Encapsulation of single-walled carbon nanotubes

Chemical Engineering Science 187 (2018) 406–414

Contents lists available at ScienceDirect

Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces

Encapsulation of single-walled carbon nanotubes with asymmetric pyrenyl-gemini surfactants Xianyu Song a, Hao Guo a, Jiabo Tao a, Shuangliang Zhao a,c,⇑, Xia Han b, Honglai Liu b a Shanghai Key Laboratory of Multiphase Materials Chemical Engineering and School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China b State Key laboratory of Chemical Engineering and School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China c Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Surfactant-based encapsulation of

asymmetric surfactants and SWCNT is reported. Asymmetric pyrenyl-gemini surfactants (APGSs) form doublelayer films on SWCNTs. APGS chains change from folding to unfolding structure as increasing SWCNT radius. APGSs with high surface charge enhance the selective dispersion of SWCNTs.

a r t i c l e

i n f o

Article history: Received 10 January 2018 Received in revised form 27 April 2018 Accepted 5 May 2018 Available online 7 May 2018 Keywords: Single-walled carbon nanotubes Asymmetric pyrenyl-gemini surfactants Molecular simulation Highly selective dispersion Self-assembly

a b s t r a c t Whereas the encapsulation technology of surfactants coating on the surface of single-walled carbon nanotubes (SWCNTs) attracts much attention, the mechanisms of asymmetric surfactants interacting with the SWCNTs and their molecular structures are rarely reported. Herein, we report a molecular dynamics (MD) simulation study on the investigation of surfactant adsorption and induced colloidal stability onto different SWCNTs. The surfactant-based encapsulation system, originating from the complementary p-p stacking, is validated through two-dimensional number density maps. We find that the asymmetric pyrenyl-gemini surfactants (APGSs) form double-layer assembled architecture films on nanotube surface. The inner layers of the films are packed with the pyrenyl groups of APGSs, which interact with SWCNTs via p-p stacking, while the outer layers composed of alkyl chains of APGSs coat at the nanotube surface through van der Waals interactions. In addition, we observe a configurational transformation of APGSs on the SWCNT surface from the folding configuration to unfolding patterns, when increasing the nanotube radius or the surface coverage of APGSs. Our study provides helpful insights into the encapsulation mechanism of APGSs on the SWCNT surface and the design of highly selective dispersants for SWCNTs. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author at: Shanghai Key Laboratory of Multiphase Materials Chemical Engineering and School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail address: [email protected] (S. Zhao). https://doi.org/10.1016/j.ces.2018.05.009 0009-2509/Ó 2018 Elsevier Ltd. All rights reserved.

Due to their extraordinary mechanical, chemical and electronic properties (Cao et al., 2013; De Volder et al., 2013; Streit et al., 2014), single-walled carbon nanotubes (SWCNTs) appear to be ideal candidates for various functional materials and have wide applications in material engineering, such as supercapacitors

X. Song et al. / Chemical Engineering Science 187 (2018) 406–414

(Niu et al., 2013; Yu et al., 2014); catalyst (Qin et al., 2014), biosensors (Giraldo et al., 2014; Iverson et al., 2013), microelectronics (Bottacchi et al., 2015; Kunai et al., 2017). The encapsulation technology of surfactants coating on the surface of SWCNTs currently represents a common and crucial strategy for manipulating interfacial properties and nanotubes interactions (Berton et al., 2014; Frise et al., 2010). For instance, the carbon nanotubeencapsulated nanomaterials, significantly strengthening the surface electron density of cathode surface (Zhao et al., 2014), are promising in building cathode materials, such as Li-oxygen batteries (Huang et al., 2014) or Li-sulfur batteries (Sun et al., 2014). Furthermore, the encapsulating nanomaterials of SWCNTs have high drug loading efficiency, good biocompatibility, immunochemical biosensors (Mehra and Palakurthi, 2016) and nonimmunogenicity, and thus can be used as pharmaceutical nanocarriers (Farka et al., 2017). Nevertheless, the success of these techniques not only requires accurate descriptions of their encapsulation at molecular level, but also demands quantitative understanding of interfacial interactions for optimizing functional nanomaterials (Mitchell et al., 2015; Subbaiyan et al., 2014). Many groups have investigated the encapsulation mechanism and processes of amphiphilic surfactants on SWCNTs, including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), hexadecyltrimetylammonium bromide (CTAB) and octyl phenol ethoxylate (Triton X-100) (Clark et al., 2011; Das et al., 2016; Duan et al., 2011; Nish et al., 2007; Suttipong et al., 2011). The surface of SWCNT is highly hydrophobic, and thus the aliphatic chain of the surfactant presents an energetically favorable interaction with SWCNTs in aqueous solutions, while the hydrophilic heads of the surfactant orient into the aqueous solutions (Tummala and Striolo, 2009). Thus, these surfactants provide both steric repulsion and colloidal stability to the dispersion on the nanotube surface (Xu et al., 2010). Arnold et al. (2008) studied the hydrodynamic properties of sodium cholate-SWCNT complexes, and found these complexes have an anhydrous partial specific volume with 0.53 ± 0.03 g/cm3 and an adsorbed linear surfactant density with 3.6 ± 0.8 molecules/nm. Recently, the adsorption and affinities of small molecules on carbon nanotube surfaces were also investigated (Comer et al., 2015; Delport et al., 2017; Wang et al., 2017). The adsorbed molecules are determined not only by the depth of free energy well but also by their molecular shapes. Moreover, the adsorption of aromatics molecules from aqueous solution onto nanotube surface could be enhanced by the strong p-p interactions between them. Pramanik et al. (Pramanik and Maiti, 2017) explored the dispersion efficiency of DNA on carbon nanotubes by mediating the binding affinities of different nucleic bases on the nanotube surface. Despite large number of studies have been contributed to illustrating the encapsulation behavior, the understanding of encapsulation behavior is still poor, inhibiting the rational design of effective and desired encapsulation systems (Tummala and Striolo, 2009; Xu et al., 2010). For studying the structure and dynamic information at molecular level, molecular dynamics (MD) simulation provides a powerful tool (Palmer and Debenedetti, 2015; Perilla et al., 2015), and it enables the investigations on the surfactant monolayer structures and the dispersion behaviors of surfactants-stabilized SWCNTs (He and Zhou, 2014; Xu et al., 2017). In this work, all-atom molecular dynamics (AAMD) simulations are performed to simulate the surfactant-based encapsulation. We firstly present the simulation results of surfactant-based encapsulation using APGSs. The surfactant-based encapsulation systems are evaluated, and the encapsulation mechanism of APGSs are also elucidated through the potential of mean force, number density map, charge density, thickness of monolayer film and the radius of gyration.

407

2. Molecular structure and computational method Inspired by previous studies, here we propose a surfactantbased encapsulation based on the p-p stacking interactions from the complexes of asymmetric pyrenyl-gemini surfactants (APGSs) and SWCNTs. Gemini surfactants contains two typical singlehead and single-tail surfactant molecules linked by a spacer chain, and they have attracted considerable attention because of their unique chemical structure and potential applications (Banno et al., 2013; Cai et al., 2012). In this study, the APGSs with one of the alkyl chains being substituted with aromatic ring are employed to design the surfactant-based encapsulation. The molecular structure of APGS is depicted in Fig. 1a. A series of APGSs is synthesized by Wang et al. (2007) used as synthetic vectors for the delivery of genes (Keyes-Baig et al., 2011; Tang et al., 2017). In all simulations, one SWCNT is fixed at the center of a rectangular simulation box with the axis aligned along the Z-direction. The structures of SWCNTs used in simulations are listed in Figure S1 of the Supporting Information (SI). To construct the initial configuration, the Packmol software tool is used (Martinez et al., 2009; Sarode et al., 2017), and the desired number of surfactants are placed around the SWCNT with their tails parallel to the nanotube axis (Xu et al., 2010). Thereafter, a certain amount of water molecules and chloride ions are added into the simulation box to reproduce bulk water density and maintain electrical neutrality. Further details of simulated systems are collected in Table 1. The typical initial configuration of simulation system is given in Figure S2. All simulations are performed with the GROMACS 5.0.2 suite (Allen et al., 2015; Zhu et al., 2017) using OPLS all-atom force field (Robertson et al., 2015; Robertson et al., 2017) for SWCNTs and APGSs. The simple point charge extended (SPC/E) water model is used upon the OPLS all-atom force field (Suttipong et al., 2011; Tummala and Striolo, 2009; Xu et al., 2010), and periodic boundary conditions are employed in the X, Y and Z directions. Before initiating the MD simulations, an energy minimization is performed to relax the system. Leapfrog algorithm MD is employed in all simulations with 2 fs time step (Tummala and Striolo, 2009). All bonds are constrained with the LINCS algorithm (Tieleman and Marrink, 2006). In each simulation; a time step of 2 fs is applied with SHAKE constraints on covalent bonds. The particle mesh Ewald (PME) method is used to treat the long-range electrostatic interactions with a cutoff of 12 Å (Suttipong et al., 2011). The number of particles (N), the simulation box volume (V), and the temperature (T) are maintained constant during our simulations. The temperature is kept at 300 K using a stochastic term (v-rescale modified Berendsen) with a collision frequency of 1.0 ps1. All simulations are conducted for 80 ns, and only the last 5 ns are used for data analysis. The systems are visualized using VMD 1.9.1 (Barroso et al., 2014).

3. Results and discussion 3.1. Surfactant-based encapsulation system The typical surfactant-based encapsulation for SWCNT (6, 6) covered by APGSs is plotted in Fig. 1. Basing on the molecular structure of APGSs, two advantages can be listed: (i) the SWCNT interacts with the alkyl tails of APGSs via van der Waals interaction, and with the aromatic rings of APGSs via p-p interactions. Due to the strong p-p stacking interactions between the pyrenyl groups of APGSs and the carbon nanotube, the aromatic rings can efficiently disperse nanotube species with a high degree of selectivity (Nogueira et al., 2016; Xiao et al., 2014; Burattini et al., 2009); as shown in Fig. 1b. (ii) Two hydrophilic head groups of the APGSs enhance the steric repulsion and colloidal stability of

408


Fig. 1. Schematic representation of surfactant-based encapsulation system for SWCNT (6, 6). (a) Schematic illustration of APGS with a special chemical pyrenyl group. (b) The principle of surfactant-based encapsulation, and the strong p-p stacking interactions between the aromatic ring of APGSs and a SWCNT. (c) Typical aqueous solutions of SWCNTs stabilized by self-assembled APGSs. (d) The potential of mean force (PMF) curves of APGSs-SWCNT complexes in aqueous solutions with different concentrations of APGSs, displaying two low-energy points. The surface coverage corresponding to the molecule number of APGSs is shown in Table 1. The hydrogen atoms, carbon atoms and nitrogen atoms of APGSs are represented in white, cyan and blue colors, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Parameters for the simulation systems studied in this work.a

a

Nanotube

Number of APGS

Surface coverage (molecules/nm2)

Number of water molecules

Box size (nm3)

(6,6) (12,12) (20,20) (6,6) (12,12) (20,20) (6,6) (12,12) (20,20)

12 12 12 24 24 24 56 56 56

0.624 0.318 0.191 1.248 0.625 0.382 2.915 1.603 1.034

3248 3720 5061 4482 4392 6066 6480 7535 9736

4.0 4.0 7.5 4.5 4.5 7.5 6.0 6.0 7.5 4.5 4.5 7.5 5.0 5.0 7.5 6.5 6.5 7.5 5.5 5.5 7.5 6.0 6.0 7.5 7.5 7.5 7.5

The diameter of the SWCNT is about 0.80 nm for SWCNT (6, 6), 0.94 nm for SWCNT (12, 12), 1.57 nm for SWCNT (20, 20).

the APGSs-SWCNT complex in Fig. 1c, and meanwhile, it also can be used as genes delivery by electrostatic interactions (Wang et al., 2007). To assess the surfactant-based encapsulation system, we calculate the radial distribution function (RDF) and potential of mean force (PMF). These results are displayed in Fig. 1d. The RDF is calculated by using the following equation (Duan et al., 2017):

gij ðrÞ ¼

fDNij ðr ! r þ DrÞgV ; 4p r 2 DrNi Nj

ð1Þ

where fDN ij ðr ! r þ DrÞg represents the averaged number of atom j around atom i within a shell from r to r + Dr, V is the system volume, and Ni and Nj are numbers of atom i and j, respectively. Potential of mean force (PMF) represents the mean interacting energy between two objects in a solution (Wu et al., 2017). With the help of RDF, the

PMF in the low density limit can be calculated as (Headen et al., 2009):

W ¼ kB T ln½gðrÞ;

ð2Þ

The PMF is equivalent to the Helmholtz free energy (plus a constant), and thus the free energy of aggregates can be computed by taking the difference of the potentials of mean force at sufficiently remote separation and at equilibrium separation (Headen et al., 2009). From the inset of Fig. 1d, we can find that the peak of RDF between the nanotube and surfactants ranges from 0.48 to 1.25 nm. The first peak locates at 0.48 nm and it is originated from the interactions between the nanotube and pyrenyl group of APGSs. The second peak at 12.5 nm is attributed to the interactions between the nanotube and alkyl chains of APGSs. From the PMF of


Fig. 1d, two low-energy points are observed. This result is induced by the orientation of the aliphatic chains of APGSs at the nanotube surface and the p-p stacking interactions between the aromatic ring and nanotube surface, respectively. All these results are confirmed by the representative snapshots and two-dimensional (2D) number density maps shown in Fig. 2. The 2D number density maps are obtained by taking ensemble average of the conformations over 5 ns time in the last equilibrium stage (Wei et al., 2016). As presented in Fig. 2a and b, the pyrenyl groups of APGSs are adsorbed at the nanotube surface, and form diamond ring-like wrapped film around the nanotube surface. In Fig. 2c–h, a thin cylinder-like density maps composited of the aromatic groups of APGSs are observed, while the CH2 or CH3 groups in the surfactant tails of APGSs form a bright double-ring film structure as shown in Fig. 2e–f. The nitrogen atoms of APGSs surround the backbone of APGSs with an outer ring-like structure (as shown in Fig. 2g–h). Furthermore, the value of 2D number density of CH group (aromatic group) is overall larger than that for the CH2 or CH3 groups in the surfactant tails and nitrogen atoms of APGSs, illustrating that the aromatic groups of APGSs highly centralize at the nanotube surface. However, the radius of 2D number density map formed by aromatic groups of the APGSs is smaller than that for the CH2 or CH3 groups (alkyl segments) in the surfactant tails of APGSs. Thus, a double-layer assembled architecture film of APGSs is identified at the nanotube surface. The inner layer of this film is composed of pyrenyl group interacting with the SWCNT via p-p stacking, and the outer layer consists of alkyl chains of APGSs interacting with the SWCNT via van der Waals

409

interaction. In fact, competitive adsorption between the aromatic groups and alkyl segments occurs during the encapsulation process. The double-layer assembled film elucidates that the p-p stacking interaction is stronger than the van der Waals interaction. The similar results are presented in Figures S3 and S4 of SI for SWCNTs (12, 12) and (20, 20) covered by the same number of APGS molecules.

3.2. Density map of charges The surface charge of APGSs-SWCNT complex is essential to predict the colloidal stability of SWCNTs solutions (Chatterjee et al., 2005). Though the electrostatic interactions between ionic surfactants and SWCNTs can enhance the dispersion of SWCNTs, some studies demonstrated other selective dispersants could be developed through strong charge repulsions (Chatterjee et al., 2005; Vaisman et al., 2006; Vaisman et al., 2006; Dou et al., 2009). The 2D number density maps of APGSs’ charges for SWCNT (12, 12) with different surface coverages are given in Fig. 3. At the surface coverage of 0.318 molecules/nm2, an annular aperture with scattered facula is observed from Fig. 3a and b. At high surface coverage (0.625 and 1.603 molecules/nm2), a double-layered and dense aperture is continuously found in Fig. 3d and f. This double-layered charge map illustrates the increase of surface coverage enhances the density of positive charges at the nanotube surface, thus improving the dispersion of SWCNTs. All these results can be confirmed by the distribution of charge density of APGSs with respect to the SWCNT axis (see Fig. 4a). In addition, the aper-

Fig. 2. Representative snapshots (a and b) and the side (c, e, h) and front (d, f, h) 2D number density maps of APGSs’ charges with different structure groups. The left panel represents the density maps of CH group (aromatic group) of the APGSs, the middle panel refers to the density maps of CH2 or CH3 groups of the APGSs’ tails, and the right panel stands for the nitrogen atoms of the APGSs’ polar heads. The hydrogen atoms of APGSs are omitted in the density maps. These results are obtained from the same simulation system of Fig. 1, and namely, the nanotube (6, 6) is used. The surface coverage is 1.248 molecules/nm2.

410


Fig. 3. Side (first row) and front (second row) 2D number density maps of APGSs’ charges with different surface coverages. The left panel represents the density maps of aromatic carbon atoms of the APGSs, the middle panel refers to the density maps of CH2 or CH3 groups of the APGSs’ tails, and the right panel stands for nitrogen atoms of the APGSs’ polar heads. The hydrogen atoms of APGSs are omitted in the density map for clarity. SWCNT (12, 12) are used in these simulation systems, and the surface coverage is 0.318, 0.625 and 1.603 molecules/nm2, respectively.

Fig. 4. Adsorption on the SWCNT surface with different surface coverages. (a) charge density of APGSs with respect to the SWCNT axis. (b–d) representative simulation snapshots for SWCNT (12, 12) covered by APGSs. Red spheres are CH group in the nanotube, and blue and cyan spheres represent nitrogen atom of the APGSs’ polar heads and CH/CH2/CH3 groups of the APGSs’ tails, respectively. Water molecules and chloride ions are not shown for clarity. These results are from the simulation systems of Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ture thickness of the positive charge increases with the increase of surface coverage. The enriching aggregation of APGSs coating at

the nanotube surface increases the charge aperture thickness in Fig. 4c–d.


The surface charge brings both steric repulsion and colloidal stability, and thus improving charge density is an effective method to enhance the dispersion of SWCNTs. The charge density per volume is calculated, as shown in Fig. 4a, with the help of which, the surface charge density is also computed and listed in Figure S5 of SI. As shown in the Figure S5, the surface charge of APGSs is larger than the non-Gemini surfactants due to the inclusion of double polar heads (Suttipong et al., 2011). Thus, APGSs are a highly efficient dispersant for SWCNTs in aqueous solutions compared with the ordinary ionic surfactants (i.e., CTAB, SDS, SDBS, etc.). Furthermore, the SWCNT-APGSs complex has a wide potential application in gene delivery owing to its strong electrostatic interactions with DNAs. On one hand, the large surface positive charge of SWCNTAPGSs complex strengthens the electrostatic interaction with negatively charged DNAs, and on the other hand, the high specific surface area of SWCNT-based composites leads to a high drug loading efficiency (Farka et al., 2017; Wang et al., 2007). 3.3. Encapsulation behavior of APGSs For quantifying the encapsulation behavior of APGSs coating on SWCNTs, the thickness of self-assembly film is computed, as presented in Fig. 5. The thickness of self-assembly film ranges from 0.42 to 1.425 nm at various surface coverages, similar to the reported results with other surfactants in previous experimental and numerical observations (Suttipong et al., 2011; Rastogi et al., 2008). As illustrated in Fig. 5, the thickness of self-assembly film increases as increasing the surface coverages of APGSs, and decreases as increasing the radius of nanotube. Furthermore, the aggregate morphology of APGSs on the SWCNT surface depends on not only the surface coverage but also, to some extent, the diameter of SWCNTs (Suttipong et al., 2011). As illustrated in Fig. 4a, at low surface coverage (i.e., 0.624 molecules/nm2), both the aromatic rings and alkyl chains of APGSs recline on the nanotube surface, and a self-assembled monolayer is observed with small thickness. At high surface coverage (i.e., 2.915 molecules/ nm2), however, a cylindrical micelle around SWCNT is observed (illustrated in Fig. 4c). All these results are likely related to the adsorption phenomenon of feeding mechanism (Song et al., 2016; Srinivas et al., 2006). When the surface coverage is small, the adsorption area is relatively redundant. Thus, both the aromatic ring segments and alkyl chain parts of the APGSs contact with the nanotube surface, while the headgroups of APGSs extend

Fig. 5. The thickness of self-assembly film of APGSs adsorbed on SWCNTs (6, 6), (12, 12), and (20, 20) with different surface coverages. The illustrations (right panel) are the representative simulation snapshots for SWCNT (6, 6) at different surface coverages. Water molecules and chloride ions are not shown for clarity. The surface coverage corresponding to the molecule number of APGSs is shown in Table 1.

411

to the aqueous solutions. When at high surface coverage, the available surface area is insufficient for all alkyl chain parts of the APGSs to be adsorbed on the nanotube surface. Due to the strong p-p interaction between the aromatic rings of APGSs and nanotube, the aromatic ring segments aggregate at the SWCNT surface while the alkyl chain accumulate at the outer layer. Correspondingly, at high surface coverage, the thickness of self-assembly film increases dramatically, and cylindrical micelles around SWCNT are formed. Similar results on structural morphology of monolayer adsorption can be found for other ionic surfactants in previous experimental and simulation studies (Das et al., 2016; Xu et al., 2010). The radius of SWCNT also has an effect on the packing structure of APGSs around the nanotube surface. The radius of gyration (Rg ) is introduced to characterize the configuration transformation. The radius of gyration (Rg ) can be calculated as follows (Cai et al., 2016):

P Rg ¼

2 i kr i k mi

P

i mi

!1=2 ;

ð3Þ

where mi is the mass of atom i, and ri the position of atom i. The computed Rg is depicted in Fig. 6. We can find that the value of total radius of gyration remains unchanged after 65 ns, illustrating the simulation time of 80 ns is sufficient for equilibrating the simulation system. The total Rg of APGSs ranges from 2.45 to 3.0 nm (shown in Fig. 6 and Figure S6 of SI), which agrees well with previous observations (Wu et al., 2006). Though the values of Rgx and Rgy are approximately equal at different surface coverages of APGSs at equilibrium, significantly different values for Rgz can be found. As presented in Fig. 6, Rgz dramatically increases as increasing the radius of nanotube. The variation of Rgz demonstrates a configurational transformation, which also can be confirmed by the representative snapshots in the right panel of Fig. 6. Some redundant APGS molecules around the nanotube surface are hidden for clarity in Fig. 6. From the representative snapshots, folding configurations of APGSs (i.e., ‘‘U shaped” and ‘‘V shaped”, shown in Fig. 6b and d) are observed at the nanotube surface of SWCNTs (6, 6) and (12, 12). For SWCNT (20, 20) which has a larger radius, however, the adsorption configuration of APGSs on the nanotube surface presents unfolding patterns (i.e., ‘‘I shaped” and ‘‘L shaped”, shown in Fig. 6f). The phenomenon of such a configurational transformation can be interpreted as the dependence of Rgz . Indeed, a larger Rgz allows for more stretchable APGSs at the z-axis direction of nanotube surface. Interestingly, the unfolding and spiral adsorption configuration can be found in the adsorption of polymer onto the nanotube surface (Berton et al., 2014; Lemasson et al., 2011). Furthermore, for SWCNTs with the same radius, larger surface coverage of APGSs on the nanotube surface also increase the values of Rgz , as shown in Figure S5 of SI. A larger value of Rgz demonstrates more order and stretchable self-assembled film forms at the nanotube surface, thus resulting in more available and effective adsorption area. In addition, the binding energy is dominated by the number of aromatic C-C contacts in the APGSs-SWCNT complex. The increasing tendency of the unfolding and the spiral wrapping behavior exhibits that the molecular geometry of APGSs at the nanotube surface may affect these contact patterns. As shown in Fig. 6, an increasing p-p interaction is identified with the reduced steric requirements of a two-tail chains at the nanotube surface (Lemasson et al., 2011). The total number density maps also reveals a smart self-assembly behavior of APGSs. The total number density maps for SWCNTs (6, 6), (12, 12) and (20, 20) covered by APGSs at different surface coverages are presented in Fig. 7. From the top side view of Fig. 7, it is illustrated that the fragments of scattered light color spots with lower number density gradually approach to the inner nanotube surface when increasing the radius

412


Fig. 6. The radius of gyration varying with simulation time (left panel) and three representative simulation snapshots (right panel) for SWCNTs (6, 6), (12, 12) and (20, 20) covered by 24 APGS molecules. Rg represents the value of total radius of gyration, while Rgx , Rgy and Rgz represent the values of radius of gyration at x-axis, y-axis and z-axis directions. In the snapshots, each APGS is represented with one individual color. Water molecules and chloride ions are not shown for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Side (first row) and front (second row) 2D total number density maps for SWCNTs (6, 6), (12, 12) and (20, 20) covered by 24 APGS molecules. The results of density map (a and b), (c and d) and (e and f) are obtained from SWCNTs (6, 6), (12, 12) and (20, 20), respectively. The total number density maps are computed by summing up all carbon atoms and nitrogen atoms of APGSs while the hydrogen atoms are not included. The surface coverages are (a and b) 1.248 molecules/nm2, (c and d) 0.625 molecules/nm2 and (e and f) 0.382 molecules/nm2.


of SWCNT. Thus, APGSs mainly distribute at the inner surface of SWCNT with large radius. The increasing high packing density of APGSs at the inner SWCNT surface originates from their compact and ordered self-assembly configuration at the SWCNT surface, as verified in Fig. 6. 4. Conclusions A new strategy is proposed for constructing the surfactantbased encapsulation of SWCNTs using APGSs. By analyzing the 2D number density maps, we demonstrate that the p-conjugated pyrenyl group of APGSs and the hydrophobic tails decreases the potential of mean force between the surfactants and SWNCTs. Moreover, double-layer assembled films formed on the SWCNT surface enhance the dispersion of SWCNTs: (i) double headgroups of APGSs promote the steric repulsion and colloidal stability; (ii) the ordered self-assembled monolayer of APGSs on the SWCNT surface acts as stable protective films, and disperse the nanotubes. At low surface coverage (i.e., 0.382 molecules/nm2), the APGSs tend to self-assemble on the nanotube surface with folding structure. At high surface coverage (i.e., 1.248 molecules/nm2), the APGSs stretch at the nanotube surface and present unfolding configuration. Ordered self-assembled film and high packing density reveal the mechanism of configurational transformation, and namely, the transformation originates from the increase of p-p interaction or the decrease of steric requirements of Gemini surfactants at the nanotube surface. The configurational transformation also demonstrates a smart self-assembly behavior of APGSs on nanotube surface, providing more available adsorption area for generating more ordered monolayer film. Conflict of interest The authors declare no competing financial interest. Acknowledgment This work is supported by National Natural Science Foundation of China (Nos. 91534103, U1707602), by PetroChina Innovation Foundation (2017D-5007-0204), and by the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology. SZ acknowledges the support of Fok Ying Tong Education Foundation (151069). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ces.2018.05.009. References Allen, W.J., Yi, H.A., Gochin, M., Jacobs, A., Rizzo, R.C., 2015. Small molecule inhibitors of HIVgp41 N-heptad repeat trimer formation. Bioorg. Med. Chem. Lett. 25 (14), 2853–2859. Arnold, M.S., Suntivich, J., Stupp, S.I., Hersam, M.C., 2008. Hydrodynamic characterization of surfactant encapsulated carbon nanotubes using an analytical ultracentrifuge. ACS Nano 2 (11), 2291–2300. Banno, T., Miura, S., Kuroha, R., Toyota, T., 2013. Mode changes associated with oil droplet movement in solutions of gemini cationic surfactants. Langmuir 29 (25), 7689–7696. Barroso, T., Branco, R.J.F., Aguiar-Ricardo, A., Roque, A.C.A., 2014. Structural evaluation of an alternative Protein A biomimetic ligand for antibody purification. J. Comput. Aided Mol. Des. 28 (1), 25–34. Berton, N., Lemasson, F., Poschlad, A., Meded, V., Tristram, F., Wenzel, W., Hennrich, F., Kappes, M.M., Mayor, M., 2014. Selective dispersion of large-diameter semiconducting single-walled carbon nanotubes with pyridine-containing copolymers. Small 10 (2), 360–367. Bottacchi, F., Petti, L., Spaeth, F., Namal, I., Troester, G., Hertel, T., Anthopoulos, T.D., 2015. Polymer-sorted (6,5) single-walled carbon nanotubes for solutionprocessed low-voltage flexible microelectronics. Appl. Phys. Lett. 106 (19).

413

Burattini, S., Colquhoun, H.M., Fox, J.D., Friedmann, D., Greenland, B.W., Harris, P.J.F., Hayes, W., Mackay, M.E., Rowan, S.J., 2009. A self-repairing, supramolecular polymer system: healability as a consequence of donor-acceptor pi-pi stacking interactions. Chem. Commun. 44, 6717–6719. Cai, B., Li, X., Yang, Y., Dong, J., 2012. Surface properties of Gemini surfactants with pyrrolidinium head groups. J Colloid Interface Sci 370 (1), 111–116. Cai, L., Lv, W.Z., Zhu, H., Xu, Q., 2016. Molecular dynamics simulation on adsorption of pyrene-polyethylene onto ultrathin single-walled carbon nanotube. Phys. E 81, 226–234. Cao, Q., Han, S.-J., Tulevski, G.S., Zhu, Y., Lu, D.D., Haensch, W., 2013. Arrays of single-walled carbon nanotubes with full surface coverage for highperformance electronics. Nat. Nanotechnol. 8 (3), 180–186. Chatterjee, T., Yurekli, K., Hadjiev, V.G., Krishnamoorti, R., 2005. Single-walled carbon nanotube dispersions in poly(ethylene oxide). Adv. Funct. Mater. 15 (11), 1832–1838. Clark, M.D., Subramanian, S., Krishnamoorti, R., 2011. Understanding surfactant aided aqueous dispersion of multi-walled carbon nanotubes. J. Colloid Interface Sci. 354 (1), 144–151. Comer, J., Chen, R., Poblete, H., Vergara-Jaque, A., Riviere, J.E., 2015. Predicting adsorption affinities of small molecules on carbon nanotubes using molecular dynamics simulation. ACS Nano 9 (12), 11761–11774. Das, S.K., Sengupta, S., Velarde, L., 2016. Interfacial surfactant ordering in thin films of SDS-encapsulated single-walled carbon nanotubes. J. Phys. Chem. Lett. 7 (2), 320–326. De Volder, M.F.L., Tawfick, S.H., Baughman, R.H., Hart, A.J., 2013. Carbon nanotubes: present and future commercial applications. Science 339 (6119), 535–539. Delport, G., Vialla, F., Roquelet, C., Campidelli, S., Voisin, C., Lauret, J.-S., 2017. Davydov splitting and self-organization in a porphyrin layer noncovalently attached to single wall carbon nanotubes. Nano Lett. 17 (11), 6778–6782. Dou, W.-L., Xin, X., Xu, G.-Y., 2009. Dispersion of carbon nanotubes by amphiphilic molecules. Acta Phys. Chim. Sin. 25 (2), 382–388. Duan, M., Song, X.Y., Zhao, S.L., Fang, S.W., Wang, F., Zhong, C., Luo, Z.Y., 2017. Layerby-layer assembled film of asphaltenes/polyacrylamide and its stability of water-in-oil emulsions: a combined experimental and simulation study. J. Phys. Chem. C 121 (8), 4332–4342. Duan, W.H., Wang, Q., Collins, F., 2011. Dispersion of carbon nanotubes with SDS surfactants: a study from a binding energy perspective. Chem. Sci. 2 (7), 1407– 1413. Farka, Z., Jurik, T., Kovar, D., Trnkova, L., Skladal, P., 2017. Nanoparticle-based immunochemical biosensors and assays: recent advances and challenges. Chem. Rev. 117 (15), 9973–10042. Frise, A.E., Edri, E., Furo, I., Regev, O., 2010. Protein dispersant binding on nanotubes studied by NMR self-diffusion and cryo-TEM techniques. J. Phys. Chem. Lett. 1 (9), 1414–1419. Giraldo, J.P., Landry, M.P., Faltermeier, S.M., McNicholas, T.P., Iverson, N.M., Boghossian, A.A., Reuel, N.F., Hilmer, A.J., Sen, F., Brew, J.A., Strano, M.S., 2014. Plant nanobionics approach to augment photosynthesis and biochemical sensing. Nat. Mater. 13 (5), 400–408. He, Z., Zhou, J., 2014. Probing carbon nanotube-amino acid interactions in aqueous solution with molecular dynamics simulations. Carbon 78, 500–509. Headen, T.F., Boek, E.S., Skipper, N.T., 2009. Evidence for asphaltene nanoaggregation in toluene and heptane from molecular dynamics simulations. Energy Fuels 23 (3–4), 1220–1229. Huang, X., Yu, H., Tan, H., Zhu, J., Zhang, W., Wang, C., Zhang, J., Wang, Y., Lv, Y., Zeng, Z., Liu, D., Ding, J., Zhang, Q., Srinivasan, M., Ajayan, P.M., Hng, H.H., Yan, Q., 2014. Carbon nanotube-encapsulated noble metal nanoparticle hybrid as a cathode material for Li-oxygen batteries. Adv. Funct. Mater. 24 (41), 6516– 6523. Iverson, N.M., Barone, P.W., Shandell, M., Trudel, L.J., Sen, S., Sen, F., Ivanov, V., Atolia, E., Farias, E., McNicholas, T.P., Reuel, N., Parry, N.M.A., Wogan, G.N., Strano, M.S., 2013. In vivo biosensing via tissue-localizable near-infraredfluorescent single-walled carbon nanotubes. Nat. Nanotechnol. 8 (11), 873–880. Keyes-Baig, C., Duhamel, J., Wettig, S., 2011. Characterization of the behavior of a pyrene substituted gemini surfactant in water by fluorescence. Langmuir 27 (7), 3361–3371. Kunai, Y., Liu, A.T., Cottrill, A.L., Koman, V.B., Liu, P., Kozawa, D., Gong, X., Strano, M. S., 2017. Observation of the marcus inverted region of electron transfer from asymmetric chemical doping of pristine (&ITn&IT,&ITm&IT) single-walled carbon nanotubes. J. Am. Chem. Soc. 139 (43), 15328–15336. Lemasson, F.A., Strunk, T., Gerstel, P., Hennrich, F., Lebedkin, S., Barner-Kowollik, C., Wenzel, W., Kappes, M.M., Mayor, M., 2011. Selective dispersion of singlewalled carbon nanotubes with specific chiral indices by poly(N-decyl-2,7carbazole). J. Am. Chem. Soc. 133 (4), 652–655. Martinez, L., Andrade, R., Birgin, E.G., Martinez, J.M., 2009. PACKMOL: a package for building initial configurations for molecular dynamics simulations. J. Comput. Chem. 30 (13), 2157–2164. Mehra, N.K., Palakurthi, S., 2016. Interactions between carbon nanotubes and bioactives: a drug delivery perspective. Drug Discov. Today 21 (4), 585–597. Mitchell, M.J., Castellanos, C.A., King, M.R., 2015. Surfactant functionalization induces robust, differential adhesion of tumor cells and blood cells to charged nanotube-coated biomaterials under flow. Biomaterials 56, 179–186. Nish, A., Hwang, J.-Y., Doig, J., Nicholas, R.J., 2007. Highly selective dispersion of singlewalled carbon nanotubes using aromatic polymers. Nat. Nanotechnol. 2 (10), 640–646. Niu, Z., Dong, H., Zhu, B., Li, J., Hng, H.H., Zhou, W., Chen, X., Xie, S., 2013. Highly stretchable, integrated supercapacitors based on single-walled carbon

414


nanotube films with continuous reticulate architecture. Adv. Mater. 25 (7), 1058–1064. Nogueira, S.L., Sahoo, S.K., Jarrosson, T., Serein-Spirau, F., Lere-Porte, J.P., Moujaes, E. A., Marletta, A., Santos, A.P., Fantini, C., Furtado, C.A., Silva, R.A., 2016. A new designed pi conjugated molecule for stable single walled carbon nanotube dispersion in aqueous medium. J. Colloid Interface Sci. 464, 117–125. Palmer, J.C., Debenedetti, P.G., 2015. Recent advances in molecular simulation: a chemical engineering perspective. AIChE J 61 (2), 370–383. Perilla, J.R., Goh, B.C., Cassidy, C.K., Liu, B., Bernardi, R.C., Rudack, T., Yu, H., Wu, Z., Schulten, K., 2015. Molecular dynamics simulations of large macromolecular complexes. Curr. Opin. Struct. Biol. 31, 64–74. Pramanik, D., Maiti, P.K., 2017. DNA-assisted dispersion of carbon nanotubes and comparison with other dispersing agents. ACS Appl. Mater. Interfaces 9 (40), 35287–35296. Qin, X., Peng, F., Yang, F., He, X., Huang, H., Luo, D., Yang, J., Wang, S., Liu, H., Peng, L., Li, Y., 2014. Growth of semiconducting single-walled carbon nanotubes by using ceria as catalyst supports. Nano Lett. 14 (2), 512–517. Rastogi, R., Kaushal, R., Tripathi, S.K., Sharma, A.L., Kaur, I., Bharadwaj, L.M., 2008. Comparative study of carbon nanotube dispersion using surfactants. J. Colloid Interface Sci. 328 (2), 421–428. Robertson, M.J., Tirado-Rives, J., Jorgensen, W.L., 2015. Improved peptide and protein torsional energetics with the OPLS-AA force field. J. Chem. Theory Comput. 11 (7), 3499–3509. Robertson, M.J., Tirado-Rives, J., Jorgensen, W.L., 2017. Improved treatment of nucleosides and nucleotides in the OPLS-AA force field. Chem. Phys. Lett. 683, 276–280. Sarode, A., Ahmed, Z., Basarkar, P., Bhargav, A., Banerjee, D., 2017. A molecular dynamics approach of the role of carbon nanotube diameter on thermal interfacial resistance through vibrational mismatch analysis. Int. J. Therm. Sci. 122, 33–38. Song, X., Zhao, S., Fang, S., Ma, Y., Duan, M., 2016. Mesoscopic simulations of adsorption and association of PEO-PPO-PEO triblock copolymers on a hydrophobic surface: from mushroom hemisphere to rectangle brush. Langmuir 32 (44), 11375–11385. Srinivas, G., Nielsen, S.O., Moore, P.B., Klein, M.L., 2006. Molecular dynamics simulations of surfactant self-organization at a solid-liquid interface. J. Am. Chem. Soc. 128 (3), 848–853. Streit, J.K., Bachilo, S.M., Ghosh, S., Lin, C.-W., Weisman, R.B., 2014. Directly measured optical absorption cross sections for structure-selected single-walled carbon nanotubes. Nano Lett. 14 (3), 1530–1536. Subbaiyan, N.K., Cambre, S., Parra-Vasquez, A.N.G., Haroz, E.H., Doorn, S.K., Duque, J. G., 2014. Role of surfactants and salt in aqueous two-phase separation of carbon nanotubes toward simple chirality isolation. ACS Nano 8 (2), 1619–1628. Sun, L., Li, M., Jiang, Y., Kong, W., Jiang, K., Wang, J., Fan, S., 2014. Sulfur nanocrystals confined in carbon nanotube network as a binder-free electrode for highperformance lithium sulfur batteries. Nano Lett. 14 (7), 4044–4049. Suttipong, M., Tummala, N.R., Kitiyanan, B., Striolo, A., 2011. Role of surfactant molecular structure on self-assembly: aqueous SDBS on carbon nanotubes. J. Phys. Chem. C 115 (35), 17286–17296.

Tang, L., Yang, J., Yin, Q., Yang, L., Gong, D., Qin, F., Liu, J., Fan, Q., Li, J., Zhao, W., Zhang, W., Wang, J., Zhu, T., Zhang, W., Liu, J., 2017. Janus particles selfassembled from a small organic atypical asymmetric gemini surfactant. Chem. Commun. 53 (62), 8675–8678. Tieleman, D.P., Marrink, S.J., 2006. Lipids out of equilibrium: energetics of desorption and pore mediated flip-flop. J. Am. Chem. Soc. 128 (38), 12462– 12467. Tummala, N.R., Striolo, A., 2009. SDS surfactants on carbon nanotubes: aggregate morphology. ACS Nano 3 (3), 595–602. Vaisman, L., Marom, G., Wagner, H.D., 2006. Dispersions of surface-modified carbon nanotubes in water-soluble and water-insoluble polymers. Adv. Funct. Mater. 16 (3), 357–363. Vaisman, L., Wagner, H.D., Marom, G., 2006. The role of surfactants in dispersion of carbon nanotubes. Adv. Colloid Interface Sci. 128, 37–46. Wang, P., Kim, M., Peng, Z., Sun, C.-F., Mok, J., Lieberman, A., Wang, Y., 2017. Superacid-surfactant exchange: enabling nondestructive dispersion of fulllength carbon nanotubes in water. ACS Nano 11 (9), 9231–9238. Wang, C., Wettig, S.D., Foldvari, M., Verrall, R.E., 2007. Synthesis, characterization, and use of asymmetric pyrenyl-gemini Surfactants as emissive components in DNA - Lipoplex systems. Langmuir 23 (17), 8995–9001. Wei, Y.Y., Liu, G.K., Wang, Z.N., Yuan, S.L., 2016. Molecular dynamics study on the aggregation behaviour of different positional isomers of sodium dodecyl benzenesulphonate. RSC Adv. 6 (55), 49708–49716. Wu, H.G., Li, Y., Kadirov, D., Zhao, S.L., Lu, X.H., Liu, H.L., 2017. Efficient molecular approach to quantifying solvent-mediated interactions. Langmuir 33 (42), 11817–11824. Wu, H., Xu, J.B., He, X.F., Zhao, Y.H., Wen, H., 2006. Mesoscopic simulation of selfassembly in surfactant oligomers by dissipative particle dynamics. Colloid Surf. A 290 (1–3), 239–246. Xiao, D., Sun, W., Dai, H., Zhang, Y., Qin, X., Li, L., Wei, Z., Chen, X., 2014. Influence of charge states on the pi-pi interactions of aromatic side chains with surface of graphene sheet and single-walled carbon nanotubes in bioelectrodes. J. Phys. Chem. C 118 (35), 20694–20701. Xu, Y., Luo, Z., Li, S., Li, W., Zhang, X., Zuo, Y.Y., Huang, F., Yue, T., 2017. Perturbation of the pulmonary surfactant monolayer by single-walled carbon nanotubes: a molecular dynamics study. Nanoscale 9 (29), 10193–10204. Xu, Z., Yang, X., Yang, Z., 2010. A molecular simulation probing of structure and interaction for supramolecular sodium dodecyl sulfate/single-wall carbon nanotube assemblies. Nano Lett. 10 (3), 985–991. Yu, D., Goh, K., Wang, H., Wei, L., Jiang, W., Zhang, Q., Dai, L., Chen, Y., 2014. Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage. Nat. Nanotechnol. 9 (7), 555–562. Zhao, Y., Wu, W., Li, J., Xu, Z., Guan, L., 2014. Encapsulating MWNTs into hollow porous carbon nanotubes: a tube-in-tube carbon nanostructure for highperformance lithium-sulfur batteries. Adv. Mater. 26 (30), 5113–5118. Zhu, W., Khalifa, I., Peng, J., Li, C., 2017. Position and orientation of gallated proanthocyanidins in lipid bilayer membranes: influence of polymerization degree and linkage type. J. Biomol. Struct. Dyn., 1–14