J IRAN CHEM SOC DOI 10.1007/s13738-015-0665-1

ORIGINAL PAPER

Study of dispersion of carbon nanotubes by Triton X‑100 surfactant using molecular dynamics simulation S. Mahmood Fatemi1 · Masumeh Foroutan1 

Received: 20 November 2014 / Accepted: 11 May 2015 © Iranian Chemical Society 2015

Abstract  In this study, the dispersion mechanism of aggregated carbon nanotubes (CNTs) using Triton X-100 surfactant under various concentrations is investigated with and without water molecules via molecular dynamics simulation. The obtained results showed that because of interaction between water molecules and hydrophilic segments of surfactant, water molecules play a significant role in the manner of adsorption of the surfactant on the CNT surface. In the presence of water molecules, the surfactant molecules are not able to wrap the CNTs, and they are located in the neighborhood of the CNTs. The results suggested that the creation of space between two CNTs in the absence of the surfactant is performed slowly, while, in the presence of the surfactant molecules, the creation of space between two CNTs which leads to the dispersion of the CNTs is remarkably rapid. The surfactant molecules cause to introduce more numbers of water molecules in the vicinity of and between the CNTs, and with the increasing radial distances between two CNTs, the number of water molecules is rapidly increased. The interfacial angle between two CNTs, surfactant gyration radius, and diagrams of radial distribution function between water molecules, the CNTs, and the surfactant molecules were calculated for a better description of dispersion mechanism of the CNTs by the surfactant and water molecules. Electronic supplementary material  The online version of this article (doi:10.1007/s13738-015-0665-1) contains supplementary material, which is available to authorized users. * Masumeh Foroutan [email protected] 1



Department of Physical Chemistry, School of Chemistry, College of Science, University of Tehran, Tehran, Iran

Keywords  Aggregated carbon nanotube · Triton X-100 surfactant · Dispersion · Interfacial angle · Radial distribution function

Introduction Carbon nanotube (CNTs) have been investigated extensively because of their unique potential applications [1], but unfortunately CNTs in organic [2] and aqueous media [3] are aggregated because of strong interactions of van der Waals forces between CNTs walls, and these interactions reduce the efficiency of these materials [4]. There are several methods to disperse the CNTs in the solution, like the usage of super acid [5–8] and modification of the surfaces of CNTs as covalent and non-covalent [9, 10]. Abundant studies about dispersing and stabilizing of CNTs in aqueous medium have been reported with different types of amphiphilic molecules, surfactants such as sodium dodecylsulfate (SDS) [11], sodium dodecyl benzenesulfonate (SDBS) [12], polymers [13–15], DNA [16, 17], ionic liquids [18], and polypeptides [19, 20]. The conditions of the arrangement and order of situation of surfactant molecules around the CNTs are dependent on the conditions of adsorption of a particular surfactant on a CNT with a special length, diameter, and chirality, which may have effect on the interaction of the CNTs [21]. Adsorption of a surfactant plays a significant role in the standard procedure of production of high weight fraction and exclusively dispersing the CNTs [22, 23]. Experimental studies indicated that during the adsorption process of an anionic surfactant, like SDBS on the surface of CNTs, Columbic force does not have an important role, but hydrophobic interaction between the tail of surfactant and the wall of CNTs is remarkable and has a prominent effect

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in stabilizing the structure of the CNT in aqueous medium [24]. On the basis of recent MD simulations, it was shown that the surfactant molecular structure strongly affects the packing of surfactants on the nanotubes, thereby reducing an effective nanotube–nanotube interaction which is in qualitative agreement with experimental studies, so no strong effects due to nanotube diameter were observed [25]. In another work [26], it is revealed that the cohesive energy per one adsorbed surfactant molecule of the CNT–surfactant complex depends on the type and the number of the adsorbed surfactant molecules on the CNT surface. Furthermore, those authors found that the reactions of adsorption and desorption of surfactant molecules occur on the CNT surface. Hence, the CNTs are thought to be coated by the amphiphilic molecules composed by surfactant. A nondestructive sorting method to separate single-walled CNTs by diameter in aqueous solution was recently employed by Carvalho and dos Santos [27]. As per this method, CNTs of different diameters are distributed according to their densities along the centrifuge tube. Recently, Arnold et al. [28] have reported on the sorting of CNTs by diameter, band gap, and electronic type using SDS and SDBS surfactants. Also, they produced bulk quantities of CNTs of predominantly a single electronic type using different mixtures of surfactants. They have shown that the energetic balance among nanotube– surfactant, water–surfactant, and surfactant–surfactant interactions as well as the packing density, orientation, ionization, and the resulting hydration of the surfactants, and the capacity of a surfactant to disperse CNTs in aqueous solution are critical parameters affecting the buoyant density and the quality of sorting. Rastogi et al. [29] have reported a comparative analysis on dispersion of multiwalled CNTs with four surfactants: Triton X-100, Tween 20, Tween 80, and SDS, using UV–Vis spectroscopy and transmission electron microscopy. They have shown that Triton X-100 and SDS provide maximal and minimal dispersions, respectively, and also have determined an optimal CNT-to-surfactant ratio for each surfactant. According to their experimental results, the dispersing power of the four surfactants in terms of percentage extractability follows the trend: SDS < Tween 20 < Tween 80 < Triton X-100. Wang et al. [30] have investigated the dispersion of single-walled CNTs in heavy water with Triton X-100 surfactant using small-angle neutron scattering. Their results indicate an optimal surfactant concentration for dispersion, which, they suggest, results from competition between maximization of surfactants’ adsorption onto CNTs’ surfaces and a depletion interaction between CNT bundles mediated by surfactant micelles. In a solution containing CNT and surfactant, surfactant molecules can be found in one of three states: in micelles, adsorbed onto CNT surface, and as individual molecules, and

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depending on the temperature and surfactant concentration, surfactants can be found in two or three of the above states. In Triton X-100 case, it has been shown that, at a surfactant loading below 0.5 %, most surfactant molecules do not exist in the state of micelles, whereas at high concentrations, the micellar volume fraction takes asymptotically the solution value. Adsorption increases with surfactant concentration and saturates around 0.5–1 % by mass. In the present research, on the basis of nondestructive and non-covalent approach, dispersing of aggregated CNTs by Triton X-100 surfactant via MD simulation was studied. This study can provide insight into better understanding of the intricacies of dispersing CNTs using Triton X-100 surfactant.

Simulation details MD simulations were performed in the Tinker molecular-modeling package (version 5.0) [31] using the AMBER99 force field [32], which has recently been evaluated by us [33]. All Simulations are carried out in a canonical (NVT) ensemble. The simulation box was of dimensions 45 × 45 × 45 Å3. In all simulations, 1200 water molecules were used. The intermolecular three-point potential (TIP3P) [34] model was employed to represent water molecules. At the starting point of simulations, the Triton X-100 surfactant was initially placed alongside the CNT and then aligned parallel to the CNT wall, so that the surfactant was within the cutoff distance of the van der Waals interactions with the CNTs’ atoms (nearly 10 Å from the CNT wall).The Nose–Hoover thermostat with a time constant of 0.1 ps was employed to regulate and maintain the temperature at 300 K. A cutoff distance of 16.0 Å was used for the van der Waals potentials, and Lorentz-Berthelot mixing rules were used for cross interactions. The equations of motion were integrated by using the velocity form of the Verlet algorithm method. After 500 ps of equilibration time, the simulations were continued for another 2 ns to obtain enough statistical sampling to perform the calculations. Since the target was consideration of dispersing the aggregated CNTs by the surfactant at the start of the dispersion process was on the focus of attention, the times more than 2 ns were not simulated. We used two open-ended finite-length (5, 5) armchair CNTs with a length of about 25 Å, the tubes of which are saturated at the ends with hydrogen atoms. The diameter of the CNT is 6.68 Å. The initial C–C and C–H bond lengths for the CNT were 1.42 and 1.09 Å, respectively. In this research work, the neutrally charged CNTs were considered. The structure of Triton X-100 which is illustrated in Fig. 1 consists of 100 atoms.

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Fig. 1  Structure of Triton X-100 surfactant (hydrogen atoms are omitted for clarity)

Fig. 3  Representative snapshot of MD simulation including two CNTs and 1200 molecules of water, (colors assigned to each atom: red oxygen, white hydrogen, green carbon)

Fig. 2  Representative snapshot of MD simulation of equilibrium system configurations of the CNT/Triton X-100 without solvent (right side lateral view, left side cross-sectional view)

The stage of energy minimizations was performed to obtain the stable morphology of the surfactant molecule with minimum potential energy, which were used as the initial confirmations of the surfactant molecule in simulations. In this present research, the CNTs similar to other molecules were not fixed and can be displaced through interaction energy available in the system, in contrast to what was considered in the other papers [35]. To investigate the effects of CNTs on Triton X-100 surfactant behavior and vice versa, we have considered the partial charges of Triton X-100 surfactant. A full natural bond orbital (NBO) analysis was obtained, and corresponding atomic partial charges were obtained through summation over natural atomic orbitals. Thus, using fully optimized geometries of Triton X-100 surfactant, the amount of NBO partial charges was computed and then assigned to Triton X-100 surfactant structures for MD simulations. Atomic labels and the partial charges of atoms of Triton X-100 surfactant are given in Figure S1 and Table S1, respectively (see supporting information).

Results and discussion For investigating the dispersion mechanism of CNTs in the triton surfactant aqueous solution, three systems as stated below were considered:

Fig. 4  Representative snapshot of MD simulation including two CNTs and two numbers of Triton X-100 surfactant and 1200 molecules of water (colors assigned to each atom: red oxygen, white hydrogen, green carbon)

1. The system consisting of one CNT and one surfactant in the absence of water molecules. A snapshot of the system simulation has been shown in Fig. 2. 2. A system consisting of two CNTs in the water in the absence of the surfactant. A snapshot of the system simulation has been shown in Fig. 3. 3. A system consisting of two CNTs in the water and in the presence of two and four numbers of the surfactant. The latter system offers a study about the effect of concentration of the surfactant on dispersing the CNTs. The snapshots of simulations of the mentioned systems have been shown in Figs. 4 and 5.

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Displacement of center of mass of the CNTs

Fig. 5  Representative snapshot of MD simulation including two CNTs and four numbers of Triton X-100 surfactant and 1200 molecules of water (colors assigned to each atom: red oxygen, white hydrogen, green carbon)

Before studying the effect of the surfactant in dispersing the CNTs, the adsorption of the surfactant onto the CNT in the absence of water molecules was considered by simulating the system including a CNT and a Triton X-100 surfactant molecule. At first, the Triton X-100 surfactant was located in the neighborhood of the CNT, and at the end of the simulation, it was seen that the strong interaction between the surfactant and the CNT leads to wrapping up by the surfactant around the CNT [36, 37]. In the process of the simulation, the surfactant approaches the CNT and wraps it up assuming a helical form (Fig. 2). The values of these interaction energies are directly attributed to strong π–π interactions between the nanotube and surfactant; the same kind of interactions, called “π-stacking” [38], are responsible for the tendency of CNTs to “stick together.” In the case of the arrangement of Triton X-100 surfactant adsorption onto the CNTs, the explanations would be given in the next subsections. In order to study the effect of the surfactant in dispersing the aggregated CNTs, variations of the distance and the angle between two CNTs have been considered in the next two sections. Fig. 6  The diagrams of variations of center of mass of CNTs for the cases of (a) without Triton X-100 surfactant and (b) with four numbers of Triton surfactant. “r” indicates the distance from center of mass of two CNTs

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Figure  6 indicates the variation of the centers of mass of two CNTs in the first 2 ns of simulation in the absence and in the presence of four numbers of the surfactant. As shown in Fig. 6, at the start moment of simulation, the distance of the center of mass of two CNTs is about 8.50 Å. Comparison of Fig. 6a, b indicates that, in the absence of the surfactant and after expiry of 800 ps till 2000 ps, the CNTs stay together, and the center of mass of CNTs changes dramatically, whereas in the presence of the surfactant (Fig. 6b) in the times between 800 and 2000 ps, the two CNTs slowly get supplanted into the cell. It appears that the surfactant molecules with the establishment of proper interaction with two CNTs and water molecules cause the creation of a space between two CNTs and likewise attenuate the speed of the CNTs displacement.

Interfacial angle along the CNTs To understand the effect of the surfactant in dispersing the CNTs, interfacial angle (θ) along the two CNTs corresponding to Fig. 7 were simulated for the last 1 ns. Table  1 represents the interfacial angle between two CNTs (θ) under various concentrations of the surfactant. The table indicates that with the increasing concentration of the surfactant, the interfacial angle between the CNTs increases, and the CNTs are displaced further away from each other.

Surfactant radius of gyration To further consider the role of the surfactant in dispersing the aggregated CNTs, variation of the surfactant radius of gyration was calculated. The variation diagram of the surfactant radius of gyration in the presence and in the absence of water is shown in Fig. 8. As shown in Fig. 8, the surfactant radius of gyration during interaction with the CNTs in the absence of water in the

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of water molecules, the surfactant molecules behave in a different manner compared to that in the absence of water molecules. As snapshots in Fig. 9a–d shows, the surfactant molecules are mainly adsorbed onto the grooves present on the periphery of the CNTs, and the exterior surface of the outermost CNTs. In other words, the surfactant molecules prefer to align parallel to the CNTs rather than wrapping around the CNTs.

Number of aggregated water molecules around the CNT Fig. 7  Interfacial angle (θ) between two CNTs

Table 1  The effect of Triton X-100 surfactant on the interfacial angle of two CNTs in the last 1 ns of simulation Number of Triton X-100 surfactant molecules

0

2

4

Angle θ (°)

6

10

21

Fig. 8  Surfactant radius of gyration evolution during interaction with the CNTs in the presence and in the absence of water molecules

system remains constant and without fluctuation. As was mentioned in the previous section, in the absence of water molecules, because of the surfactant interacting with the CNTs and wrapping around them, this radius of gyration remains constant at 8.02 Å throughout the simulation (see Fig. 2), but in the presence of water molecules, the radius of gyration undergoes some fluctuations. The point which can be deduced from this behavior is that after attracting the surfactant molecules onto the CNT surface, the desorption does not occur. During the 2-ns simulation, molecules of the surfactant are unlikely to return to its original bared form. Figure 9a–d shows snapshots of equilibrium system configurations, including two CNTs and two and four surfactant molecules. It seems that the surfactant molecules with their hydrophilic group interact with the molecules of water, and the surfactant molecules cannot be wrapped around the CNTs as the Fig. 9 indicates, in the presence

To find the effect of the surfactant in dispersing the CNTs, the mean numbers of water molecules in the last 1 ns of simulation in different radii from the center of the CNTs were calculated in the absence of the surfactant and also with the various concentrations of the surfactant. For this purpose, the number of water molecules at different radii around the CNTs (R) was considered corresponding to Fig. 10. The results are given in Fig. 11. As the diagram indicates with the increasing number of surfactant molecules, more numbers of water molecules are located around the CNTs, and this would lead to dispersing the CNTs. It seems that following the displacement of each surfactant molecule, some molecules of water are dragged. The molecules of the surfactant in the neighborhood of water would drag more numbers of water molecules toward the region between two CNTs. In the further R, more numbers of water molecules are assembled around the CNTs, and this leads to the enhancement of the interfacial angle between two CNTs. Table 1 confirms this argument. Whenever the number of surfactant molecules is increased, more numbers of water molecules are aggregated around the CNTs.

Radial distribution functions (RDFs) Figure 12 illustrates the effects of the number of surfactant molecules on RDFs of water atoms with respect to the carbon atoms of the CNTs. As is represented in the RDF diagrams shown in Fig. 12, increasing number of the surfactant molecules leads to the increase of the water molecules around the CNTs, and this enhancement of the surfactant molecules enables the water molecules get closer to surface of the CNT. This behavior is deducted from shifting the diagram to fewer digits. As is seen from the Fig. 12, when there is no surfactant molecule in the system, the molecules of water are located at a distance of 2.75 Å from the CNT wall, and when there are four surfactant molecules in the system, the water molecules are located at a distance of 2.61 Å from the CNT

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Fig. 9  Lateral and crosssectional views of equilibrium system configurations, including two CNTs and two surfactant molecules (a, b) and four surfactant molecules (c, d). For better understanding of the surfactant adsorption onto the CNTs, molecules of water were omitted

Fig. 10  Distance from center of the CNT(R)

Fig. 12  RDF diagrams of oxygen atom of water molecules with respect to the carbon atom of the CNTs on the basis of the numbers of the surfactant molecules

Fig. 11  Diagram of the mean numbers of water molecules in the last 1 ns of simulation at different radii from center of the CNTs both in the absence and in the presence of the surfactant

wall. Table 2 indicates the distance of atoms (in terms of Å) based on RDFs of the oxygen atoms of water molecules

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with respect to the carbon atom of the CNT based on different amounts of the surfactant molecules. As is shown in Table 2, with the increasing surfactant molecules, the oxygen atoms of water molecules get closer to surface of the CNT. This result shows that more surfactant molecules cause more dense packing of surfactant molecules on the surface of the CNT. Other RDF diagrams that have been calculated are those of the RDFs of hydrophobic and hydrophilic segments of the surfactant with respect to the carbon atom of the CNT, in the presence and in the absence of water molecules, as can be seen in Fig. 13.

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Figure  13a indicates the RDF diagrams of the oxygen atom of hydrophilic segment and the carbon atom of the benzene ring of hydrophobic segment of the surfactant with respect to the carbon atom of the CNT in the absence of water. As the RDF diagrams show, the hydrophilic and hydrophobic segments of the surfactant molecules are located at distances of 2.65 and 2.95 Å from the CNT wall. As is seen in Fig. 13b, in the presence of water molecules, the hydrophobic segments are closer to the CNT than hydrophilic segments due to their wrapping around the CNTs. The diagram of Fig. 13b shows that the carbon atom of the benzene ring of hydrophobic segment and the oxygen atom of hydrophilic segment are located at distances of 3.05 and 3.97 Å, from the carbon atom of the CNT wall, respectively. It can be deduced from the higher peak of hydrophobic segments that there are more numbers of hydrophobic segments of the surfactant around the CNTs. Both of the hydrophilic and hydrophobic segments of the surfactant in the absence of water are closer to the CNT wall than those in the presence of water. The hydrophobic segment has greater affinity toward the surface of the CNT,

Table 2  The distance between oxygen atoms of water molecules and carbon atom of the CNT wall based on different amounts of Triton X-100 surfactant molecules Number of Triton X-100 surfactant

Distance (Å)

0 2

2.75 2.71

4

2.61

and with this segment, each surfactant molecule approaches toward the CNT. The hydrophilic segment of the surfactant has greater affinity toward water molecules. Hence, on the basis of hydrophobic and hydrophilic interactions between atoms of the surfactant, water molecules, and the CNT, the surfactant molecules play an important role in dispersing the CNTs. On comparison of RDF diagrams in Fig. 13, it can be found that the presence of water molecules increases the peak height in the RDF diagrams between hydrophobic segments and the CNTs. Table 3 indicates the distances of atoms (in terms of Å) based on RDFs of hydrophobic and hydrophilic segments of the surfactant with respect to the carbon atom of the CNT in the presence and in the absence of water. Table 3 reveals that in the presence and in the absence of water, hydrophilic segments of the surfactant are closer to the carbon atom of the CNT than hydrophobic segments of the surfactant. To consider the amphiphilic behavior of the surfactant in dispersing the CNTs, the RDF diagrams between hydrophilic and hydrophobic segments of the surfactant and water molecules were calculated. Figure 14 represents the RDF diagram of oxygen atom of the water–oxygen atoms of hydrophilic segment and the RDF diagram of oxygen atom of water–carbon atoms of the benzene ring of the hydrophobic segment of the surfactant. As is obvious from Fig. 14, the distance between oxygen atoms of water molecules and oxygen atom of hydrophilic segment is 1.39 Å. Also, the distance between the oxygen atom of water molecules and the carbon atom of the benzene ring of hydrophobic segment is 1.90 Å. Diagrams of Fig. 13b show that the total area under the RDF

Fig. 13  RDF diagrams of oxygen atom of hydrophilic segment and carbon atom of benzene ring of hydrophobic segment of the surfactant with respect to carbon atom of the CNT for the cases of (a) in the absence and (b) in the presence of water

Table 3  Distances between CNT and hydrophobic and hydrophilic segments of the surfactant molecules in different systems

Number of Triton X-100 surfactant molecules

Number of water molecules

Distance between CNT and hydrophobic segments of the surfactant (Å)

Distance between CNT and hydrophilic segments of the surfactant (Å)

1 2

0 1200

2.95 4.05

2.65 3.12

4

1200

3.97

3.05

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interactions with the surfaces of the CNTs and molecules of water, respectively, leading to the formation of hydration layers in the surrounding region and consequently to the increase in the number of molecules of water around the CNTs, which would result in the further separation of the CNTs. Hence, the increase of the surfactant molecules intensifies this effect as was proven by the application of radius of gyration diagram, the variation of the center of mass of the CNTs, RDF diagrams, and calculations of the angles between the center of the CNTs and the number of water molecules. Fig. 14  RDF diagrams of oxygen atom of water–oxygen atoms of hydrophilic segment (hydrophilic) and oxygen atom of water–carbon atoms of benzene ring of hydrophobic segment (hydrophobic) of the surfactant

Acknowledgments  The authors acknowledge the support by Iran National Science Foundation through grant #91058102.

References of the hydrophobic segment of the surfactant and the CNT is more than that under the RDF of hydrophilic segment of the surfactant and the CNT. Also, diagrams of Fig. 14 show that the total area under the RDF of the hydrophobic segment of the surfactant and water is more than that under the RDF of hydrophilic segment of the surfactant and water. Hence, hydrophobic segments of the surfactant play an important role in creating the strong interactions of the surfactant with both water and the CNT, and therefore it would result in dispersing the CNTs in aqueous solution. Because of the interaction of the hydrophobic segment of the surfactant with the CNT, the surfactant molecules reach the CNT. On the other hand, the interaction of hydrophilic segment of the surfactant with water molecules can be considered as a factor that would result in making the water molecules move toward the region between two CNTs and leading to the creation of longer spatial distances between the two CNTs. It appears that dispersions of the CNTs by the surfactant in the absence and in the presence of water have different mechanisms. In the absence of water molecules, the surfactant by adsorbing onto and wrapping around the CNTs prevent aggregation of the CNTs. In the presence of water molecules, water molecules act as intermediate molecules between the surfactant and the CNT and cause the surfactant molecules locate at the distances further away from the CNTs. On the other hand, in the presence of water molecules, larger surface areas of the CNTs are covered by hydrophilic and hydrophobic groups of the surfactant molecules, leading to further separation of the CNTs.

Conclusion The Triton X-100 surfactant molecules with hydrophobic and hydrophilic groups perform hydrophobic and hydrophilic

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