Separation of Single-Walled Carbon Nanotubes Using the Amino Acid

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Separation of Single-Walled Carbon Nanotubes Using the Amino Acid Surfactant N‑Cocoyl Sarcosinate Yan Zhang,†,‡ Meng Wang,§ Dechun Li,‡ Hui Zhao,† and Ying Li*,† †

Key Laboratory of Colloid and Interface Chemistry of State Education Ministry and ‡School of Information Science and Engineering, Shandong University, Jinan 250100, China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

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S Supporting Information *

ABSTRACT: Understanding the interaction nature between dispersants and single-walled nanotubes (SWNTs) is crucial for separating SWNTs. In this work, it was found that amide groups of the amino acid surfactant N-cocoyl sarcosinate (NCS) molecules could be anchored on SWNTs by a π−πstacking interaction, offering the opportunity for alkyl chains to closely adsorb on SWNTs under an hydrophobic effect, with the winding directions found to be adjusted to match the chiral angles of SWNTs, which made N-CS molecules not only uniformly wrap around SWNTs but also recognize the chiral angle of SWNTs, guaranteeing the effective individualization and the multiple separation of SWNTs. Resorting to the scalable technique of density gradient ultracentrifugation, the mixture of SWNTs was first separated by diameter because the amount of N-CS adsorbed on SWNTs is proportional to the surface area of SWNTs. Afterward, when sodium dodecylsulfate (SDS) was added as the cosurfactant, a different number of SDS molecules were inserted into the SWNT/N-CS assemblies to enhance the density difference among SWNTs with different chiral angles, ensuring the separation of SWNTs by chiral angles. On the basis of the recognition of SWNTs by dispersants, the difference among SWNTs could be effectively established, providing a precondition for the accurate separation of SWNTs. KEYWORDS: nanotube chiral angle, diameter, s-SWNTs, amide acid surfactant, density gradient ultracentrifugation

1. INTRODUCTION With miniaturization of electronic circuitry, single-walled nanotubes (SWNTs) have been known as one of the most promising nanomaterials in molecular electronics because of their superb mechanical, electronic, and optical properties, hopeful to break through the performance limit of the existing electronic and optical devices.1,2 Although commercialization of SWNTs has begun, the promise of triggering epoch-making innovation of related technologies has not materialized, because of restriction of the bottleneck problem of nanotube purity.3 Considerable efforts have been made to obtain homogeneous SWNT samples either by a controllable synthesis or by sorting of the as-produced mixtures of SWNTs.4,5 Some creative breakthroughs in the controllable growth of SWNTs have been acquired,6−9 but the large-scale production of uniform materials remained challenging, far from satisfying the request for practical applications. In recent years, several postsynthetic separation techniques such as polymer wrapping,10−14 density gradient ultracentrifugation (DGU), 15−20 aqueous two-phase extraction,12,22−24 and gel chromatography25−28 have been developed to sort the mixture of SWNTs by diameter,29−32 electronic type,10,15,21 chirality,11,18,25 and handedness (optical isomer).12,13,20 In these methods, the subtle differences among © XXXX American Chemical Society

SWNTs with different structures were distinguished by specific dispersants such as conjugated polymers (CPs), 10−14 DNA,11,12 and diverse surfactants. 15,18,19 It has been demonstrated that the electronic nature of the CP backbone may influence selectivity of the CP−SWNT interaction.3 Electron-rich CPs with extensive π-conjugated structures could selectively extract semiconducting SWNTs (s-SWNTs) by a π−π-stacking interaction.10−14 Surfactants such as sodium cholate (SC), 2,5-dimethoxy-4-chloroamphetamine, and sodium dodecylsulfate (SDS) adsorb on SWNTs by a hydrophobic effect, debundling SWNTs by steric and electrostatic repulsions. The molecular structures of the surfactants were thought to be closely correlated with the dispersion and separation efficiency of SWNTs.15,33 Relying on the hydrophobic interaction, linear surfactants such as SDS or sodium dodecylbenzenesulfonate (SDBS) tended to adsorb on SWNTs in random forms, resulting in poor performances in the debundling and separation of SWNTs,33 while facial SC wrapped around SWNTs like a ring, resulting in strong individualization of SWNTs and uniform sidewall coverage,5,33 Received: June 3, 2019 Accepted: August 20, 2019 Published: August 20, 2019 A

DOI: 10.1021/acsanm.9b01054 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 1. (a) Molecular structure of the amino acid surfactant N-CS. The planar structure was centered on the conjugated AG marked by the red dotted circle. (Inset) Simplified small molecule centering on the AG. (b) Initial conformation of the AG molecule on SWNTs. (c) Optimized conformation of the AG/SWNT system. (d) Side and (e) top views of the charge density difference isosurface of the AG/SWNT system. The blue and yellow contours indicated electron accumulation and depletion, respectively, at an −7.48 × 10−4 e/Å isosurface value. Atom coloring scheme: C, green; H, white; O, red; N, blue; Na, purple. performed on a PerkinElmer Lambda 750 spectrophotometer using a 10-mm-path-length quartz microcuvette. Raman spectra were recorded on a Horiba Jobin-Yvon LabRAM ARAMIS system with excitation wavelengths of 532 and 633 nm lasers. The photoluminescence (PL) 2D excitation−emission maps were collected on a Horiba Jobin-Yvon Nanolog-3 spectrofluorometer equipped with a 450 W xenon arc lamp and a liquid-nitrogen-cooled InGaAs detector. An 830 nm filter was set in front of the detector to cut off the Rayleigh scattering. 2.2. Methods. 2.2.1. Preparation of SWNT Suspensions. A total of ∼1.00 mg CoMoCAT SWNTs was dispersed in 20.0 mL of a ∼0.300 wt % N-CS solution in a 50 mL centrifuged tube by 30 min of tip sonication at a power of 480 W. The centrifuged tube was kept in an ice−water bath during the sonication process, and the sonication and stop times were all set to 2 s. Then the dispersion was centrifuged at 60000g for 1 h with a S80AT3 fixed-angle rotor on a Hitachi CS150GX II instrument. The supernatant was carefully decanted and used for optical characterization and further separation by using DGU. Also, 20.0 mL of a ∼1.00 wt % SDBS solution was used to disperse ∼1.00 mg of CoMoCAT SWNTs, and the supernatant was achieved at the same sonication and centrifugation conditions. 2.2.2. Formation of Density Gradients. Our density gradient medium was an aqueous solution of 60.0% (w/v) iodixanol (OptiPrep, from Sigma-Aldrich). The density gradient column was formed by stacking a series of discrete layers in 4.00-mL-capacity polypropylene ultracentrifuge tubes with the following iodixanol concentrations and volumes: 30.0% (w/v) (480 μL), 27.5% (w/v) (400 μL), 25.0% (w/v) (518 μL), 22.5% (w/v) (633 μL), 20.0% (w/ v) (633 μL), 17.5% (w/v) (690 μL), and 15.0% (w/v) (750 μL). For diameter selectivity, the N-CS concentration in the gradient was 0.300% (w/v), and for the chiral-angle- and electronic-type selectivity, the mixed surfactants of 0.200% (w/v) N-CS and 0.167% (w/v) SDS were used instead. A total of ∼1.00 mL of supernatant of SWNTs after 60000g centrifugation was injected above the lowest density layer [15.0% (w/v) iodixanol solution] using a syringe. A S52ST swing bucket rotor was used. After centrifugation at 180000g for 18 h at 293 K, the SWNT supernatant was separated into different colored bands, and a 1.00 mL syringe was used to withdraw the separated fractions.

which was crucial for the effective separation of SWNTs. The effective individualization of nanotube bundles is a prerequisite of the successful separation of SWNTs. Macromolecular CPs displayed selectivity for s-SWNTs by π−π-stacking interaction,2,14 while large molecular sizes are unfavorable for the thorough debundling of SWNTs, thus affecting the separation efficiency of SWNTs. The facial bile surfactants exhibited structural selectivity by stacking with the semirigid, hydrophobic, cholesterol-type backbone on SWNTs,34 and the flexible alkyl chains of linear surfactants such as SDS or SDBS possess adjustability of their adsorbed morphologies on SWNTs. Although there is some understanding of the interaction between dispersants and SWNTs, the processes of the recognition and separation of SWNTs remain unclear. In our work, the amino acid surfactant N-cocoyl sarcosinate (NCS), combining the characteristics of the above molecules, was demonstrated to enable the separation of SWNTs by diameter, chiral angle, and electronic type by DGU, and the recognition and separation mechanisms were illustrated in detail by investigating the interaction nature between N-CS molecules and SWNTs using molecular dynamics (MD) simulations and first-principles density functional theory (DFT) calculations, providing guidance in the search for suitable dispersants to separate SWNTs.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. CoMoCAT SWNTs were purchased from Sigma-Aldrich Inc. and used as received without further treatment. Information on the SWNT product used is displayed in Table S1. N-Cocoyl sarcosinate (N-CS; Sarcosinate CN30) was obtained from Nikko Chemicals Co., Ltd. (Tokyo, Japan), and used as received. Sodium dodecylbenzenesulfonate (SDBS) and sodium dodecylsulfate (SDS) were purchased from Sigma-Aldrich Inc. and used as received. Iodixanol was purchased from SigmaAldrich Inc.. DGU was carried out on a Hitachi CS150GX II, with a S52ST swing bucket rotor. UV−vis−near-IR absorbance measurements were B


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Figure 2. (a) Front view of the initial configuration of the SWNT/N-CS system with 10 N-CS molecules. Corresponding representative simulation snapshots of the (b) (7, 6) SWNT/N-CS and (c) (9, 4) SWNT/N-CS systems. (d) Side view of the initial configuration of the SWNT/N-CS system with 10 N-CS molecules. Corresponding representative simulation snapshots of the (e) (7, 6) SWNT/N-CS and (f) (9, 4) SWNT/N-CS systems. (g) Initial and (h) postequilibrium conformations of the carboxyl group of the N-CS molecule. (i) Alkyl chains aligning with the hexagonal path of the SWNTs for the anticlockwise wrapping case of (9, 4) tubes and with the SWNT hexagonal path on the opposite side of the N-CS molecule. The SWNT hexagon path is labeled in yellow. The (7, 6) and (9, 4) nanotubes represent SWNTs with larger and smaller chiral angles, respectively. Water molecules are not shown in parts b−i for clarity.

further confirmed the occurrence of the π−π-stacking interaction between the AG and SWNTs.36 So, the planarconjugated AGs of N-CS molecules could be anchored on the surface of SWNTs by π−π-stacking interaction. Anchoring of the AGs of the N-CS molecules on SWNTs was also confirmed by the results of MD simulations. Parts a and d of Figure 2 are the initial configurations of the N-CS/ SWNTs system with 10 surfactant N-CS molecules and 2000 water molecules; the equilibrated configurations are displayed in Figure 2b,c,e,f. The shortest distance between the AGs (a N atom was chosen as the label atom) and the surface of the SWNTs ranged from 3.27 to 4.07 Å (Figure 2b,c), demonstrating the existence of a π−π-stacking interaction between them. (7, 6) and (9, 4) nanotubes were chosen to represent the SWNTs with larger and smaller chiral angles, respectively. For the system containing larger-chiral-angle (7, 6) SWNTs, the shortest distance between the AGs (a N atom was chosen as the label atom) and the surface of the SWNTs mainly focused on 3.52−4.07 Å (Figure 2 b), while for the system containing smaller-chiral-angle (9, 4) SWNTs, the shortest distance mainly focused on 3.37−3.67 Å, as presented in Figure 2c, which demonstrated a more compact wrapping in comparison with the (7, 6) SWNT/N-CS system. Simultaneously, we found that N-CS molecules uniformly wrapped around the SWNTs just like a film, but the winding directions of the alkyl chains on the SWNTs with different chiral angles were obviously different, as observed in Figure 2e,f. The nearlinear adsorption conformations on the (7, 6) nanotube and

3. RESULTS AND DISCUSSION 3.1. Recognition of SWNTs by the Amino Acid Surfactant N-CS. The amino acid surfactant N-CS molecule possesses both a planar conjugated amide group (AG) and a linear alkyl chain, and a methylene links the amide and carboxyl groups, as shown in Figure 1a. The interaction nature between the N-CS molecule and SWNTs was investigated by MD simulations and DFT calculations. For simulation and calculation details, see the Supporting Information. In order to explore the interaction between the AG of the N-CS molecule and SWNTs by DFT calculations, a simplified small molecule centering on the AG was designed, as presented in the inset of Figure 1a. The AG molecule was placed parallel to the tube axis at a distance of 3.539 Å beyond the bonding range in the initial conformation, as shown in Figure 1b. The optimized conformation after geometry optimization is shown in Figure 1c, and the side and top views of the calculated charge-density difference isosurface of the AG/SWNT system are displayed in parts d and e of Figure 1d, respectively. Both the optimum distance of 3.438 Å and the small binding energy of 3.413 kcal/ mol (for the calculation method, see the Supporting Information) were consistent with weak interactions like physisorption, which suggested that the AG formed a weak π−π-stacked complex with SWNTs.35,36 The shape of the electron density redistribution of the AG/SWNT system determined by analysis of the isosurface of the charge density difference accorded with a typical “sandwich” electrostatic model of the π−π interaction, as shown in Figure 1d,e, which C


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Figure 3. (a) Optical absorbance spectra of the SWNT supernatants dispersed by N-CS (∼0.300 wt %) and SDBS (∼1.00 wt %) after double dilution. (b) PL 2D excitation−emission map of the SWNT supernatant dispersed by N-CS after 4-fold dilution.

the anticlockwise helical wrapping around the (9, 4) nanotube of the alkyl chains were found to be well matched to the chiral angle of the SWNTs, as displayed in Figure 2i, which conveyed important information that the wrapping forms of the hydrophobic tails could be modulated according to the rollup angle of the SWNTs. However, in fact, there might be no opportunity to adjust the wrapping directions of the alkyl chains of surfactants on SWNTs by hydrophobic interaction alone, just like the surfactants SDS and SDBS. Although SDBS molecules have benzene groups, hydrophobic interactions between the tail segments and SWNTs dominate the agglomeration of SDBS on SWNTs.37 Striolo and co-workers38 also pointed out that the surfactant headgroups protrude extensively toward the aqueous phase, effectively pulling the benzene rings, present within the SDBS molecules, away from the nanotube surface according to the results of all-atom MD simulations. A previous report pointed out that the interactions between SWNTs and polymers with aromatic groups proceeded in stages: first the interactions between the electrons of the graphene lattice and the aromatic groups and then the closepacking stage depended on the underlying chirality of SWNTs, and the wrapping process can be guided by the interaction between the aromatic groups and SWNTs.39 Furthermore, the key motif for (n, m) SWNT enrichment was thought to hinge on the π−π-stacking interactions between the underlying graphene sidewall and aromatic moieties of the surfactants.40,41 The above calculations showed that planar-conjugated AGs of the N-CS molecules were anchored on SWNTs by π−πstacking interaction and the alkyl chains closely winded around SWNTs in terms of the chiral angles by hydrophobic interactions. Compared with the SDS (or SDBS)/SWNTs system, the only significant difference was that there existed a π−π-stacking interaction between the N-CS molecules and SWNTs, which could make the N-CS molecules closely anchor onto the surface of SWNTs independent of the hydrophobic interaction between the alkyl chains and SWNTs. Therefore, the anchoring of AGs onto SWNTs could offer the opportunity for alkyl chains of the N-CS molecules to adjust the organization and directions to match the chiral angles of SWNTs, reaching the lowest energy of the SWNTs/N-CS assembly. However, for SDS or SDBS molecules, the alkyl chains could only maximize contact with the surfaces of SWNTs to decrease the total energy, and fine adjustment was not possible when only relying on the hydrophobic interaction between them. So, π−π-stacking interaction between the N-CS

molecules and SWNTs was vital to recognizing the chiral angle of SWNTs, which was consistent with the previous speculations.36,39,40 In addition, the methylene between the AG and carboxyl group could effectively buffer the effect of the hydrophilic pull force from the carboxyl group on the π−π-stacking interaction, as displayed in Figure 2h, which further ensured anchoring of the AGs on SWNTs. Except for the advantage of the molecular structure, the relatively weaker hydrophilicity of the carboxyl group compared with the sulfate group in SDS and the sulfonate group in SDBS also benefited the π−π-stacking interaction. So, the unique molecular structure and appropriate hydrophilicity of the carboxyl groups guaranteed strong π−πstacking interaction between the AGs of the N-CS molecules and SWNTs, which offered the opportunity for the alkyl chains to closely and uniformly wind around SWNTs by the chiral angle, realizing recognition of the SWNTs. 3.2. Effective Individualization of SWNTs and Their Diameter Separation. MD simulations showed that N-CS molecules uniformly wrapped around the SWNT just like a “sheath”, hindering the approach of other SWNTs. Simultaneously, we found that the conformations of carboxyl groups varied from being nearly parallel to the surface of SWNTs in the initial conformation (Figure 2g) to being perpendicular to the surface in the postequilibrium conformation (Figure 2h), effectively expanding the volume of the N-CS/SWNT assemblies. So, SWNTs could be well individualized by steric and electrostatic repulsions. The effective individualization and uniform encapsulation of SWNTs by N-CS molecules would allow the diameter separation of SWNTs by DGU.33 Figure 3a displays the absorbance spectra of the SWNT supernatants dispersed by N-CS (∼0.300 wt %; black line) and SDBS (∼1.00 wt %; red line), respectively. The better resolved peaks and stronger absorption intensity of the black line compared with the red line demonstrated more thorough individualization of SWNTs dispersed by the amino acid surfactant N-CS, despite the lower mass concentration of the N-CS solution. In Figure 3b, the intense PL signals also confirmed the effective individualization of SWNTs, and the distinct PL peaks were assigned to various species of s-SWNTs by a Kataura plot.42,43 DGU is a very effective method for separating SWNTs by diameter. Different concentrations and volumes of iodixanol solutions containing ∼0.300% (w/v) N-CS were manually laid in ultracentrifuge tubes, and the detailed preparation process of the density gradient column was described in the Experimental D


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Figure 4. (a) Image of the DGU tube after centrifugation. (b) Optical absorbance spectra of the fractions. PL 2D excitation−emission maps of the fractions in (c) TP 1, (d) BG 1, (e) TP 2, and (f) BG 2.

Figure 5. (a) Optical absorbance spectra of the fractions. (b) Distribution of the chiral angles of SWNTs. PL 2D excitation−emission maps of the fractions from the (c) up and (d) middle layers.

reflected by the change of the absorbance intensity from TP 1 to BG 2 in Figure 4b, which might be caused by the blend of small SWNT bundles with small diameter and the individual large-diameter SWNTs in the down layer. 3.3. Chiral Angle Separation of SWNTs. Despite recognition of the nanotube chiral angles by N-CS molecules, as demonstrated in the theoretical calculations section, no separations of SWNTs by the chiral angles were achieved in the single N-CS system. In order to improve the separation efficiency, the cosurfactant SDS was added to amplify the density difference among SWNTs. An iodixanol solution containing 0.200% (w/v) N-CS and 0.167% (w/v) SDS was used as a density gradient medium. The density gradient column was prepared as described in the Experimental Section. After DGU, the optical absorbance spectra of the fractions in Figure 5a clearly indicate that SWNTs have been separated by chiral angles; the smaller-chiral-angle SWNTs were enriched in the up layer, and the larger-chiral-angle SWNTs were gathered

Section. After DGU treatment, two groups of colored bands were observed and labeled as the up and down layers, as displayed in Figure 4a. The up layer consisted of the top purple 1 (TP 1) and bottom green 1 (BG 1), and the down layer consisted of the top purple 2 (TP 2) and bottom green 2 (BG 2). For the up layer, the obvious decrease of the absorbance intensity of BG 1 compared to that of TP 1 at ∼1000 nm in Figure 4b demonstrated that SWNTs of increasing diameter were enriched in the increasing density regions, which also was confirmed by the change of the PL intensity of the fractions from TP 1 to BG 1 in Figure 4c,d. For the down layer, the change of the amplitudes of the optical absorbance in Figure 4b and the change of the PL intensities of the TP 2 and BG 2 in Figure 4e,f also demonstrated the diameter selectivity of SWNTs. Simultaneously, a general distribution trend of increasing diameters with increasing density from TP 1 to BG 2 was observed by the change of the PL intensity, as shown in Figure 4c−f, while the distribution trend was not well E


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Figure 6. (a) Optical absorbance spectra of the fractions. (b) Image of the DGU tube after centrifugation at 180000g for 18 h at 293 K. (c) Distribution of the chiral angles of SWNTs.

Figure 7. Raman spectra of the extracted fraction from the up layer with excitations at (a) 532 and (b) 633 nm.

centrifuge tube is visually displayed in Figure 6c, demonstrating the distribution trend of SWNTs of increasing chiral angle with increasing density. Simultaneously, we found that the larger the chiral-angle difference between SWNTs, the larger their density difference [such as (9, 4) and (6, 5)] and vice versa, such as (7, 5) and (6, 5) nanotubes. According to the distribution trend, SWNTs with the largest chiral angle, such as armchair (6, 6) and (7, 7) tubes, should enrich ML 3, which was supported by the increasing relative absorbance intensity of metallic (6, 6) and (7, 7) tubes from ML 1 to ML 3 in Figure 6a. The presence of (6, 6) and (7, 7) tubes in ML 1 and ML 2 was attributed to the capillary force of the syringe. The PL maps of the fractions in Figure S1 further verified the fact that SWNTs of increasing chiral angle were distributed in the increasing density regions. 3.4. Small-Chiral-Angle s-SWNT Separation. The absorbance peaks in the ranges of 400−550 and 550−900 nm originate from the first interband transitions of metallic SWNTs (M11) and the second interband transitions of sSWNTs (S22).44 In Figure 5a, a flat peak valley was observed in the M11 region of the up layer, while the obvious absorbance peaks occurred in the middle layer, the down layer, and the pristine SWNT sample, which demonstrated that only sSWNTs with small chiral angles occurred in the up layer. Likewise, no peaks of metallic tubes were detected in the UL 1 and UL 2 in Figure 6a. Raman spectra of the up layer in Figure 7 also demonstrated the removal of metallic tubes. The largediameter (8, 4) and (9, 4) s-SWNTs enriched in the up layer are very suitable for preparing high-performance optoelectronic devices. 3.5. Mechanism Discussion of Separating SWNTs by the Chiral Angles. On the basis of the recognition of the chiral angles of SWNTs by surfactant N-CS, the successful

in the middle layer, as observed in Figure 6b, and the visual diagram of the chiral-angle distribution of SWNTs is plotted in Figure 5b. The 2D excitation−emission PL maps of the up and middle layers further confirmed the separation of SWNTs by chiral angles, as presented in Figure 5c,d. Simultaneously, we found that the absorption spectrum of the pristine dispersion of SWNTs was decomposed into two perfectly complementary spectra of the up and middle layers, although there was only a few nanometer difference of the peak positions, which also proved the excellent individualization of SWNTs. By scrutinizing the three groups of colored bands in Figure 6b, we found that each group contained at least two closely adjacent bands. The thinner layers were withdrawn more carefully from top to bottom (0.00−1.30 cm, labeled on the centrifuge tube), and the separated fractions from the up layer were labeled as UL 1 (0.00−0.10 cm) and UL 2 (0.10−0.25 cm), from the middle layer as ML 1(0.60−0.70 cm), ML 2 (0.70−0.80 cm), and ML 3 (0.80−0.90 cm), and from the down layer as DL 1 and DL 2. The absorption spectra of the refined fractions in Figure 6a demonstrated that SWNTs of increasing chiral angles were enriched in the increasing density regions from top to bottom in the centrifuge tube. The UL 1 enriched the smallest chiral-angle fractions (5.0−17°) of SWNTs, such as (9, 1), (10, 2), (8, 3), and (7, 3) tubes; the UL 2 gathered SWNTs in the chiral-angle range from 17 to 20°, including (8, 4), (9, 4), and (9, 5) tubes; (6, 4) and (7, 5) tubes (20−25°) were mainly remained in the ML 1; SWNTs with the chiral angle from 25 to 28° such as (6, 5), (7, 6), and (8, 7) tubes were separated in the ML 2. In addition, the absorbance peak of (6, 5) tubes in ML 1 was also very obvious, which might be due to the strong capillary force of the syringe and its high abundance in the CoMoCAT sample. The position distribution of the chiral angles of SWNTs in the F


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Figure 8. (a) Images of the DGU tube before (left) and after (right) addition of the cosurfactant SDS. (b) Radius distribution function of water molecules around the sulfate group (the S atom was chosen to be the label atom) of the SDS molecule and the carboxyl group (the C atom of carboxyl was chosen to be the label atom) of the N-CS molecule. (insets) Hydration of the surfactant headgroup from the equilibrium snapshot of MD simulations. Atom coloring scheme: C, gray; H, white; O, red; N, blue; S, yellow.

Scheme 1. Sketch of the Separation Process of SWNTs by DGU: (a) SWNT Bundles; (b) Dispersion of SWNTs by the Amino Acid Surfactant N-CS; (c) Separation of SWNTs by Diameter in the Single N-CS System; (d) Separation of SWNTs by Chiral Angles in the Cosurfactant N-CS/SDS Systema

a

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