Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in

Article pubs.acs.org/Langmuir

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2 Muhan Wang,† Timing Fang,† Pan Wang,† Youguo Yan,† Jun Zhang,*,†,‡ Bing Liu,† and Xiaoli Sun† †

College of Science and ‡Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, 266580 Qingdao, Shandong, China S Supporting Information *

ABSTRACT: Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO2), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C8Benz). It is found that adding the C8Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C8Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C8Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C8Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO2. of anionic surfactants21,22 could promote spherical RMs growth. Eastoe and his co-workers found that replacing the Na+ of fluorinated surfactant Na(di-HCF4) by divalent Co2+ or Ni2+ could transform the spherical RMs to wormlike shape.14 Further study of hybrid surfactants indicates that exchanging the counterion enables access to adjust the columbic repulsions between the head groups, hence forming the wormlike RMs.13 Second, appropriately cutting the surfactant tail also drive a sphere-to-rod transformation.15,23 Most recently, the fluorocarbon-hydrocarbon hybrid surfactants were found that could form wormlike RMs by cutting their hydrocarbon chains.16 The adjusting surfactants packing is regarded as the key of RMs elongating. These approaches have successfully applied to generate wormlike RMs in case of fluorinated surfactant self-assembly. Further, many studies attempt to generate wormlike RMs by these approaches using the hydrocarbon carbon surfactants24−26 which are less expensive and less harmful to the environment and most suitable for the requiring of today green chemistry. However, hydrocarbon surfactants are currently unsuitable for generating stabilize rodlike or wormlike RMs. Previous study21 found that replacing the Na+ of Na(TC14)

1. INTRODUCTION Supercritical carbon dioxide (scCO2) has been considered as a potential green solvent in diverse fields such as chemical synthesis, dry cleaning, extraction, and enhanced oil recovery,1−4 owing to its great advantages of being nontoxic, nonflammable, easily recyclable, and environmentally friendly.5−9 However, due to the low dielectric constant, CO2 is generally a poor solvent.10,11 As a result, it suffers from many unfavorable characteristics which has restricted the full potential of scCO2 in terms of industrial applications.11 Therefore, over the past decades, designing additives to modify the physicochemical characteristics of scCO2 (solubility, interfacial tension, wettability, viscosity, etc.) has attracted a great deal of attention. Surfactant (small molecules containing polar head-groups and nonpolar tails) is one of the most effective additives that can self-assemble into reverse micelles (RMs) in scCO2 that accommodate polar materials. When the RMs present a rodlike or wormlike shape, they could effective thicken the scCO2 fluid and thus improve the viscosity and solubility of scCO2.12 Drive by this issue of practical importance, numerous experimental13−16 and theoretical17−20 work has been devoted to find surfactants with an appropriate aggregation behavior and generating wormlike RMs. In terms of generating wormlike RMs in scCO2, two approaches have been used: first, exchanging the counterion © 2017 American Chemical Society

Received: April 6, 2017 Revised: May 5, 2017 Published: May 9, 2017 5291

DOI: 10.1021/acs.langmuir.7b01176 Langmuir 2017, 33, 5291−5297

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field of Optimized Potentials for Liquid Simulations (OPLS-UA) force field.37,38 The OPLS-UA force field is a widely used MD simulation force field which is based on experimental properties of liquids and gas phase, it united carbon and connected hydrogen on hydrocarbon (nonpolar hydrogen, e. g. − CH3, − CH2 groups) as a bead to save computation. It is able to accurately predict liquids properties for the surfactant/CO2 systems.39,40 This force field could be composed by the pairwise and the bonding interaction. The pairwise interaction include two parts: the short-range van der Waals (vdW) and the longrange electrostatic interaction. The vdW interaction is represented by 12−6 Lennard-Jones potential and the electrostatic interaction is represented by Coulombic potential. For the bonding interaction, there are three components including bond stretching, angle bending and dihedral torsion. All of the pairwise and the bonding interaction potential function formulas are described in section S1 in the Supporting Information. The force field parameters of TC14 and C8Benz are modeled using the all-atom OPLS parameters41,42 and the work of Canongia Lopes et al.43 scCO2 was described by the EPM2 model.44,45 To describe water, the SPC/E model46 was used. We carried out two series of self-assembly system. First, TC14 selfassembly system consists of 128 TC14 molecules, 640 water molecules and 12 600 CO2 molecules. Second, TC14 and C8Benz self-assembly system consists of 128 TC14 molecules, 16 C8Benz molecules, 640 water molecules and 12600 CO2 molecules ([moles of surfactant] = 0.2 mol/mL, [moles of surfactant] = 0.025 mol/mL). The initial simulation structure of all molecules were randomly initialized in the simulation box with the dimensions of 100 Å × 100 Å × 100 Å. Periodic boundary conditions were applied in all directions. These models were built by moltemplate software.47 The initial configuration of simulation box were shown in Figure 1b. The self-assembly process is separated in three parts. First 10 ns was simulated in the NPT ensemble at 318 K and 40 MPa. This process will get a reasonable system density. And then, a 90 ns NVT ensemble were calculated. The last 10 ns were used for statistical averages of all relevant observables. The temperature and the pressure were controlled via the Nosé48,49 thermostat and barostat with the bath coupling time of 100 steps for the thermostat and 1000 steps for the barostat. For all the simulations, a cutoff distance of 10.0 Å was applied for the short-range pair interactions, the long-range electrostatic interactions were treated using the pppm summation,50 the time step was 1 fs, the trajectory was saved every 1 ps for the analysis. Three independent initial configurations were chosen for analyzing the TC14 surfactant and cosurfactant (C8Benz) self-assembly in scCO2. After the simulation, all the snapshots were displayed by VMD software.51

surfactant by divalent Co2+ or Ni2+ induce the spherical RMs with little change. Moreover, adjusting the tails of surfactants also fail to transform the spherical RMs to wormlike form.27 Accordingly, it is urgently needed to obtain a method for design the wormlike RMs using the hydrocarbon surfactants. An alternative strategy is employed here, by introducing cosurfactant in hydrocarbon surfactant self-assembly system. Recently, the cosurfactant has been proved to transform the shape of hydrocarbon surfactant self-assembly.28,29 TC14 surfactants25 is one of the most appreciated hydrocarbon surfactant that can form stable reverse micelles (RMs) in scCO2. And the research30 suggests that the TC1425 surfactant shows shape transition with adding cosurfactants of C8Benz. Hence, TC14 surfactant is one of the most representative hydrocarbon surfactants and has a great application prospect. But, along with this strategy, generating long rodlike or wormlike RMs remains difficulty in scCO2 fluid.30 Herein, we intend to present a guideline for designing the wormlike RMs in scCO2. First of all, it is necessary to understand the underlying mechanism of TC14 RMs shape transition with introducing C8Benz (molecular structures are shown in Figure 1a). Interpreting it by experimental techniques, such as NMR, SANS, X-ray scattering, is challenging and requires thorough theoretical.

Figure 1. (a) Molecular structures of surfactant (TC14) and cosurfactant (C8Benz). (b) Initial configuration of simulation box. Color scheme: red = oxygen; green = carbon; white = hydrogen; yellow = sulfur; blue = sodium.

In recent decades, the molecular dynamics (MD) simulations have been successfully applied for micelles system to study the surfactants self-assembly microscopic information which is not available in experiments.31−35 Therefore, performing MD simulation studies on the self-assembly of TC14 with its cosurfactants in scCO2 is expected to reveal the RMs shape formation mechanism. In this work, the MD methods was used for discussing and designing the wormlike RMs. First, RMs shape transition mechanism was investigated by the studies of microscopic equilibrium morphologies. We found that adding cosurfactants in TC14 self-assembly system will lead to a trend of sphere-to-rod transition of RMs shape. The unique multistage formation mechanism is regarded as the origin of the shape transition. Moreover, the affinity of TC14 and cosurfactant was clarified by the evidence of free energy and order parameter calculation. Thus, a design strategy was finally proposed and the long wormlike RMs was generated. Predictability, this work will facilitate the design of wormlike RMs and broaden the application of scCO2 fluid.

3. RESULTS AND DISCUSSION To unveil the effects of C8Benz on the shape of TC14 selfassembly, two comparative simulations were conducted and the equilibrium morphologies were shown in Figure 2. Without the C8Benz, some spherical RMs were formed (Figure 2a). With adding a small fractions of C8Benz, a short rodlike RM was

Figure 2. Equilibrium morphologies of TC14 surfactants self-assembly in scCO2. The snapshots are of (a) only TC14 and (b) TC14 cooperating with C8benz self-assembly system. For clarity, all the CO2 and the surfactants except the rodlike RMs in (b) are omitted. The following figures are the same.

2. MODELS AND SIMULATION DETAILS In this work, MD simulations were performed by large-scale atomic/ molecular massively parallel simulator (LAMMPS) software.36 All interatomic interactions were described by the united-atom (UA) force 5292

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molecules. The structure of the pool regions most like the spherical RM structure (which is shown in the inset figure on the Figure 3). These divisional regions in density profiles indicate that the structure of TC14 self-assembly with cosurfactants prefer to generate the dumbbell-like RM rather than generate the uniform rodlike RM. The function of C8Benz is to connect two spherical TC14 RM and thus drive the RM shape transformation. Therefore, in the TC14/C8Benz selfassembly system, there are only a few amounts of C8Benz that significantly affect the shape of self-assembly because they are only needed to locate at the middle region of rodlike RM. In addition, the density profiles also indicate that the C8Benz molecules need to possess two characteristics for forming the dumbbell shape RMs: First, the C8Benz molecules must suitable to self-aggregate. Because it was aggregated in a region instead of dispersing in the whole of RMs. The self-aggregation of C8Benz was proved by the free energy difference between TC14 and C8Benz molecular binding and C8Benz molecular self-binding (Supporting Information S2). We found that the free energy of C8Benz molecular self-binding is much bigger than the TC14 and C8Benz binding (about 28 kJ/mol). And most importantly, this self-aggregation C8Benz must easy to fusion to TC14 RMs. Only if a C8Benz RM fuses with two TC14 RMs, the dumbbell-like RM will appear. To analysis the function of RMs fusion, we analysis the selfassembly process of the TC14 and C8Benz in scCO2 that is shown in Figure 4. In the first 1 ns, an initial fast clustering of water and TC14 and C8Benz head-groups was found. After that, a 10 ns fusion process was followed and then, forming several spherical RMs. These spherical RMs will further fuse with each other and finally form the rodlike RMs (Figure 4c). In contrast, the self-assembly process was stopped at the 10 ns without the further fusion step of spherical RMs (Supporting Information S3). We can conclude that the function of adding C8Benz is adjusting the RMs fusion. In order to quantitatively prove the function of C8Benz for the fusion, we perform the potential of mean force (PMF) profiles calculation on the process of the fusion of two TC14 RMs and the fusion of TC14 RM with C8Benz RM. In this paper, the PMFs clarify the free energy profiles in the process of two RMs fusion which are performed by using the adaptive biasing force (ABF) method53 that are given in Supporting Information S4. In the process of two TC14 RMs close to each other, the PMF shows a clear barrier at 26 Å. This is also close to the radius of a TC14 RM. The free energy barrier inhibited the fusion of two TC14 RMs, which indicates this combination process is difficult to achieve. In contrast, the PMF shows a decline when C8Benz and TC14 RMs close to each other.

formed (Figure 2b). These simulation results were highly consistent with the reported results of experiment.30 Why does the cosurfactant could induce the transformation from spherical RMs to rodlike RMs? Previously experimental study28,29 suggests that the cosurfactants could insert into the surfactant film for modulating its packing curvature and thus facilitate the sphere-to-rod transformation. But, the fraction of cosurfactant is very small and limited to change the packing behavior. Therefore, the cosurfactant has to work on the key location for driving the shape transformation. To clarify the localization of C8Benz in the surfactant films, we study the density distributions of all components in the rodlike RM along with its backbone52 in Figure 3. The definition of the backbone for rodlike RM is shown in the inset figure on the top of Figure 3.

Figure 3. Density distributions of all components in the rodlike RM along with its backbone. The top inset snapshot diagram shows the definition of the rodlike RMs backbone.

According to the density distributions, we found a unique mechanism of forming rodlike RMs base on a dumbbell-like structure. The rodlike RM can be divided into three regions: a region for connection which is appeared on the middle of the rodlike RM and two regions of water pool which is appeared on both sides of rodlike RM along with the backbone. For the connection region, only the C8Benz molecules are located with a single peak. Others components in RM (i.e., water, TC14 and Na+) show very low density in this region. In the two pool regions on both side of RMs, there are three components of TC14, Na+ and water molecules without any C8Benz

Figure 4. Time evolution of the self-assembly of TC14 and C8Benz in scCO2. Snapshots are taken at (a) 1 ns, (b) 10 ns, and (c) 100 ns. 5293

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Langmuir Therefore, C8Benz and TC14 RMs can fuse with little inhibition. The C8Benz RM is easier to fuse with TC14 RM than the TC14 RMs self-fusion. Therefore, when the C8Benz RM fuses with two TC14 RMs on it both side, the dumbbelllike RM will be expected. The strong ability of fusion of C8Benz RM is the reason for why the shapes of TC14 RMs are growth after adding a small fraction of C8Benz. The inset of Figure 5

Figure 6. Order parameter of TC14 and C8Benz surfactants as a function of the atom carbon number in the surfactant tail-group (starting at the carbon atom connect to headgroup). The “C8Benzenhance” is the enhance fusion ability C8Benz molecular order parameter. The inset figure shows the molecular structure of the C8Benz and the TC14 RMs.

an axis of line-segment. The both ends of the line-segment are not shield by CO2-philicity groups. The conformation of C8Benz RM indicates that the surfactant head groups do not completely shield the aqueous core from the nonpolar exterior, and the largely fractions of polar component are exposed. Previous studies56,57 show that the exposure cores of RMs are beneficial to RMs fusion. Accordingly, these unshielded ends of line-segment possess high activity to fuse surrounding TC14 RMs. In conclusion, the orderly arrangement of C8Benz molecules on the RMs is the reason for that it can connect two TC14 RMs and thus generate dumbbell-like form. Inspired by above perspective, an origin idea is that whether we can use this function of C8Benz to connect two dumbbelllike RMs and further generate a long wormlike RM. Unfortunately, the wormlike RMs are not shown in both simulation17−19,39,58 and experimental25,30 studies. To design wormlike RMs, it is necessary to enhance the active of C8Benz in this self-assembly system. Therefore, it is needed to further enhance the fusion ability of C8Benz aggregation. Although there are many ways to enhance the fusion ability. We further try another C8Benz RM with a higher Sm, which is realized by a technical method. To ensure the parallel arrangement of C8Benz molecules, we add a very slight force on the both side of the C8Benz RM. This force will compel the C8Benz molecules to close to each other and thus lead a parallel conformations (see Figure S7-1). After treatment, the Sm of the C8Benz molecules is shown in Figure 6. The Sm is increased. Consequently, we performed comparative simulations of two RMs fusion models: (1) the free self-assembly (FS) system and (2) enhance fusion ability self-assembly (EFS) system. The comparison of their equilibrium morphologies is shown in Figure 7. In the initial system, there are two dumbbell-like RMs and a C8Benz RM. The difference is that, in the EFS system, the Sm of C8Benz molecules is increased to enhance its fusion ability. After 30 ns simulations, the final structure of FS system shows two separate dumbbell-like RMs. By contrast, in the EFS system, the two initial dumbbell-like RMs are connected by the C8Benz RM in end-to-end and the final structure shows only a long wormlike RMs. Different from the general uniform wormlike RMs, the TC14 RMs and the C8Benz RMs are alternately arranged in this wormlike RM. And the beads-on-astring structure is generated. This structure is attributed to the multistage fusion between RMs. Also, the simulation result is a proof that the order of C8Benz molecules on RMs is the key factor for generating rodlike RMs. In the experiment, according

Figure 5. Potential of mean force (PMF) distribution of two RMs fusion. The r in abscissa is the distance of two RMs centroids. In the inset figure, the difference between the two TC14 RMs self-fusion and the TC14/C8Benz RMs fusion is shown. The dumbbell-like RM is also clarified.

describes the process of the dumbbell-like RM forming. The process of forming dumbbell-like RMs is very similar to the multistage self-assembly. TC14 and C8Benz respectively selfassembled and then cooperated to generate the dumbbell-like RMs. The middle C8Benz RM supplied a higher fusion freeenergy and thus stable the dumbbell-like RM. In these process, the TC14 and C8Benz molecules are playing specific role: the TC14 molecules supply the solubility (which is proved in S5 by the free energy of dissolution, the higher free energy was found in the process of TC14 dissolution) and the C8Benz molecules are self-assembled as a bridge to connect two TC14 RMs. The high fusion ability of C8Benz RMs are attributed to the self-assembly structure of their monomer. The order parameter54,55 (Sm) was calculated for understanding the reason why C8Benz RM is easy to fuse with TC14 RM. The order parameter is relevant parameter to evaluate the molecular arrangement and the aggregation structures which is explained in Supporting Information S6. The value of Sm is 1 as for parallel surfactants and the value is 0 for isotropic distributing surfactants. The Sm’s for the TC14 and C8Benz tail-group are shown in Figure 6. In TC14 molecules, the Sm is very close to 0 which indicates that the ordering of TC14 molecules is independent. But, nearly 1 of Sm in the C8Benz molecules implies that the C8Benz molecules have a large relativity with each other. This relativity is expressed in the molecular arrangement of C8Benz in the RMs that the C8Benz molecules is prefer to parallel arrange in the C8Benz RMs. In the snapshot of Figure 6, the self-assembly structures of C8Benz RMs are most likely arranged along with 5294

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Figure 7. Comparison of two self-assembly models between the pre- and post-enhance, fusion ability of C8Benz RMs. Both models have the same initial system with two dumbbell-like RMs and a C8Benz RM.

the chemical structure of C8benz surfactants, we could infer that the stacking benzene might improve the order of molecules because of the π−π interaction. Therefore, inducing “benzene” in the surfactant molecules may be an effective way.

4. CONCLUSIONS In this work, MD simulations were taken for investigating and designing the shape of hydrocarbon surfactants self-assembly with inducing the cosurfactants. The effects of C8Benz on TC14 self-assembly structure was clarified. The self-assembly structure shows a sphere-to-rod transition after adding C8Benz. Through the analysis of microconformation of RMs, we found that the self-assembled RMs can be composed in three parts: two TC14 aggregate on the both ends and a C8Benz aggregates in the middle of RM which is difference from the suggestion of previous studies.28−30 This dumbbell-like shape of rodlike RMs is regarded as origin mechanism of the sphere-to-rod transition of RMs. Further, the free energy profiles indicates that the high fusion ability of C8Benz aggregation is the crucial to generate this structures. We suggest that improving the affinity of C8Benz aggregation is a potential strategy for generating wormlike RMs using the hydrocarbon surfactants which is difficult to achieve in present studies. Adjusting the affinity of cosurfactant by MD techniques, a wormlike RM with a beadson-a-string structure was finally generated with a multiple structure, which was little report in scCO2 system at molecular level.19,20,39,58 This investigation leads to possible new direction for designing wormlike RMs that are self-assembled by hydrocarbon surfactants. In the future, it will be desirable to reveal what kinds of chemical structures of cosurfactant possess the properties of generating multiple structural wormlike RMs and if there are different phase behavior of the beads-on-a-string structure RMs with traditional wormlike RMs. These works of the MD simulations provide an excellent approach for observing and designing structure of self-assembly. This strategy of design hydrocarbon wormlike RMs may facilitate to broaden the application of scCO2 with economic and environmental credentials.

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Pairwise and the bonding interaction potential function formula; potential of mean force profiles of molecules close; detail of adaptive biasing force method; calculation of free energy of dissolution; method of the order parameter calculation; method of enhancing the order parameter of C8Benz molecules (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0532-86983366. E-mail: [email protected]. ORCID

Jun Zhang: 0000-0001-7786-4825 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2014CB239204, 2015CB250904), the National Natural Science Foundation of China (U1262202), the Shandong Provincial Natural Science Foundation, China (ZR2014EEM035), the Fundamental Research Funds for the Central Universities (14CX02222A, 15CX08003A, 15CX05049A, YCXJ2016080). We thank Dr. Jie Zhong, who is from University of Nebraska at Lincoln, for helpful discussions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01176. 5295

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DOI: 10.1021/acs.langmuir.7b01176 Langmuir 2017, 33, 5291−5297