Dispersion of SWCNTs with Imidazolium-Rich Surfactants - Langmuir

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Dispersion of SWCNTs with Imidazolium-Rich Surfactants Antonello Di Crescenzo,† Sofie Cambré,‡ Raimondo Germani,§ Pietro Di Profio,† and Antonella Fontana*,† †

Dipartimento di Farmacia, Università “G. d’Annunzio”, Via dei Vestini, I-66100 Chieti, Italy Experimental Condensed Matter Physics Laboratory, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium § CEMIN, Centro di Eccellenza Materiali Innovativi Nanostrutturati, Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy ‡

S Supporting Information *

ABSTRACT: Starting from previous evidence on the crucial role of imidazolium ions, long alkyl chains, and aromatic rings in favoring the adsorption of surfactants onto carbon nanotube (CNT) walls, we have synthesized novel gemini surfactants with the aim to optimize and identify a reference structure for CNT dispersants. The efficiency of the novel surfactants has been evaluated, discussed, and compared with already well-investigated dispersants. The good affinity of the surfactants for the CNT sidewalls is highlighted by the presence of resonant van Hove absorption and highly resolved Raman and fluorescence spectra, while the strong hydrophobic interactions and favorable packing between the two alkyl chains of the investigated gemini surfactants and the CNT sidewalls ensure good CNT dispersion. Our results show no selectivity toward specific diameters/ chiralities, confirming the twin heads of imidazolium surfactants are pointed toward the bulk water, while the alkyl chains are arranged on the CNT walls, improving water solubility at the expense of potential selectivity.

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INTRODUCTION Carbon nanotubes (CNTs) are extended, aromatic graphene cylinders with unique electronic and mechanical properties determined by the chirality and diameter of the individual tubes.1,2 Unfortunately, substantial van der Waals attractions among the tubes lead to their aggregation in large and difficult to process bundles.3 Therefore, the ability to handle CNTs as individual entities and the understanding of their solution properties are essential for characterizing their intrinsic properties, for further development of their applications,3 and for their postsynthesis chirality separation.4−6 In order to improve the dispersibility of CNTs, both chemical functionalization of nanotube sidewalls7−10 and surfactant adsorption on nanotube surfaces11−20 have been exploited. The noncovalent solubilization with proper amphiphiles has been found to be the elected approach when mechanical, electrical, or optical properties of the CNTs are to be exploited,21−27 since the covalent functionalization results in strong changes of the nanotube’s intrinsic properties. It has been demonstrated that ionic liquids (ILs) based on the imidazolium cation can effectively disentangle single-walled carbon nanotube (SWCNTs) bundles.28,29 In a previous paper15 we have demonstrated that stable homogeneous aqueous dispersions of SWCNTs could be obtained by using a water-soluble long-chain imidazolium ionic liquid (hvimBr) above its critical micelle concentration (CMC). The © 2014 American Chemical Society

introduction in the polar head of aromatic groups enhances the affinity for SWCNTs but at the same time renders the molecule less water-soluble and more prone to self-assembling.30 Some of us have previously demonstrated16 that gemini surfactants31,32 are able to disperse a larger amount of SWCNTs with respect to the related single-tailed surfactant at concentrations far lower than their CMCs. Starting from these evidence, we present herein a comparison of the efficiency of a series of newly synthesized surfactants to solubilize SWCNTs, including a comparison to one of the best surfactants currently on the market, namely sodium deoxycholate (DOC),18 and their time-domain stability, in terms of resistance of the suspension to aggregation over time. On the basis of published results,16 our principal goal was to increase the aromatic moieties present in the amphiphilic structure of previously investigated surfactants {i.e., N-[p-(n-dodecyloxybenzyl)]-N,N,N-trimethylammonium bromide (pDOTABr), 2,5-bis(n-dodecyloxy)-1,4-bis(N,N,Ntrimethylammoniomethyl)phenyl bromide [pXDo(TA)2Br], and 2,5-dimethoxy-1,4-bis[N-(n-dodecyl)-N,Ndimethylammoniomethyl]phenyl bromide [pXMo(DDA)2Br]} without excessively affecting the water solubility of the new Received: January 15, 2014 Revised: March 13, 2014 Published: March 24, 2014 3979

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Chart 1. Structure of the Investigated Surfactants T1−T4

formation of small aggregates at the lowest investigated concentrations of surfactant and the comparable conductivity of these oligomers with that of mature aggregates. CMC values are 0.38, 0.40, and 0.02 mM for T1, T3, and T4, respectively (see Figures S1, S3, and S4 in the Supporting Information, respectively). Spectrofluorometric measurements by using pyrene as the probe on T2 sample allowed to determine a CMC of 0.32 mM. (see Figure S5 in the Supporting Information). It is interesting to note that the investigated T1 and T2 gemini surfactants do not display a CMC much lower than that of the investigated single-tailed T3. Vice versa, the CMC of T4 is 24 times lower than that of the corresponding single chain derivative, i.e., pDOTABr (0.51 mM).34 This evidence is not surprising because it points out that while pDOTABr is exactly the single-tailed analogue of T4, T1, and T2 are not simply obtained by covalently binding two molecules of single-tailed surfactant T3. Indeed, T1 is obtained by exploiting one phenyl group as the spacer, whereas T2 contains imidazolium ions substituted by long alkyl chains. Degrees of ionization, α, higher than those of the corresponding monocationic bromide have been determined for micellar solutions of T1 (0.39 for T1 and 0.26 for T3). On the other hand, α is relatively small for T4 (0.21) probably due to strong interactions with the counterions favored by the relatively short spacer.35 Vice versa in T1 the presence of two cationic headgroups and two bromide counterionsas compared to only one cationic headgroup and one counterion in monomeric surfactantassociated with the rigidity of the spacer and a consequent hydrophobic screening of the positive head groups do favor an higher ionization degree as is typical of gemini surfactants.36 Dispersing Ability of the Surfactants toward SWCNTs. The dispersing ability of the newly synthesized surfactants was compared to a well-accepted standard surfactant for SWCNT solubilization, namely sodium deoxycholate, which was found to be the best surfactant for SWCNT solubilization among

amphiphilic systems. For this reason the aromatic component was increased by introducing imidazolium rings as head groups, in substitution of the trialkylammonium groups, keeping the same hydrophobic tails and hopefully increasing the affinity for CNT sidewalls as previously demonstrated.15,28 The 3-[p-(ndodecyloxybenzyl)]-1-methylimidazolium bromide (T3) single chain surfactant was synthesized as comparison system for two imidazolium gemini surfactants: 2,5-bis(n-dodecyloxy)-1,4-bis(3-methylimidazolium)benzene bromide (T1) and 2,5-dimethoxy-1,4-bis[N-(n-dodecyl)-imidazolium methyl]benzene bromide (T2). The last investigated gemini surfactant, bis{2-N,Ndimethyl-N-[p-(n-dodecyloxybenzyl)]ammonium bromide− ethyl ether (T4), is characterized by the absence of any imidazolium ring and a less rigid spacer with respect to those containing phenyl moieties (i.e., T1 and T2) (Chart 1). The idea is that the synergistic effects of imidazolium rings, phenyl groups, and long alkyl chains could promote the dispersion of the CNTs, whereas rigidity and length of the gemini spacer33 could play a crucial role in enhancing the selectivity toward CNTs of different diameter or chirality paving the way for applications in CNT enrichment or fractionation.

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RESULTS AND DISCUSSION Characterization of T1−T4. The investigated gemini surfactants have a solubility in water comparable with that of the investigated single-tailed cationic analogues, the only exception being T2 which appears much less soluble in agreement with typical gemini surfactants. Solubilities at 25 °C of 0.125, 0.0005, 0.137, and 0.0492 M have been determined for T1, T2, T3, and T4 at room temperature, respectively. The CMCs and the degrees of ionizations were determined by conductometric studies for T1, T3, and T4 (see Figures S1−S4 in the Supporting Information). The conductometric technique was not sensible enough for CMC determination of T2 (see Figure S2 in the Supporting Information) probably due to the 3980

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increased electronic intertube coupling, which broadens and upshifts van Hove transitions for the surfactants under study2,17,38 and π-stacking of the dispersants on the tubes. However, compared with previously investigated imidazoliumbased surfactants30 and dodecyloxybenzyl-N,N,N-trimethylammonium analogues,16 a significant improvement of the intensity of van Hove transitions can be clearly observed, indicating the crucial role of the concurrent presence in the dispersant of methylimidazolium ions and dodecyloxy chains for ensuring a good adsorption of the surfactant onto the nanotube sidewalls. Indeed, while the role of long alkyl chains and phenyl rings in favoring SWCNTs dispersions is well recognized, very recently the effect of imidazolium rings has been demonstrated to be comparable with that of phenyl rings.30 While not yet reaching the solubilizing power of DOC, i.e., 55−66% of dispersed SWCNTs as compared to DOC, these new synthesized surfactants have certainly improved the efficiency of the previously investigated imidazolium surfactants.30 T2 demonstrated a much lower ability to disperse and exfoliate SWCNTs, as indicated by the lower intensity and severe broadening and further upshift of the absorption bands. This different capacity of T2 to adsorb onto SWCNTs sidewalls can be ascribed to the demonstrated30 tendency of amphiphilic molecules formed by imidazolium ions substituted with long alkyl chains to expose the imidazolium polar head and the proximal alkyl chain toward the bulk water. The detachment of the proximal alkyl chain and presumably of the spacer phenyl from the CNT surface reduce van der Waals and π−π interactions among the alkyl chain and the nanotube surface and the phenyl spacer and SWCNT walls, respectively. Therefore, it can be evidenced that surfactants containing Nmethylimidazolium head groups and n-dodecyloxy chains appear more effective than those containing N-dodecylimidazolium ions and methoxy-substituted phenyl rings.16 It is important to stress at this stage that previously published data39 have demonstrated that dodecyl appears to be the minimum alkyl chain length necessary to suspend SWCNT with a series of 1,2-dimethyl-3-alkyl-substituted imidazolium bromide; shorter chains do not yield suspended SWCNT whereas longer ones demonstrate a much higher efficiency. However, T1, T2, and T4 demonstrate to be relatively effective dispersants also below their CMC or at very low surfactant concentrations (see Figure S6 in the Supporting Information). This evidence confirms previously highlighted capacity of gemini surfactants16,40,41 to favor adsorption onto SWCNT sidewalls due to their characteristic compact alignment on the nanotube surface. Indeed, in the conventional surfactant, the single ammonium ion is exposed to the aqueous solution, favoring the dispersion of the nanotube in the aqueous medium, whereas the benzene ring and alkyl chain interact with the nanotube backbone by π−π stacking and van der Waals interactions, respectively. In the case of gemini surfactants, the dispersion is ensured by stronger hydrophobic interactions between the two alkyl chains and the CNT sidewalls and favorable packing, due to the capacity of the spacer to force the pair of ionic groups to reside in a less space-filling geometry relative to that of two conventional (i.e., single-chained) surfactant molecules. Nevertheless, the concentration that ensures a maximum dispersion of SWCNTs seems not to be correlated to the CMC or the ionization degree of the surfactant but rather to the water solubility of the dispersant, as already reported in a previous paper.30

many others and has been shown to isolate the SWCNTs into perfectly ordered micellar structures that provide a very homogeneous surrounding of the SWCNTs, resulting in enhanced spectral resolutions in optical spectroscopy.18 For this specific comparison with DOC, 1 mg of raw SWCNTs was dispersed in 5 mL of aqueous surfactant solutions. After 5 h of subsequent bath sonication, the suspensions were centrifuged for 30 min at 9500g, and the supernatant was collected. This minor centrifugation removes any undissolved species but, as shown by Raman spectroscopy (see below), is not sufficient to remove all the bundles present in the solution. The UV−vis−NIR absorption spectra of the so-obtained solutions (Figure 1) clearly show resonant van Hove absorption

Figure 1. UV−vis−NIR absorbance spectra of sonicated aqueous SWCNT dispersions prepared with 1 mM T1 (black), 1 mM T2 (blue), 1 mM T3 (red), and 1 mM T4 (green). For the sake of comparison, UV-vis-NIR absorbance spectra of aqueous SWCNT dispersions prepared with 24 mM DOC are presented in clear blue. The initial amount of SWCNTs was 1 mg in each sample.

of the SWCNTs superimposed on the 1/wavelength-like background arising from bundles and impurities scattering.37 Therefore, to quantify the amount of dispersed SWCNTs, absorption spectra were background subtracted (see Experimental Section), and the resonant van Hove absorption was integrated over a well-defined fixed wavelength range, i.e. 910− 1800 nm,18 corresponding to the first van Hove transition of the semiconducting SWCNTs present in the HiPco sample. Because of the shifts and line broadening, only the integration allows a proper comparison.37 We use the intensity of the van Hove transitions as a relative measure of SWCNT concentration. The spectra reported in Figure 1 were obtained from samples with a surfactant concentration above the CMC, where the surfactants were found to work at their best for SWCNT dispersion. As a matter of fact, lower CNT concentrations (∼50% for T1, 20% for T3) were obtained for surfactant concentrations below the CMC as confirmed by UV−vis−NIR spectroscopy (Figure S6 in the Supporting Information). All spectra of the T-samples have the same diameter distribution; thus, no chiral selectivity can be evidenced for any of the investigated surfactants. T1, T3, and T4 demonstrated to be quite good SWCNT dispersants as evidenced by the presence of resonant van Hove absorption (i.e., excitonic interband transitions for SWCNT chiralities; see Figure 1), the most effective dispersant being T1. Compared to the reference DOC sample, peak positions are red-shifted and line widths are severely broadened, indicative of 3981

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Figure 2. Fluorescence intensity of (A) 0.1 mM T1 aqueous solution (λexc = 303 nm); (B) 1 mM T2 aqueous solution (λexc = 302 nm); (C) 1 mM T3 aqueous solution (λexc = 275 nm); and (D) 0.1 mM T4 aqueous solution (λexc = 273 nm) in the absence (solid line) and in the presence (dashed line) of SWCNTs (5 μg/mL). The concentrations of T1−T4 were chosen with the aim of comparing samples with a similar SWCNTs content and the lowest possible surfactant concentration (see Tables S1 in the Supporting Information) without considering the CMC values (sample A contains a concentration of T1 below the CMC; samples B−D contain T2−T4 above the CMC).

The reduced affinity of T2 for the nanotubes is confirmed by ζ-potential measurements. The ζ-potential values for the SWCT dispersions prepared with T1, T2, T3, and T4 were 18 ± 3, 15 ± 1, 21 ± 2, and 25 ± 2 mV, respectively. The positive ζ-potential values measured for all the dispersions indicate that the positively charged surfactants do adsorb onto SWCNT surface, but among them, the T2 appears to be the less effective. The optimum sonication time was chosen by adding to 1 mg of SWCNTs the appropriate amount of surfactant and sonicating (at 35 kHz) the sample for different time intervals (i.e., from 1 to 6 h). As highlighted in Figure S7 in the Supporting Information, SWCNT absorption increased mainly in the first few hours of sonication, remaining essentially unchanged afterward (up to 6 h). Consequently, we chose a sonication time of 5 h, which can be considered a fair compromise between SWCNT dispersibility and sonicationinduced cutting of the SWCNTs. It is interesting to note that sonication appears essential for the investigated surfactants to act as dispersants. On the other hand, DOC, used as the reference surfactant (see Figure S8 in the Supporting Information), disperses CNTs also in the absence of any sonication, thereby resulting in undamaged, and

thus pristine, SWCNTs in the solution. To test the solubilizing power of our surfactants without sonication, we prepared a solution (1 mg/5 mL) with the best among the studied surfactants, T1 (see Figure S8 in the Supporting Information). Even using a much higher concentration of T1 (100 mM), the efficiency of DOC is more than 1 order of magnitude better without sonication even in terms of debundling. In order to assess whether the different behavior of the investigated surfactants could be attributed to a different adsorbing capacity of the surfactants toward the SWCNT sidewalls, we monitored the resulting dispersions by emission spectroscopy. The four surfactants are fluorescent and show a characteristic emission spectrum in the 200−500 nm range [λexc = 303, 302, 275, and 273 nm for T1, T2, T3, and T4, respectively]. A substantial decrease of fluorescence emission of these dispersants can be monitored when SWCNTs are present in the dispersion (see Figure 2). This decrease is due to the static quenching of the molecules whose chromophore group interacts by π−π stacking interactions with the carbon nanotube backbone. It seems worth noting that the quenching evidenced for the less efficient T2 is less marked with respect to the other surfactants, independent of the CMC value. This datum can be associated either with a lower adsorption capacity 3982

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Figure 3. RBM bands in the Raman spectra for SWCNTs dispersed in 1 mM T1 (black), T2 (red), T3 (green), T4 (blue) and DOC (clear blue) upon excitation at 785 nm (left graph) or 725 nm (right graph). Assignments are based on experimental Kataura plots. The RRS in all the spectra is normalized on the maximum intensity.

Figure 4. (A) 1D PL spectra excited at λ = 650 nm for all the investigated samples. Inset shows the normalized PL data for T1 and DOC samples in order to allow line widths and peak position comparison. Other panels show 2D PL-EX maps for T1, T4-, and DOC-dispersed SWCNTs. Predicted peak positions for different chiralities are indicated on the maps according to Cambré and Wenseleers.45

570 nm in Figure S9 of the Supporting Information). The RBMs in the range 200−260 cm−1 are due to isolated tubes that are in resonance with the specific wavelength, i.e., (9,7), (10,5), (11,3), and (12,1) for 785 nm excitation and (8,7), (8,6), (9,4), and (10,2) for 725 nm excitation. The Raman peaks located at 268 cm−1 (for excitation at 785 nm) or 304 cm−1 (for excitation at 725 nm) originate from (10,2) and (9,1) tubes, respectively, that only come into resonance when the SWCNTs are bundled, resulting in a red-shift of the electronic transitions.43 It is interesting that the most intense bundle peak is that of the less efficient dispersant T2; vice versa the less intense peak is that of the well-recognized good dispersant DOC.18 For the RBMs, the DOC sample also has the narrowest

or with a reduced area of contact between the surfactant molecules and SWCNTs, evidence of the presence of bundled SWCNTs. In order to better characterize the obtained surfactant-coated SWCNT dispersions, we performed resonant Raman scattering spectra (RRS) experiments at different laser wavelengths, corresponding to resonant excitation with the thickest and thinnest diameters of HiPco samples42 and compared them to the DOC sample. Absolute intensities cannot be compared because of different optical densities, and thus spectra were normalized to the maximum Raman intensity. The radial breathing mode vibrations (RBM) of SWCNTs upon excitation at 785 and 725 nm are reported in Figure 3 (see excitation at 3983

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The ζ-potential is often used as an index of the magnitude of electrostatic interactions between colloidal particles, and it is thus considered a further measure of the colloidal stability of a suspension. Colloidal solutions with a ζ-potential of less than −15 mV or more than 15 mV are particularly stable because of electrostatic repulsion interactions.46 The relatively high positive values measured further confirm the stability over time of dispersions made from T1, T3, and T4, whereas highlight the borderline stability of dispersions made from T2.

line widths, while in the T2 sample the lines are broad and therefore less-resolved [see (11,3) and (12,1) tubes]. The T1, T3, and T4 surfactants give a very nice spectral resolution, almost comparable to DOC, quite unexpected for aromatic surfactants18 and are only slightly blue-shifted with respect to DOC, which can be attributed to the specific binding interaction to the SWCNT walls. Analogous results are obtained at an excitation wavelength of 570 nm (see Figure S9 in the Supporting Information), which is particularly used to highlight the presence in the solutions of small diameters SWCNTs. For the small diameter SWCNTs, the resolution of the T-sonicated samples is even sufficient to differentiate between empty and filled tubes,44 a remarkable feat for aromatic surfactants. While RRS and absorption spectroscopy are relatively insensitive to the presence of bundles, the fluorescence− excitation (PL) of SWCNTs can be drastically quenched due to intertube interactions. Therefore, we use PL spectroscopy to investigate the exfoliation power up to individual SWCNTs. The PL of SWCNTs is very sensitive to the local environment of the tubes.17 The design of the T-molecules with exceeding aromatic components results in strong stacking on the nanotube walls as evidenced by previously quenching experiments of the surfactant’s PL (see Figure 2), thereby influencing the PL properties. First, the samples were excited at 650 nm, i.e., a wavelength where the highest emission is expected for HiPco SWCNTs, and the different samples were compared. Figure 4a reports these 1D spectra for the different samples. Fluorescence data evidenced a 6-fold higher PL from the DOC sample compared to T1 and T4 samples, whereas T2 (130 times lower PL) and T3 (12 times lower PL) highlight lower PL. When normalizing the PL intensities (inset of Figure 4a), it can be seen that DOCdispersed SWCNTs have narrower line widths and blue-shifted emission energies, indicative of lower interactions of the surfactants with the CNT walls. Second, for the samples that do show PL (T1 and T4) also 2D wavelength-dependent PL-EX maps were collected (Figure 4b−d). From these plots it is clear that there is no chirality-dependent solubilization with the T1 and T4 samples (i.e., relative intensities in the PL-EX maps among different chiralities do not change). This lack of selectivity may be due to the fact that, contrary to our expectations, T1 behaves very similarly to the flexible T4, and no evidence of increased π−π stacking can be highlighted for the former surfactant. The much lower PL quantum yield for the T-dispersed CNTs can be due to (1) the presence of bundles rather than isolated tubes and/or (2) the strong adsorption of the surfactants on the nanotube wall and consequently the strong interaction with the nanotubes. Raman spectra evidenced that bundles were mostly present in the T2 sample, in agreement with the lowest PL quantum yield observed for this specific sample. It is very likely that, in agreement with quenching experiments of the surfactants PL and lower peak position in 2D PL maps, surfactant stacking onto the SWCNTs framework is the cause of reduced PL. Stability of the Obtained SWCNT Dispersions. The dispersions appear relatively stable over time as demonstrated by measuring the absorption over the time of the SWCNTs reported in Figure S10−S13 of the Supporting Information. The reported graphs highlight that only T2-dispersed SWCNTs samples are not very stable.

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CONCLUSIONS In conclusion, the designed imidazolium-rich surfactants T1− T3 demonstrate to be good dispersants for SWCNTs. The dispersion protocol requires sonication whose mildness is evinced by the presence of empty tubes in the Raman spectra. Nevertheless, the sonication appears essential as no dispersion could be obtained without sonication unless the concentration of the dispersant is increased by 100-fold. Contrary to our expectations following the well-known affinity of imidazolium-based ionic liquids for CNTs,15,28,39 the presence into surfactant molecules of imidazolium ions ensures an increase of their water solubility and a consequent improvement of their CNTs dispersing ability. Optical absorption, Raman, and PL spectra provide an estimate of the quality of the SWCNT suspensions and indicate that T1, T3, and T4 allow to obtain good dispersion of SWCNTs although bundles still persist in the solutions. On the other hand, T2 appears much less effective, its dispersing ability depending mainly on suspension of agglomerated SWCNTs, as evidenced also by its short-lived dispersions. Unlikely Raman, absorption and PL data evidence no diameter-dependent solubilization with T-samples. Despite the good adsorption of the surfactants onto the nanotube sidewalls evidenced by improvement of the intensity of the van Hove transitions and PL data, there is no evidence of selectivity, the arrangement of the surfactants onto the nanotube surface being ruled by the tendency of the twin heads of imidazolium surfactants to expose toward the bulk water,16,30 thus reducing the potential selectivity in favor of solubility.

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EXPERIMENTAL SECTION

Materials. Pristine HiPco SWCNTs (lot #0339, 37% of iron content) were provided by Carbon Nanotechnologies, Inc., Houston, TX, and were used as received. All organic reagents and solvents were purchased from Sigma-Aldrich and were used without further purification unless otherwise specified. Sodium deoxycholate (DOC, 99%) was purchased from Acros Organics. Ultrapure Milli-Q water (Millipore Corp. model Direct-Q 3) with a resistivity of >18.2 MΩ·cm or deuterated water (D2O, Cortecnet, 99.89% D atom) was used to prepare all solutions. 2,5-Bis(n-dodecyloxy)-1,4-bis(3-methylimidazolium)benzene Bromide (T1). In a 500 mL round-bottom flask equipped with a reflux condenser protected by a CaCl2 tube were introduced 0.032 mol (20.0 g) of 1,4-dibromomethyl-2,5-didodecyloxybenzene, synthesized following the literature procedure,47 0.119 mol (9.45 mL) of freshly distilled imidazole and 200 mL of anhydrous CH3CN. The heterogeneous reaction mixture was refluxed for 24 h. After this time, the resulting pale yellow homogeneous solution was concentrated under reduced pressure to obtain a white solid which was then purified twice by crystallization from acetone−methanol mixture. The solid was filtered, washed several times with diethyl ether, and finally dried under vacuum at room temperature. Yield 93%, mp 160−162 °C, CMC = 0.38 mM (by conductivity). 1H NMR (CD3OD 400 MHz): δ = 0.92 (tr, 3J = 6.8 Hz, 6H, 2RCH3); 1.32−1.40 (m, 36H, 18CH2); 1.80 (m, 4H, −2CH2− in β); 3.94 (s, 6H, 2N+CH3); 4.08 (tr, 3J = 6.4 3984

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Hz, 4H, 2CH2−O); 5.41 (s, 4H, 2Ar−CH2−N); 7.30 (s, 2H, Ar); 7.57 (s, 2H, Im); 7.61 (s, 2H, Im); 9,00 (s, 2H, in C2 Im). 13C NMR (CD3OD 400 MHz): δ = 14.44 (CH3 chain); 23.73, 27.14, 30.31, 30.48, 30.52, 30.77, 30.80; 33.07 (−CH2− chain); 36.57 (N−CH3); 49.87 (N+(CH2)Ar); 70.21 (Ar−O−CH2−); 116.55 (o-Ar−O−CH2); 123.86 (C4 Im); 124.76 (C5 Im); 124.91 (Ar−CH2N+-ipso); 152.47 (O−Ar-ipso). HRMS (ESI-TOF) m/z: [M-Br] + Calcd for C40H68BrN4O2 717.451; Found 717.449. 2,5-Dimethoxy-1,4-bis-[N-(n-dodecyl)imidazolium methyl]benzene Bromide (T2). In a 500 mL round-bottom flask equipped with a reflux condenser were introduced 0.03 mol (10.0 g) of 1,4dibromomethyl-2,5-dimethoxybenzene, synthesized following the literature procedure:47 0.063 (15.0 g) of 1-dodecylimidazole48 and 150 mL of anhydrous CH3CN. Followed the same procedure of surfactant T1. Yield 90%, mp 204−206 °C, CMC = 0.35 mM (by conductivity). 1H NMR (CD3OD 400 MHz): δ = 0.92 (tr, 3J = 7.2 Hz, 6H, 2CH3); 1.30−1.36 (m, 36H, 18CH2); 1.89 (m, 4H, 2CH2 in β Im); 3.91 (s, 6H, 2OCH3); 4.24 (tr, 3J = 7.2 Hz 4H, 2CH2 in α Im); 5.41 (s, 4H, 2CH2−Ar); 7.31 (s, 2H, Ar); 7.64 (m, 4H, Im); 9.13 (s, 2H, in C2 Im). 13C NMR (CD3OD 400 MHz,): δ = 14.43 (CH3 chain); 23.74, 27.24, 30.08, 30.47, 30.55, 30.65 30.74; 31.19; 33.07 (−CH2− chain); 50.89 (N+(CH2)Ar); 56.81 (O−CH3); 115.61 (oAr−O−CH3); 123.57 (C4 Im); 123.89 (C5 Im); 125.19 (Ar−CH2N+ipso); 153.22 (O−Ar-ipso). HRMS (ESI-TOF) m/z: [M-Br]+ Calcd for C40H68BrN4O2 717.451; Found 717.449. 3-[p-(n-Dodecyloxybenzyl)]-1-methylimidazolium Bromide (T3). In a 500 mL round-bottom flask equipped with a reflux condenser were introduced 0.086 mol (38.8 g) of p-dodecyloxybenzyl bromide, synthesized following the literature procedure,15 0.095 mol (7.5 mL) of freshly distilled imidazole, and 200 mL of CH3CN. The mixture was left under reflux overnight. When TLC confirmed the complete disappearance of the reagent, the solution was concentrated by using the rotavapor until thick oil was obtained. The oil was then washed several times with diethyl ether to become a solid and crystallized from acetone. The solid was filtered, washed several times with diethyl ether, and finally dried under vacuum at room temperature. Yield 94%, mp 51−53 °C, CMC = 0.40 mM (by conductivity). 1H NMR (CD3OD 400 MHz): δ = 0.92 (tr, 3J = 6.8 Hz, 3H, R−CH3); 1.31 (m, 16H, 8CH2); 1.49 (m, 2H, CH2); 1.78 (m, 2H, CH2); 3.94 (s, 3H, N+(CH3)); 4.00 (tr, 3J = 6.8 Hz, 2H, R−CH2−O); 5.35 (s, 2H, Ar−CH2−N); 6.98 (d, 3J = 6.9 Hz, 2H, Ar); 7.40 (d, 3J = 7 Hz, 2H; Ar, in ortho to CH2−N+); 7.59 (d 3J = 7 Hz, 2H, Im); 8.98 (s, 1H, Im). 13C NMR (CDCl3 400 MHz): δ = 14.46 (CH3 chain); 23.75, 27.15, 30.32, 30.48, 30.50, 30.72, 30.76, 30.78, 33.08 (−CH2− chain); 36.57 (N−CH3); 53.76 (N+(CH2)Ar); 69.16 (Ar−O−CH2−); 116.26 (o-Ar−O−CH2); 123.50 (C4 Im); 125.18 (C5 Im); 126.80 (Ar−CH2N+-ipso); 131.43 (o-Ar−CH2N+); 161.44 (O−Ar-ipso). HRMS (ESI-TOF) m/z: [M-Br]+ Calcd for C23H37N2O 357.291; Found 357.295. Bis{2-N,N-dimethyl-N-[p-(n-dodecyloxybenzyl)]ammonium Bromide Ethyl Ether (T4). In a two-necked 500 mL round-bottom flask containing a magnetic stirring bar, equipped with a reflux condenser and a dropping funnel, were introduced 0.049 mol (17.4 g) of p-dodecyloxybenzyl bromide and 150 mL of CH3CN. The mixture was heated at reflux, while 0.024 mol (3.80 g) of bis-[2-(N,Ndimethylamino)ethyl] ether was added, under stirring, from the dropping funnel; a white solid precipitates at the first drops. The reaction mixture was refluxed for 1 h, and then methanol was added until complete dissolution of the solid. Cooling to room temperature separates a white crystalline solid. The solid was filtered, washed several times with acetone, and finally dried under vacuum at room temperature. Yield 94%, mp 205−207 °C, CMC = 0.02 M (by conductivity). 1H NMR (CD3OD 400 MHz): δ = 0.92 (tr, 3J = 7.2 Hz, 6H, 2R−CH3); 1.32 (m, 32H, 16CH2); 1.50 (m, 4H, 2CH2); 1.81 (m, 4H, 2CH2); 3.11 (s, 12H, 4N+CH3); 3.69 (m, 4H, CH2OCH2); 4.03 (tr, 3J = 6.4 Hz, 2H, R−CH2−O); 4.13 (m, 4H, 2 −CH2N+); 4.60 (s, 4H, 2 (Ar−CH2−N+)); 7.04 (d,d 3J = 6.8, 4J = 1.6 Hz, 4H, Ar); 7.52 (d 3J = 8.4 Hz, 4H; Ar in ortho to CH2−N+). 13C NMR (CDCl3 400 MHz): δ = 14.46 (CH3 chain); 23.75, 27.16, 30.29, 30.49, 30.51, 30.74, 30.77, 30.79, 33.09 (−CH2− chain); 50.91 (N+−CH3); 64.81

(N+(CH2)Ar); 65.79 (N+(CH2)CH2); 69.26 (Ar−O−CH2−); 69.95 (O−CH2−); 116.10 (o-Ar−O−CH2); 120.25 (Ar−CH2N+-ipso); 135.76 (o-Ar−CH2N+); 162.57 (O−Ar-ipso). HRMS (ESI-TOF) m/ z: [M-Br]+ Calcd for C46H82BrN2O3 791.550; Found 791.549. Characterization of Newly Synthesized Surfactants. For characterization of the surfactants melting points were determined with a Barloworld Scientific Stewart SMP3 apparatus. GC analyses were performed with an Agilent 6850 Series II Network GC instrument (column DB-35MS l 30 m, d 0.32 mm, film 0.25 μm). TLC was performed on silica gel on aluminum foil. 1H and 13C NMR were recorded with a Bruker AVANCE DRX 400 instrument using CD3OD as the solvent at 25.0 °C. Chemical shifts are given in ppm relative to the residual solvent signal. MS (ESI-TOF) analyses were performed on a Waters Xevo G2 Qtof. Preparation of SWCNTs Dispersions. Aqueous dispersions of SWCNTs were prepared by adding aqueous dispersant solutions (5 mL) of different concentrations to pristine SWCNTs (1 mg) in a glass centrifuge tube. The sample was then sonicated with an ultrasonic bath (Transsonic 310 Elma, 35 kHz) for several hours (see Results and Discussion). The obtained suspension was centrifuged for 30 min at 9000 rpm (9509g) using a Universal 32 (Hettich Zentrifugen) centrifuge in order to favor the separation of the supernatant aqueous solution from the precipitate. DOC reference sample was prepared using a 24 mM concentration of the surfactant in D2O and sonicated for 5 h using a bath sonicator (Bransonic 1510E-MTH, 42 kHz). Afterward, it was centrifuged for 30 min at 10 700 rpm (9500g) using a Sigma 2-16KCH centrifuge with swing-out rotor. NIR-PL Measurements. Wavelength-dependent fluorescence− excitation (PL-EX) experiments were performed in backscattering geometry in a home-built setup comprising a pulsed xenon lamp for excitation (Edinburgh Instruments, Xe900/P920) and an extended (up to 2.2 μm) liquid-nitrogen cooled InGaAs detector (OMAV:1024/LN-2.2). The backscattering geometry was chosen to avoid dilution of the samples. Spectra were corrected for detector sensitivity, lamp excitation power, and emission filter. Spectra were recorded with 5 nm steps in excitation and an instrumental resolution of 8 nm in excitation and a 25 nm resolution in emission wavelength. Raman Measurements. Resonant Raman spectra were recorded in backscattering geometry, using a Dilor XY800 triple spectrometer with liquid nitrogen cooled CCD detection, and subwavenumber spectral resolution. Resonant excitation was achieved using a tunable Ti:sapphire laser or the 568.2 nm originating from our krypton-ion laser. UV−vis−NIR Measurements. The absorption spectra of the suspended SWCNTs were recorded with a Varian Cary 100 Bio UVvis spectrophotometer (range 300−800 nm), a Jasco V-570 UV-visNIR spectrophotometer (range 300−1850 nm), or a Varian Cary 5E UV−vis−IR spectrometer in the range of 200−2500 nm, using 1 cm or 1 mm path length quartz cuvettes. To compare the efficiency of different surfactants to disperse the SWCNTs, we subtract a 1/ wavelength background which can be attributed to scattering in order to set the absorption at 1800 nm on zero and perform integration over the first optical transition of the semiconducting tubes, in the range of 910−1800 nm.18 This technique was used since, as evidenced in spectra recorded in Figure 1, positions and line widths of the SWCNT absorption peaks change upon varying the surfactant, thus affecting SWCNTs quantification when determinations are performed by using the widely used peak-to-valley ratios.37 The background, arising from bundles and impurities, depends not only on the dispersing ability of the surfactant (i.e., some surfactants preferentially solubilize the impurities instead of the isolated tubes) but also on the centrifugation conditions (i.e., longer and/or higher speed centrifugations will remove bundles and impurities at a faster rate than isolated SWCNTs). Therefore, background subtraction appears to be essential in order to monitor and quantify isolated SWCNTs. Nevertheless, we experimentally observed that the relative ratio between van Hove transitions and background hardly changes (example in Figure S7 of the Supporting Information) if samples are prepared from the same surfactant and under identical centrifugation and preparation protocols. This allowed us to use the absorption at a fixed wavelength, 3985

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Thermodynamics Properties. Int. J. Hydrogen Energy 2010, 35, 4543− 4553. (9) Singh, P.; Campidelli, S.; Giordani, S.; Bonifazi, D.; Bianco, A.; Prato, M. Organic Functionalisation and Characterisation of SingleWalled Carbon Nanotubes. Chem. Soc. Rev. 2009, 38, 2214−2230. (10) Herrero, M. A.; Prato, M. Recent Advances in the Covalent Functionalization of Carbon Nanotubes. Mol. Cryst. Liq. Cryst. 2008, 483, 21−32. (11) Britz, D. A.; Khlobystov, A. N. Noncovalent Interactions of Molecules with Single Walled Carbon Nanotubes. Chem. Soc. Rev. 2006, 35, 637−659. (12) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. High Weight Fraction Surfactant Solubilization of Single-Wall Carbon Nanotubes in Water. Nano Lett. 2003, 3, 269−273. (13) Wang, H.; Zhou, W.; Ho, D. L.; Winey, K. I.; Fischer, J. E.; Glinka, C. J.; Hobbie, E. K. Dispersing Single-Walled Carbon Nanotubes with Surfactants: A Small Angle Neutron Scattering Study. Nano Lett. 2004, 4, 1789−1793. (14) Bai, Y.; Park, I. S.; Lee, S. J.; Bae, T. S.; Watari, F.; Uo, M.; Lee, M. H. Aqueous Dispersion of Surfactant-Modified Multiwalled Carbon Nanotubes and Their Application as an Antibacterial Agent. Carbon 2011, 49, 3663−3671. (15) Di Crescenzo, A.; Demurtas, D.; Renzetti, A.; Siani, G.; De Maria, P.; Meneghetti, M.; Prato, M.; Fontana, A. Disaggregation of Single-Walled Carbon Nanotubes (SWNTs) Promoted by the Ionic Liquid-Based Surfactant 1-Hexadecyl-3-Vinyl-Imidazolium Bromide in Aqueous Solution. Soft Matter 2009, 5, 62−66. (16) Di Crescenzo, A.; Germani, R.; Del Canto, E.; Giordani, S.; Savelli, G.; Fontana, A. Effect of Surfactant Structure on Carbon Nanotube Sidewall Adsorption. Eur. J. Org. Chem. 2011, 5641−5648. (17) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes. Science 2002, 297, 593−596. (18) Wenseleers, W.; Vlasov, I. I.; Goovaerts, E.; Obraztsova, E. D.; Lobach, A. S.; Bouwen, A. Efficient Isolation and Solubilization of Pristine Single-Walled Nanotubes in Bile Salt Micelles. Adv. Funct. Mater. 2004, 14, 1105−1112. (19) Di Crescenzo, A.; Bardini, L.; Sinjari, B.; Traini, T.; Marinelli, L.; Carraro, M.; Germani, R.; Di Profio, P.; Caputi, S.; Di Stefano, A.; Bonchio, M.; Paolucci, F. Fontana, Surfactant Hydrogels for the Dispersion of Carbon-Nanotube-Based Catalysts. A. Chem.Eur. J. 2013, 19, 16415−16423. (20) Saint-Aubin, K.; Poulin, P.; Saadaoui, H.; Maugey, M.; Zakri, C. Dispersion and Film-Forming Properties of Poly(acrylic acid)Stabilized Carbon Nanotubes. Langmuir 2009, 25, 13206−13211. (21) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S.-W.; Choi, H.; Heath, J. R. Preparation and Properties of Polymer-Wrapped Single-Walled Carbon Nanotubes. Angew. Chem., Int. Ed. 2001, 40, 1721−1725. (22) Di Meo, E. M.; Di Crescenzo, A.; Velluto, D.; O’Neil, C. P.; Demurtas, D.; Hubbell, J. A.; Fontana, A. Assessing the Role of Poly(ethylene glycol-bl-propylene sulfide) (PEG-PPS) Block Copolymers in the Preparation of Carbon Nanotube Biocompatible Dispersions. Macromolecules 2010, 43, 3429−3437. (23) Di Crescenzo, A.; Aschi, M.; Fontana, A. Toward a Better Understanding of Steric Stabilization When Using Block Copolymers As Stabilizers of Single-Walled Carbon Nanotubes (SWCNTs) Aqueous Dispersions. Macromolecules 2012, 45, 8043−8050. (24) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. DNA-Assisted Dispersion and Separation of Carbon Nanotubes. Nat. Mater. 2003, 2, 338−342. (25) Kang, Y. K.; Lee, O.-S.; Deria, P.; Hoon Kim, S.; Park, T.-H.; Bonnell, D. A.; Saven, J. G.; Therien, M. J. Helical Wrapping of SingleWalled Carbon Nanotubes by Water Soluble Poly(p-phenyleneethynylene). Nano Lett. 2009, 9, 1414−1418.

i.e. 377 nm as reported in ref 15, as a facile and quick measure of the SWCNT content in the sample. ζ-Potential Measurements. The ζ-potentials of the SWCNT solutions were measured using a Zeta Plus apparatus (Zeta Potential Analyzer, Brookhaven Instruments Corporation). Dispersions of ca. 0.1 mg/mL SWCNTs and 1 mM surfactant were diluted 1:5 in order to perform the measurements.

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ASSOCIATED CONTENT

S Supporting Information *

Conductometric and spectrofluorometric CMC determinations; optimization of surfactant concentration; effect of sonication on dispersing SWCNTs; RBM spectra upon excitation at 570 nm for SWCNTs; stability of aqueous SWCNT dispersions; 1H and 13C NMR spectra, MS (ESITOF) spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Maurizio Prato (Nanophotonics Laboratory, Università di Trieste, Dipartimento di Scienze Chimiche) for provision of the pristine MWCNTs and SWCNTs and Dr. Samantha Reale and Prof. Nicoletta Spreti (Dipartimento di Scienze Fisiche e Chimiche, Università de L’Aquila) for performing MS spectrometry analyses. This work was carried out with support from the Universities ‘G. d’Annunzio’ of Chieti-Pescara and Perugia, and MIUR (PRIN 2010-11, prot. 2010N3T9M4). A.D.C. thanks Regione Abruzzo (Reti per l’alta formazione − P.O. F.S.E. Abruzzo 2007) and MIUR (PRIN 2010-11, prot. 2010N3T9M4) that funded his biennial postdoctoral fellowships. S.C. gratefully acknowledges the Fund for Scientific Research Flanders, Belgium (FWOVlaanderen), who provided her a postdoctoral fellowship.

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