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New Concepts at the Interface: Novel Viewpoints and Interpretations, Theory and Computations
Release of Retained Single-Walled Carbon Nanotubes in Gels Lili Zhou, Xiaofeng Liu, and Huaping Li Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018
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Langmuir
Release of Retained SingleSingle-Walled Carbon Nanotubes in Gels Lili Zhou, Xiaofeng Liu, Huaping Li* Atom Optoelectronics, 440 Hindry Avenue, Unit E, Inglewood, California 90301, United States KEYWORDS Single-Walled Carbon Nanotubes, Surfactant Exchanges, Morphologic Changes, Pseudo 15-Crown-5 Ether Microstructures, Strain Induced Gel Relaxation
ABSTRACT: The separation of single-chirality, even enantiomeric single-walled carbon nanotubes (SWCNTs) has been well established using gel permeation chromatography. The successful SWCNTs separation has been considered as the selective adsorption and desorption of specific SWCNTs on the porous sites of Sephacryl gels. This work reports two nonspecific releases of SWCNTs retained on Sephacryl gels: (1) the considerable amount of SWCNTs was eluted out using low concentration SDS condition solution (0.5 weight%) from the gels exclusively eluted with high concentration SDS eluting solution (5 weight%) after stocked overnight, (2) the retained SWCNTs in Sephacryl gels can be eluted out using low concentration SDS condition solution (0.5 weight%) after stocked overnight without any treatments. Inspired by extracellular matrix systems, these releases are attributed to the strain induced gel relaxation. The roles of surfactants, especially SDS on the retention and releasing of SWCNTs on Sephacryl gels were discussed on the basis of spectral dilution and titration experiments using single-chirality (6,5) SWCNT as the probe.
Introduction Surfactants are widely used for dispersing carbon nanotubes.1,2 Surfactants are chemicals containing hydrophilic and hydrophobic portions to disperse themselves in water at low concentrations and to form micelles at threshold values (commonly as critical micelle concentration, CMC). For these hydrophilic and hydrophobic groups linked with flexible spacers, the formed spherical micelles can be elongated to ellipsoid, rod-like structures, and even to secondary complex structures like bilayer lipids with further increased concentrations. For these hydrophilic and hydrophobic groups distributed in two faces of rigid skeletons, the formed micelles are relatively stable, hardly to be altered.3 The common surfactants with rigid cores reported to disperse carbon nanotubes are steroid derivatives such as sodium cholate (SC) and sodium deoxycholate (DOC). These surfactants with rigid cores exhibit the stronger dispersion ability for carbon nanotubes than those with flexible linkers,4 ascribed to the higher zeta potential difference between pure surfactants and surfactantdispersed carbon nanotubes.5,6 The rigid steroid backbones are suggested to compactly pack on the sidewalls of carbon nanotubes.7,8 SC was considered to form the thinner hydrodynamic layers around carbon nanotube sidewalls with less moieties than DOC based on analytic ultracentrifugation results.4,9 This observation was further supported by the differentiated photoluminescence intensities of carbon nanotubes.9 The mostly employed flexible surfactant for dispersing carbon nanotube is sodium dodecyl sulfonate (SDS). The monodispersed single-walled carbon nanotubes (SWCNTs) using SDS revealed the band-gap luminescence of SWCNTs,10,11 and enabled the purification of single-chirality,12 even enantiomeric SWCNTs13,14 through gel permeation chromatography.15 Based on the band-gap luminescence of SWCNTs, SWCNTs were considered to be wrapped with rod-like SDS micelles.10,16 High resolution transmission electron microscope images show hemispheric morphologies of SDS on carbon nanotubes.17,18 While, the small angle neutron scattering results implied random structureless attachment of SDS on SWCNT surfaces.19,20 These morphologic structures of
SDS on carbon nanotube sidewalls are possible,21 and correlated to dispersed SDS molecules, spherical micelles, and rod-like micelles.22 In gel permeation chromatography, Strano et al proposed that the SDS on SWCNTs was completely stripped by Sephacryl gels.23 Recently, Herben et al considered partially and completely unsheathed SWCNTs as the active reactants to adsorb on specific sites.24 Ziegler et al reported the strong bonding of random structureless SDS on SWCNT surfaces based on nuclear magnetic resonance (NMR) investigations.25 These disparities indicate the morphologies of SDS around carbon nanotube surfaces in gel chromatography are still under debate. Moreover, the effect of SDS on the retention of SWCNTs in Sephacryl gels is not well understood. In this contribution, we use single-chirality (6,5) SWCNT to probe the SDS morphology changes by diluting SDS dispersed (6,5) SWCNT or by titrating SDS dispersed (6,5) SWNCT with SC. Based on the spectral changes of the first van Hove Singularity transition (S11) absorbance of (6,5) SWCNT, the different morphologies of SDS on (6,5) SWCNT under different concentration ranges are elucidated. In combination with newly observed automatic release of retained (6,5) SWCNT in Sephacryl gels, a new retention mechanism of SWCNTs in Sephacryl gel is proposed to interpret the carbon nanotube separation using gel permeation chromatography. In this research, we conducted the Visible-Near Infrared (VisNIR) absorption spectra of (6,5) SWCNT dispersed by SDS, SC and DOC with varied concentration through dilution, respectively. The Vis-NIR absorption spectra of (6,5) SWCNT dispersed in SDS aqueous solution titrated with SC, or vice versa, were measured. The observed spectral changes are correlated to the different morphologies of surfactants adsorbed on the sidewall of (6,5) SWCNT on basis of literature reports on high resolution transmission electron microscope images,17,18 small angle neutron scattering simulation,19,20 NMR measurements,25 and theoretical computations.21 These morphologies of surfactants are used to understand the retention and release of SWCNTs in Sephacryl gels. Experimental Section
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Langmuir Materials: Sodium dodecyl sulfate (SDS), sodium cholate (SC), sodium deoxycholate (DOC) were purchased from Sigma-Aldrich and used as received. Sephacryl gel S200 was purchased from GE health. Single-walled carbon nanotubes (SWCNTs) raw powder was produced by Rice University Mark III high pressure carbon monoxide reactor using less catalyst in a yield of 1 gram per hour. The raw SWCNTs were used for the extraction of (6,5) SWCNT using gel permeation chromatography. The (6,5) SWCNT stock solutions dispersed in SDS, SC and DOC were obtained by eluting the trapped (6,5) SWCNT in Sephacryl gels with according SDS, SC and DOC concentrations, respectively. The concentration variations of SDS, SC and DOC dispersed (6,5) SWCNT were obtained by diluting their stock solutions respectively. The titration experiments were conducted by adding weighted surfactant powders. All concentration units are weight percentage unless there are specific descriptions. Measurement: All Visible Near-Infrared absorption spectra of (6,5) SWCNT dispersed in different surfactant systems were measured on NS3 Applied Nano Spectralyzer at ambient temperature without adding acid or base chemicals.
(FWHM) of 29 nm, at 980.5 nm (FWHM: 26.5 nm) for 2% SC, and at 982 nm (FWHM: 26.5 nm) for 2% DOC, respectively. These spectral disparities infer the different microenvironments of (6,5) SWCNT in SDS, SC and DOC.30,31 The microstructures of SDS might be significantly different from those of SC and DOC (which are structurally similar) with 2% concentration in aqueous solution.32
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Results and Discussion The chemical structures of sodium dodecyl sulfate (SDS), sodium cholate (SC), sodium deoxycholate (DOC) and (6,5) single-walled carbon nanotube ((6,5) SWCNT) are presented in Figure 1 A. SDS (molecular weight: 288.4 g/mol) molecule is a hydrophobic 12 carbon n-alkyl chain tethered to a polar sulfate (OSO3-) to be dissolved in aqueous solution as free molecule at low concentration. When SDS concentration reaches to the threshold value, socalled critical concentration micelle (CMC, 8 mM, 0.23 weight%),26 the hydrophobic n-alky chains start to aggregate and to form spherical micelles. The further increase of SDS concentration leads to the morphological change from spherical to ellipsoidal micelles and eventually to rod-like micelles.22 The flexible nature of SDS results in the tunability and control of the morphological structures. SC (molecular weight:430.6 g/mol) is rigid molecule with a flattened ellipsoid shape comprising of a hydrophobic steroid core and three hydrophilic hydroxyl groups and tethered to sodium carboxylate through a propyl chain to improve the solubility in aqueous solution and to promote the formation of different micelles and aggregates. The primary micelles of SC are 2-10 molecules with hydrodynamic radii of 1-2 nm. These primary micelles of SC can form secondary micelles through hydrogen bonding when the concentration of SC is high enough. Slightly different from SC, DOC (molecular weight: 414.6 g/mol) only has two hydrophilic hydroxyl groups, showing declined solubility and larger hydrodynamic radii with more molecules in comparison to SC. Their CMCs are 5 mM (0.21 weight%) for DOC and 11 mM (0.47 weight%) for SC respectively.27 The (6,5) SWCNT aqueous solution was obtained from the separation of high pressure carbon monoxide conversion single-walled carbon nanotube (HiPCO) using gel chromatography.28,29 The separation and characterization details are reported previously.28,29 To prepare for (6,5) SWCNT aqueous solution dispersed with SDS, SC, and DOC surfactants, the (6,5) SWCNT was trapped in Sephacryl Gel S-200, then 5% SDS, SC and DOC were used to wash out (6,5) SWCNT solutions, respectively. These solutions were diluted for low concentration measurements. The Vis-Near Infrared (NIR) absorption spectra of (6,5) SWCNT dispersed in aqueous solutions using 2% SDS, SC and DOC are illustrated in Figure 1B. The inset shows the photographic image of 1 liter of (6,5) SWCNT solution. The characteristic S11 and the second van Hove Singularity transition (S22) peaks of (6,5) SWCNT are observed at near infrared and visible area, respectively.11 As shown in Figure 1B, the sideband of S11 absorbance is mixed with S11 peak of (6,4) SWCNT impurity. The S11 peak of (6,5) SWCNT appears at 978 nm for 2% SDS with full width at half maximum
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Figure 1. A) Chemical Structures of SDS, SC, DOC, and (6,5) SWCNT. B) Vis-NIR absorption spectra of (6,5) SWCNT dispersed in 2% SDS (red), SC (green) and DOC (blue) respectively. The inset shows 1 liter of (6,5) SWCNT solution in 2% SDS. S11 spectral shifts of (6,5) SWCNT in varied surfactant concentrations The Vis-NIR absorption spectra of (6,5) SWCNT aqueous solutions were measured in a variety of SDS concentrations using NS3 Applied Nano Spectralyzer. Figure 2A shows the normalized S11 peak absorbances of (6,5) SWCNT at varied SDS concentrations from 0.05% to 4%. The feasibility of (6,5) SWCNT bundling can be ruled out due to no broaden absorbances that observed. This indicates there was no (6,5) SWCNT aggregation during measurement process. The (6,5) SWCNT at low SDS concentration could form bundles with long period. The plot of varied peak wavelengths against various SDS concentrations is shown in Figure 2B. The results show the peak absorption shifted from 983 nm at 0.05% SDS to 978 nm at 0.2% SDS when more SDS molecules were bounded onto SWCNT surfaces. This trend was also observed in zeta-potential and conductivity measurements.5,6 In this case, SDS adsorbed onto (6,5) SWCNT surfaces through van der Waals forces between SDS hydrophobic tails and SWCNT sidewalls.33 The hydrophilic head (SO3-Na+) were towards the water for dispersing SWCNT (Figure 3). Though the dynamic exchanges between free SDS and SDS adsorbed on SWCNT surfaces are viable, the adsorption rate of SDS to SWCNT sidewalls is expected to be much higher than the desorption rate of
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SDS from SWCNT surfaces, leading to the strong bonding for remaining thermodynamically stable solution.34,35 Such structureless binding of SDS molecules on SWCNT sidewalls was probed using small angle neutron scattering technology.19,20 The increased SDS molecules on SWCNT surfaces limited their exposure to water molecules, thus caused the solvatochromism of SWCNT shifting the S11 absorbance peak at 983 nm to the blue at 978 nm.30,31 It is worthy to note that the S11 absorbance peaks measured during dilution process might have some deviations due to disturbed SDS molecules or hot SDS molecules. When the concentration of SDS is above CMC, for instances, 0.23%, one can observe S11 absorbance of (6,5) SWCNT shifted to 982 nm with SDS concentration to 0.5%. The phenomenon indicates (6,5) SWCNT exposed to the enhanced polar microenvironments. It is well-known that SDS can form spherical micelles in solution and hemispherical morphologies on graphite and graphene surfaces when its concentration is above CMC.17,22 In such concentration range, the similar hemispherical morphologies could be formed on (6,5) SWCNT sidewalls (Figure 3),17,18 pushing the polar heads of SDS close to (6,5) SWCNT surfaces and leading to the redshifted S11 peak absorbance. In these hemispherical micelles, SDS moieties indirectly contact with SWCNT surfaces would behave as these free spherical micelles for molecular exchanges in solution. Further increasing SDS concentration (>2%), SDS spherical micelles elongated to ellipsoid micelles and rod-like micelles in solution. While, rod-like morphologies are expected to form on (6,5) SWCNT sidewalls with hydrophobic tails toward (6,5) SWCNT surfaces (Figure 3),16 shifting S11 peak absorbances to the blue again (close to 977 nm). In these rod-like structures, the weak binding between SDS molecules and SWCNT surfaces make these rod-like SDS micelles have same thermodynamic properties for molecular exchanges with or without incorporating SWCNT.16,17,21,25 These S11 spectral results clearly elicit the flexible nature of SDS with morphological changes driven by concentration variation. The tunability and control of SDS morphologies enable low concentration SDS (2%) aqueous solution.12-15 The Vis-NIR absorption spectra of (6,5) SWCNT aqueous solutions were measured in a variety of SC concentrations using NS3 Applied Nano Spectralyzer. Figure 4A shows the normalized S11 peak absorbances of (6,5) SWCNT at varied SC concentrations from 0.005% to 5%. Once again, no obvious absorbance broadening was observable ascribed to (6,5) SWCNT 1.2
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Figure 2. A) The normalized S11 absorbances of (6,5) SWCNT dispersed in SDS with various concentrations shown in B). These different SDS concentrations were obtained by diluting the stock solution in SDS. B) The S11 peak wavelengths of (6,5) SWCNT were plotted against varied SDS concentrations.
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Figure 3. Diagram illustration of molecular exchanges of different morphologic SDS (random structureless attachment (left), hemispheric morphology (middle), and rod-like morphology (right) on (6,5) SWCNT sidewall with free SDS and free SDS micelles under different concentration ranges. bundling. The plot of S11 peak wavelengths of (6,5) SWCNT against various SC concentrations is shown in Figure 4B. One can observe that the S11 peak wavelength shifted from 983 nm with 0.005% SC concentration to 980 nm with SC concentrations range from 1% to 5%. Due to its structural nature, SC tends to form aggregates at low concentrations smaller than CMC (0.47%).27 With highly hydrophobic SWCNT surfaces, SC would desire to bind on the sidewalls of (6,5) SWCNT to assist the dissolution of SWCNT based on thermodynamic principles.35 The S11 peak absorption at 983 nm of (6,5) SWCNT in 0.005% SC implies the insufficient surface coverage of SC on SWCNT sidewalls unsheltered to polar microenvironments resulting in the first optic transition S11 of (6,5) SWCNT at red-shifted wavelength. With increase SC concentration to 1%, more SC molecules adsorbed onto SWCNT sidewalls with enhanced surface coverages and less exposure areas to polar environments, exhibiting slightly blue-shifted S11 peak absorption of (6,5) SWCNT solution. The concentration driven surface coverage changes suggest that SC exchanges among free SC molecule, SC aggregates, and SWCNT bounded SC are active in aqueous solution. When the SC concentration is over the threshold value, usually CMC increased with SWCNT,25 the SC micelles with rigid helix structures are suggested to form through hydrogen-bonding between carboxylate and hydroxyl groups of different SC moieties, and too thermodynamically stable to be altered by concentration.32 Different from
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flexible SDS morphologic changes, the molecular exchanges among free SC, SC micelles and SC micellar morphologies on SWCNT surfaces would be deactivated unable to rearrange SC for more compact coverage on SWCNT surfaces. This cooperative binding of SC on (6,5) SWCNT sidewalls was also revealed by Hertel et al. from an abrupt photoluminescence peak shifts by diluting SC dispersed (6,5) SWCNT.37
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Figure 4. A) The normalized S11 absorbances of (6,5) SWCNT dispersed in SC with various concentrations shown in B). These different SC concentrations were obtained by diluting the stock solution in SC. B) The S11 peak wavelengths of (6,5) SWCNT were plotted against varied SC concentrations. Similarly, the Vis-NIR absorption spectra of (6,5) SWCNT aqueous solutions were measured in a variety of DOC concentrations using NS3 Applied Nano Spectralyzer. Figure 5A shows the normalized S11 peak absorbances of (6,5) SWCNT at varied DOC concentrations from 0.005% to 5%. The measured S11 absorbances are perfectly overlapping. No (6,5) SWCNT bundles can be suggested based on spectral features. The plot of S11 peak wavelengths of (6,5) SWCNT against various DOC concentration is displayed in Figure 5B, showing DOC concentration independent level line. These observations, remarkably different from absorption spectral changes of (6,5) SWCNT in SDS and SC aqueous solutions, imply that the thermodynamically molecular exchanges in SDS and SC systems could not be applicable to DOC system. Slightly different from SC, DOC molecules are more hydrophobic due to one less hydroxyl group. This minor structural difference makes the thermodynamically stable SWCNT bounded DOC, energetically disfavored to exchange molecules with free DOC, DOC aggregates, or DOC micelles. Once SWCNT is dispersed, in any concentration, the SWCNT surfaces will be covered with sufficient DOC moieties that could not be reoriented. This attrib-
Figure 5. A) The normalized S11 absorbances of (6,5) SWCNT dispersed in DOC with various concentrations shown in B). These different DOC concentrations were obtained by diluting the stock solution in DOC. B) The S11 peak wavelengths of (6,5) SWCNT were plotted against varied DOC concentrations. Molecular Exchanges of SDS and SC on (6,5) SWCNT Surfaces Surfactant exchanges on SWCNTs were reported to quantify the binding strengths using either absorption or photoluminescence spectral shifts.38-40 For instances, Jagato et al. reported the recognition sequence of deoxyribonucleic acid (DNA) to (6,5) SWCNT based on absorption spectral changes after surfactant exchange with sodium dodecyl benzene sulfonate (SDBS).38 Ju et al. published the binding constants of SDS, SDBS, SC on SWCNTs by investigating the surfactant exchanges with flavin mononucleotide (FMN) wrapped SWCNTs using both absorption and photoluminescence spectral shifts.39 Ju et al. further estimated the absolute enantiomeric excess of SWCNTs from photoluminescence spectral titration of FMN-SDBS cosurfactant dispersed SWCNTs using SDBS.40 To investigate the thermodynamic exchanges of SDS and SC on (6,5) SWCNT surface, the absorption spectral changes of (6,5) SWCNT in 2% SDS by adding SC in the range from 0.05% to 10%, and (6,5) SWCNT in 0.05% SC by adding SDS in the range from 0.05% to 4% were recorded on NS3 Applied Nano Spectralyzer. Figure 6A shows the S11 absorption spectral changes of (6,5) SWCNT in 2% SDS by adding SC in the concentration range from 0.05% to 10%. The S11 absorbance initially shifted from 977 nm in 2% SDS solution to 995 nm in 2% SDS + 1.25%
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SC with progressively declined optic density. Further adding SC, the S11 absorbance of (6,5) SWCNT shifted from 995 nm in 2% SDS +1.25% to 980.5 nm in 2%+2.5% SC with gradually regressed optic density. After this point, the S11 absorbance remained the same but with further regressed optic density. The S11 peak wavelengths are plotted against the added SC as shown in Figure 6B (black curve). Initially, the (6,5) SWCNT aqueous solution would dominated by rod-like SDS micelles, exhibiting S11 peak wavelength at 977 nm. By adding SC (less than 1.25%), once one SC molecule adsorbed on the sidewalls of SWCNT, the rod-like SDS micelles on SWCNT would be partially destroyed in the segment with SC to expose uncovered SWCNT surfaces to polar microenvironments resulting in the red-shifted S11 absorbance of (6,5) SWCNT. With more SC molecules adsorbed on SWCNT sidewalls, more segments of SWCNT wrapped with rodlike SDS micelle would be demolished to open more SWCNT surfaces to polar microenvironments leading to the more redshifted S11 absorption spectra. When the rod-like micelles on SWCNT surfaces were completely replaced by SC molecules, the S11 peak wavelength of (6,5) SWCNT shifted to the maximum wavelength at 995 nm. These SDS molecules desorbed from SWCNT sidewalls would form new rod-like micelles or extend the existing rod-like micelles without SWCNT in solution (Figure 3). The observation of the replacement of SDS rod-like micelle by SC moieties can be rationalized as the slower molecular exchange rates of SC system than those of SDS system.34 Further increasing SC concentration from 1.5% to 10%, the more SWCNT surface areas were covered to prevent the exposure to polar medium and shifted the S11 absorbance back to the blue region. After SC micelles formed on the SWCNT surfaces, the S11 absorbance wavelength remained unchanged at 980.5 nm independent with SC concentration but with slowly decreased peak optical density. These results resemble the observations in (6,5) SWCNT dispersed by SC only, except the larger CMC accounted by SDS interference. The observed optical density variations are dominantly correlated to the areas exposure to polar microenvironments, these changes are consistent with peak wavelength shifts (Figure 6B blue curve). The optical density of S11 absorbance might also be influenced by the zeta-potential and the number of micelles in solutions.41
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Figure 6. A) The S11 absorbances of (6,5) SWCNT dispersed in 2% SDS titrated with SC in various concentrations shown in B). These different SC concentrations were obtained by adding SC powder to 2% SDS solution. B) The S11 peak wavelengths (black curve) and optical density (blue curve) of (6,5) SWCNT dispersed in 2% SDS were plotted against added SC in various concentrations. Figure 7A displays the S11 absorption spectral changes of (6,5) SWCNT in 0.05% SC by adding SDS in the concentration range from 0.05% to 4%. The S11 absorbance of (6,5) SWCNT initially shifted from 984.5 nm in 0.05% SC solution to 978 nm in 0.05% SC + 0.1% SDS with slightly increased optic density. These results can be illustrated with a picture of SC covered SWCNT surfaces that adsorbed SDS structurally with the diminished areas exposed to polar environments.7,8,42 Further adding SDS, the S11 absorbance of (6,5) SWCNT shifted from 978 nm in 0.05% SC + 0.1% SDS to 980.5 nm in 0.05% SC + 0.15% SDS with the depressed optic density. To explain these spectral changes, one might figure out the hemispherical SDS micelles formed on SC partially covered SWCNT sidewalls. As discussed in (6,5) SWCNT solution dispersed by SDS only, the SWCNT surfaces with hemispherical micelles experienced more polar microenvironments than those structureless SDS with maximum coverages (Figure 3). After this point, further adding SDS concentration neither perturbed the S11 peak wavelength nor S11 peak optical density. This could be the reason why the rod-like SDS micelles could not be formed on SC bounded SWCNT surfaces. The S11 peak wavelength and peak optical density against the added SDS are plotted in Figure 7B.
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Figure 7. A) The S11 absorbances of (6,5) SWCNT dispersed in 0.05% SC titrated with SDS in various concentrations shown in B). These different SDS concentrations were obtained by adding SDS powder to 0.05% SC solution. B) The S11 peak wavelengths (black curve) and optical density (blue curve) of (6,5) SWCNT dispersed in 0.05% SC were plotted against added SDS in various concentrations. The Vis-NIR absorption spectra of (6,5) SWCNT dispersed in 2% SDS + 0.5% SC and 2% SDS + 0.05% SC were measured. The (6,5) SWCNT dispersed in 2% SDS + 0.5% SC was diluted with 2% SDS to form (6,5) SWCNT dispersion in 2% SDS + 0.05% SC. Then, the (6,5) SWCNT dispersion in 2% SDS + 0.05% SC was converted back to 2% SDS + 0.5% SC dispersed (6,5) SWCNT solution by adding SC. After four times conversions, two solutions with 2% SDS + 0.5% SC dispersed (6,5) SWCNT and two solutions with2% SDS + 0.05% SC dispersed (6,5) SWCNT were obtained but with varied SWCNT concentrations. Their normalized S11 absorbances are displayed in Figure 8. One can see the well-overlapped S11 absorbances of (6,5) SWCNTs for different surfactant combinations peaked at 979 nm for 2% SDS + 0.05% SC and 989 nm for 2% SDS + 0.5% SC, respectively. These results further confirm that the SDS moieties on SWCNT can be replaced by SC molecules. The results also imply that SC molecules bounded on SWCNT can be substituted by SDS molecules, though strong binding affinity (slow molecular exchange) of SC on SWCNT was discussed in previous section. This finding is consistent with the reported two-dimensional NMR data indicating the replacement of SC bounded on SWCNT by SDS.34 1.2
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Wavelength (nm) Figure 8. The normalized S11 absorbances of (6,5) SWCNT in 2% SDS + 0.5% SC peaked at 989 nm and 2% SDS + 0.05% SC peaked at 979 nm. The starting solution was 2% SDS + 0.5% SC
which was diluted into 2%SDS + 0.05% SC using 2% SDS solution. The diluted 2% SDS + 0.05% SC solution was converted into 2% SDS + 0.5% SC by adding SC powder. The formed 2% SDS + 0.5% SC was diluted again to 2% SDS + 0.05% SC solution. The release of retained (6,5) SWCNT in Sephacryl Gels To trap (6,5) SWCNT in Sephacryl gels, the loading solution would be (6,5) SWCNT solution dispersed with
