samples, labeled as SDS, C4, C6, etc, were sonicated for 20 min and centrifuged at 20000g for 1 hour. After centrifugation the supernatant phase was collected and used for optical measurements. PL measurements were performed using a fluorometer (Horiba Nanolog) equipped with the liquid N2-cooled InGaAs detector. The spectra were obtained for an excitation range from 550 to 750 nm and the emission range from 900 to 1400 nm. The resolutions in the excitation and emission ranges are 5 nm and 1 nm, respectively. Raman spectroscopy measurements were carried out with a micro-Raman spectrometer (RenishawInVia) with resolution better than 1.0 cm-1. Laser sources at 514 (20 mW) and 785nm (30 mW) were used for excitation. The laser beams were focused on the samples with a 10x objective. Dispersions in ethanol. Saturated fatty acids are soluble in ethanol. Dispersions were prepared by adding 200 mg of fatty acid and 0.6 mg of HiPCO SWCNTs to 20 ml ethanol. The mixtures were left under magnetic stirring for 24 hours, then sonicated for 20 min using an ultrasonic tip


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processor (40 W of power). They were left at ambient conditions until all the alcohol was fully evaporated. The remaining solid consists of SWCNTs bundles and fatty acids. Raman spectra were obtained at room temperature with a Jobin Yvon T64000 spectrometer equipped with a N2-cooled Charge Coupled Device (CCD) detection system, using an excitation laser at 532 nm (laser energy, 2.33 eV), with 0.3 mW power. A lens with a focal distance f = 20.5 mm and numeric aperture NA = 0.35 was used to focus the laser beam on the surface of the samples. The slit of the spectrometer was set for a resolution of 2.0 cm-1. 2.2. Computational Details. The supramolecular structures formed in the dispersions were studied by Molecular Dynamics simulations (MD) in the NPT (constant number of particles, constant pressure, constant temperature) ensemble. The force field pcff

20

was used as

implemented in Forcite module of Materials Studio computational package.21 This force field includes valence terms (bond stretch, bond angle bend and torsion angle potentials) as well as van der Waals and Coulomb interactions (typical formal charges of atoms also given). The NoséHoover-Langevin thermostat22,23 and the Berendsen barostat24 were used to simulate T=298 K and ambient pressure. Periodic boundary conditions were applied to the simulation boxes built to represent the two types of mixtures: (i) water-surfactant-Cn-nanotube, and (ii) ethanol-Cnnanotube. In the first case, a box of approximate dimensions 52Å × 28Å × 58Å, containing a semiconducting (10,6) nanotube in the center, was filled with 16 SDS, 16 C12 and enough water molecules to represent the ambient pressure. In this case the nanotube is infinite and its interior is not accessible to the other components of the box. SDS is built as an organic anion separated -

from Na+. The charge distribution on CH3(CH2)11OSO3 was obtained using the charge equilibration scheme QEq25 and kept fixed in all calculations. Fatty acids are weak acids and


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were treated in their neutral molecular forms (without proton dissociation). Both water and fatty acid molecules were attributed the atomic charges as parametrized in pcff force field. The second set of simulations consisted of two boxes of initial dimensions 32Å × 32Å × 120Å filled with ethanol molecules and 24 molecules of C12. A cylindrical cavity was created to accommodate either a 87 Å long (7,5) nanotube or a 87 Å long (10,6) nanotube, both with open ends. In all simulations the solvents were driven to thermal equilibrium prior to the addition of other components. After a re-equilibration of the simulation box, the production MD calculations lasted for 1 ns using a time step of 1 fs.

3. RESULTS AND DISCUSSION A PL map obtained from an ensemble of SWCNTs produces a pattern of bright spots, each representing an individual (n,m) semiconducting nanotube. The spots come from a resonant absorption at E22 transition and emission at E11 for a given (n,m) semiconducting nanotube.26,27 The link between a (E11, E22) spot on the PL map and a specific chirality (n,m) is one of the most fundamental results in carbon nanotube photophysics.2,26 It is known that the PL signal is quenched when SWCNTs are aggregated into bundles containing metallic nanotubes, which offer channels for nonradioactive recombination of photoexcited carriers. Small bundles composed exclusively of semiconducting nanotubes still present PL though not as bright as that coming from individual nanotubes.28,29


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Figure 1: Photoluminescence maps of SWCNTs dispersed in (a) SDS and (b) to (i) co-dispersed with fatty acids from C4 to C18. Intensities are normalized by the highest peak corresponding to (9,4) nanotube in C18. Figure1 shows the emission vs. excitation PL maps for the SWCNTs dispersed in SDS and codispersed with fatty acids. The graphs are displayed in the sequence of increasing fatty acid chain length, having SDS suspension in the first panel, Fig. 1a. The intensities were normalized by the brightest spot, corresponding to the (9,4) nanotube in C18 sample (Fig. 1i). Peaks were assigned to individual (n,m) chiralities according to data reported in the literature.30-32


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The mixtures containing C4 and C6 (Figs. 1b and 1c) exhibit very small but detectable photoluminescence signal. These are also the most soluble fatty acids in water. C8 has low solubility in water (68 mg/100 mL), nevertheless the PL spots observed from suspensions where it has been added (Fig. 1d) are brighter than those of SDS alone. From C10 (Fig. 1e) to longer chain fatty acids (Figs. 1f, 1g, 1h, 1i), all insoluble solids in water, the PL intensities increase as the chain length increases. C18 suspensions (Fig. 1i) display bright spots that are more than twice the intensity of the peaks observed in SDS suspensions.

Figure 2: Raman spectra for the SWCNTs dispersed in SDS, and SWNTs dispersed in SDS and co-dispersed with C4 and C16, using excitation laser at: 514 nm (a and b) and 785 nm (c and d).


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Raman spectroscopy measurements were also performed in order to determine the aggregation states of SWNTs in aqueous dispersions with SDS and fatty acids. Figs. 2(a) and 2(b) reveal the RBM spectra at 514 nm for SWNTs dispersed in SDS and using fatty acids C4 and C16 as codispersants. Figures 2(c) and 2(d) show the same spectra using a laser excitation of 785 nm. The (n,m) assignment used here was based on the characteristic patterns found in the experimental and theoretical data for the pairs (ωRBM, Eii) of RBM frequencies and transition energies following the procedures described elsewhere.31,32 When using both excitation wavelengths, the Raman intensities of the suspension containing C4 are drastically reduced with respect to the SDS suspension while that containing C16 reveals Raman intensities comparable to the spectrum of the SDS suspension. A small blue shift (1 - 2 cm-1) of the RBM frequencies is also observed in the suspensions when having fatty acids as co-dispersants. This indicates that the co-micelles formed by SDS and fatty acids have a different wrapping structure on SWCNTs than SDS micelles. Previous studies have shown that major aggregation of nanotubes in aqueous solution produces an enhancement in the Raman RBM intensities.33,34 In combination with our PL data, the above results indicate that SDS dispersions have a significant amount of nanotube bundles. Addition of small chain fatty acids results in further aggregation and precipitation of nanotubes, thus decreasing the amount of individually suspended nanotubes and resulting in weak PL intensities and RBM Raman signals. Long chain fatty acids, on the contrary, seem to progressively exfoliate the bundles as the chain length increases, as seen in the evolution of PL maps and Raman RBM modes. These dispersions are stable: PL measurements were repeated after 20 months of their preparation. No significant differences were observed.


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Figure 3: (a) Calculated binding energies (kcal/mol) of fatty acids on SWCNTs (10,6) and (7,5). (b) Snapshot of the simulation box at the beginning of the molecular dynamics simulation in the NPT ensemble and (c) after 1 ns. SWCNT, SDS, Na+ and C12 are represented by spheres where: grey = C, red = O, yellow = S, white = H and purple = Na+. The small red/white structures represent water molecules. We investigated the binding of C12 and SDS on nanotubes in the presence of water by MD calculations. C12 was chosen for having an intermediate chain length among the fatty acids used in the present study, and for having the same number of carbon atoms of SDS. We evaluated the


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binding energy of fatty acid molecules and SDS to the outer surface of two representative nanotubes of "small" and "large" diameters, the nanotubes (7,5) and (10,6), using the pcff force field described above in Methods. Figure 3a displays the variation of the binding energy of fatty acids to the surface of nanotubes as a function of the number of carbon atoms in the fatty acids. The binding energy increases linearly with the fatty acid chain length, as expected for an interaction based upon van der Waals forces. Curvature effects due to the small diameter of the nanotubes are not very pronounced: binding energies for C12 are 21.0 kcal/mol and 20.3 kcal/mol, respectively, for the (10,6) and (7,5) nanotubes. For comparison, SDS anion exhibits binding energies of 22.2 kcal/mol and 21.4 kcal/mol, respectively, in these nanotubes. Figures 3b and 3c depict snapshots of the simulation box at the beginning of the MD simulation and after 1.0 ns. The evolution is consistent with an aggregation of C12 and SDS on the nanotube wall and in the liquid phase. The coverage of the nanotube wall is not uniform due to the tendency of C12 and SDS to form aggregates. We expect that shorter chain fatty acids will also form aggregates in water and, due to the reduced binding energy to nanotubes when compared to SDS, they will not be able to bind to nanotubes already covered by SDS. In addition, these fatty acid aggregates are themselves target objects to be covered by SDS. The concentration of free SDS in water should decrease and trigger the rebundling of nanotubes, since the equilibrium condition around the nanotubes is altered. This is consistent with the small PL signal observed in the dispersions of SDS and short chain Cn's. Longer chain fatty acids can replace SDS on the nanotube wall due to the larger binding energy and, as a matter of fact, the experimental results discussed above confirm that small bundles of the main SDS dispersion are dissociated leading to an increased PL signal of individualized nanotubes. The efficient wrapping


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of individual SWCNTs by the co-micelles formed by SDS and long chain fatty acids is in agreement with the picture of a denser packing of adsorbed surfactants.18 This is made possible by the hydrophobic interactions between the two surfactants and a decreased electrostatic repulsion between anionic polar heads by the intercalation with non-ionic surfactant molecules.

Figure 4: (a) Raman intensities (arbitrary units) of RBM modes at 532 nm excitation wavelength: lower black line is from the HiPCO powder and colored lines are from nanocomposites of HiPCO and fatty acids C4, C8, C12, and C16. Dotted lines and numbers on top are the peak positions of RBM modes observed in HiPCO powder. (b) Frequency shifts of RBM modes of SWCNT-Cn composites with respect to observed HiPCO frequencies as a function of nanotube diameter. Dotted line is a plot of Eq. (2) (see text).

The mixtures of fatty acids and SWCNTs in ethanol were prepared to further investigate environmental effects in the carbon nanotubes physical properties. Fatty acids are soluble in ethanol, however the addition of HiPCO SWCNT powders to the ethanol solutions did not result in a homogeneous dispersion. Instead, a two-phase system was produced with the aggregation of


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the fatty acids around the nanotubes. The mixtures were then allowed to dry, and the resulting solids were studied by Raman spectroscopy. Figure 4a shows the Raman spectra of HiPCO SWCNT powders and some of the dried solids in the RBM region, using an excitation wavelength of 532 nm. The main peaks of the HiPCO SWCNT samples are located at 186, 213, 226, 236, 245, 263, 271 and 291 cm-1, as indicated in the figure. For the solid nanocomposite powders these peaks experience a blue-shift (∆ω = ωCn ωHiPCO) ranging from 3.0 to 7.8 cm-1, as can be seen in Fig. 4b. We did not try to assign chiral indices to those peaks. Instead, we associate them to specific diameters by comparison with other SWCNT samples. The nanotubes probed by the 532 nm excitation are clearly in the low diameter range: taking the fundamental relation for the RBM mode of an isolated SWCNT,35 ω = 227/d, where d is the nanotube diameter in nm, we conclude that d ≤ 1.3 nm for the measured peaks. This is the same range of diameters reported for CoMoCAT nanotubes.36 Assuming that RBM frequencies depend only on the diameter, subjected to an environmental effect described by the relation:

√1

(1)

where the parameter C accounts for the external effects on the nanotube vibrations, we used published data on CoMoCAT powders35 to obtain C ≈ 0.03 nm-2. By inverting Eq. 1 one obtains the associated diameters used to plot Fig. 4b. Also plotted in this figure is a curve given by the expression: Δ

√1 √1 0.03

(2)

which represents the frequency shift of the dried solids with respect to pristine HiPCO SWCNT powders for some constant C'. The dotted curve in Fig. 4b corresponds to C' = 0.07 nm-2.


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The striking behavior observed in Fig.4 is the systematic blue-shift of the RBM features for SWNTs with adsorbed fatty acids. The blue-shifts can be as high as 7.8 cm-1 at low frequencies and decrease at higher frequencies. While the shifts are distributed around the curve given by Eq. 2 for d < 1.0 nm, there is a clear deviation from this curve toward higher shifts for d > 1.0 nm. Also noteworthy is that the effects on the vibrational properties of nanotubes induced by fatty acids do not follow a pattern as a function of chain length, unless for C4 and C8 acids, which systematically produced the smallest and the highest shifts, respectively. C4 is very small and should interact more effectively with ethanol. C8, on the other hand, seems to strongly bind to nanotubes, a characteristic that has also been observed in the co-dispersions with SDS in water (see Fig. 1). The reason for this is unclear and will be addressed in a future work. Two sets of MD simulations were performed in order to investigate the structures formed by SWCNT and fatty acids in ethanol. We used the (10,6) (d = 1.11 nm) and (7,5) (d = 0.83 nm) nanotubes, both present in HiPCO SWCNT samples, which represent diameters above and below 1.0 nm. The tubes are 8.7 nm long and are open at both ends, in order to allow the molecules present in the simulation box to access the nanotube interior. Since the binding energies of fatty acids to the nanotube outer surface (Fig. 3a) are very similar for the nanotubes (10,6) and (7,5), there is a need to investigate other mechanisms that might be diameter dependent. Two simulation boxes with the same content in terms of C12 and ethanol distributions were prepared to accommodate the tubes within exactly the same volume. After 1 ns of simulation time on NPT ensemble, the structures evolved to the configurations shown in Figs. 5a and 5b. These figures display two cells so that we can see the inter-tube contents. It is clear that, given enough simulation time, the tubes will collapse due to the formation of fatty acid-nanotube networks. An indication of this behavior is seen in Fig. 5a, where the (10,6) tube tilted toward the neighboring


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tube but the applied periodic boundary conditions did not allow them to join. It is also clear that fatty acids interact by hydrogen bond formation besides the van der Waals interaction - note that several structures, over the tube walls or not, are seen in the geometry of a hydrogen bond. After 1 ns of simulation the structures do not seem to evolve toward an uniform coverage of the nanotube walls. However, an interesting feature is seen in the (10,6) tube: a cluster of fatty acids formed on the tube near the left hand side end where there is a C12 molecule inside the nanotube cavity. After few picoseconds of simulation, both tubes are completely filled: the small diameter (7,5) nanotube is filled by a line of ethanol molecules and the (10,6) nanotube has one C12 molecule and a double line of ethanol molecules. This is depicted in Fig. 5c, which shows both tubes along their axes and cross sections. The filling of nanotubes is (by capillary) expected to occur when sonication is used to produce suspensions. The ultrasound waves used to break the bundles and allow encapsulating agents to bind to nanotubes could also open the nanotube caps thus exposing their inner cavity to the suspension. This has been shown to occur in aqueous suspensions of nanotubes from several sources and with several surfactants.37,38 Moreover, it has been possible to separate nanotubes that remained closed from those with open ends due to the higher nanotube densities that results upon filling. Other properties change due to the filling of the nanotube cores39 including the frequency of the RBM mode, which blue shifts with respect to the empty nanotubes.37,38


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Figure 5: Supramolecular structures of SWCNT and C12 in ethanol from NPT molecular dynamics after 1,0 ns: (a) (10,6) nanotube and (b) (7,5) nanotube. Two unit cells are shown.


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SWCNTs are represented by the green structures, C12 molecules are represented by the overlapping spheres (gray = C, red = O, white = H) and the small gray/red/white structures are ethanol molecules. (c) Close-up of the nanotubes (10,6) (top) and (7,5) (bottom) from (a) and (b), showing the filling of nanotube cavities by ethanol molecules and C12 (green/red/white spheres) in the larger diameter tube. The access of molecules into the nanotube cavity obviously depends on geometric factors. Although a fatty acid molecule in its ground state geometry, which has a rod-like form, should exhibit a cross section comparable to the size of an ethanol molecule, in a good solvent the molecule adopts bent geometries. This is the reason why ethanol molecules fill the (7,5) nanotube but C12 does not - note that several C12 molecules accumulate at the tube open tip (Fig. 5b) but do not enter the nanotube cavity. The (10,6) nanotube has a larger diameter, which allows fatty acids to get inside its core and to adopt a geometry that optimizes the interactions with the nanotube internal surface and with the other molecules inside, as seen in Fig. 5c. The abrupt change in RBM frequency shifts for nanotube diameters over 1.0 nm in the fatty acidSWCNT composites could then be interpreted as being a consequence of nanotube filling. The encapsulation of organic molecules inside carbon nanotubes has been studied both theoretically and experimentally.39-42 In many of the experiments involving nanotube filling, SWCNTs are intentionally filled with molecules to form the so-called "peapods". The Raman spectral features of the peapods in the RBM region are observed to shift to higher frequencies due to the presence of molecules inside the nanotube cavity. These blue shifts were measured as 5 cm-1 in sexithiophene encapsulated in SWCNT40 and 6 cm-1 in C60 peapod.41 In our composite powders the combination of nanotube filling and fatty acid adsorption at the outer surface shifts RBM modes of large diameter nanotubes by up to ≈ 8 cm-1.


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4. CONCLUSION In summary, SWNTs dispersed in water with SDS and co-dispersed with different saturated fatty acids were studied by PL and Raman spectroscopies in order to probe the interaction of the nanotubes with those species. The PL intensities indicate that short chain fatty acids provoke rebundling and precipitation of nanotubes while the long chain lengths species really act as a codispersant, enhancing individualization of SWCNTs dispersed in water. RRS spectroscopy data also support this conclusion. MD simulations showed the formation of fatty acids and SDS aggregates on the nanotube surface and within the liquid phase. Fatty acids having carbon chains shorter than SDS also have lower binding energies to the nanotube wall, then their aggregates compete with nanotubes for SDS and nanotubes rebundle. Calculated binding energies of longer chain fatty acids to nanotubes are higher than those of SDS. A denser packing of surfactants around SWCNTs is thus allowed by the intercalation of SDS and long chain Cn, leading to a reduction of electrostatic repulsion between anionic polar heads of SDS and more stable surfactant aggregates. As a consequence, small SWCNT bundles can be dissociated. The Raman spectra of SWCNTs adsorbed with fatty acids showed blue-shifts of the RBM frequencies with respect to RBM frequencies of pristine SWCNT powders. These blue-shifts are generally explained in terms of van der Waals forces of the adsorbed layer acting on the nanotube walls and have been accounted for by the expression Δ

√1 1,

where the constant C depends on the specific environmental effect on the nanotube properties. RBM Raman shifts observed in these dried powders showed that, for a given nanotube diameter, variation of fatty acid leads to a variation in the shift, which is expected to occur due to the different arrangements these molecules must adopt within the nanotube bundles. However, the blue-shifts depend on nanotube diameter in a unexpected form: there is a sharp increase of the


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blue-shift for diameters around and above 1.0 nm. MD simulations performed on these systems showed that nanotubes mixed with fatty acids in alcohol are filled with a line of ethanol molecules if d < 1.0 nm. Otherwise, if d > 1.0 nm, fatty acid molecules and a double line of ethanol molecules fill the nanotube core. We propose that the different fillings give rise to the observed sharp increase of blue-shifts in RBMs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Acknowledgements. JRAC thanks FAPESPA for a grant along the realization of this work and a fellowship from CAPES. Financial supports from FAPESPA (Proj. n. 112/2008), and CNPq (INCT NanobioSystems, Proc. n. 573925/2008-9) are gratefully ackowledged. CF acknowledges financial support from FAPEMIG and VALE S.A. MT thanks JST-Japan for funding the Research Center for Exotic NanoCarbons, under the Japanese regional Innovation Strategy Program by the Excellence and the Center for 2-Dimensional and Layered Materials at Penn State University. MCS thanks FAPESP for financial support and the Research Computing and Cyber-infrastructure unit of Information Technology Services at Penn State University for providing access to the advanced computational facilities and services.

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Graphic Table of Contents


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Enhanced Solubilization of Carbon Nanotubes in Aqueous

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