Atomistic Simulation of Solubilization of Polycyclic Aromatic

Apr 6, 2016 - Solubilization of two polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAP, 2-benzene-ring PAH) and pyrene (PYR, 4-benzene-ring ...
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Atomistic Simulation of Solubilization of Polycyclic Aromatic Hydrocarbons in a Sodium Dodecyl Sulfate Micelle Xujun Liang,†,‡,§ Massimo Marchi,‡,§ Chuling Guo,†,∥ Zhi Dang,†,∥ and Stéphane Abel*,‡,§ †

School of Environment and Energy, South China University of Technology, Guangzhou 510006, China Commissariat à l′Energie Atomique et aux Energies Alternatives, DRF/IBITECS/SB2SM/LBMS & CNRS UMR 9198, Saclay, France § Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Univ Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette cedex, France ∥ The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China ‡

S Supporting Information *

ABSTRACT: Solubilization of two polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAP, 2-benzene-ring PAH) and pyrene (PYR, 4benzene-ring PAH), into a sodium dodecyl sulfate (SDS) micelle was studied through all-atom molecular dynamics (MD) simulations. We find that NAP as well as PYR could move between the micelle shell and core regions, contributing to their distribution in both regions of the micelle at any PAH concentration. Moreover, both NAP and PYR prefer to stay in the micelle shell region, which may arise from the greater volume of the micelle shell, the formation of hydrogen bonds between NAP and water, and the larger molecular volume of PYR. The PAHs are able to form occasional clusters (from dimer to octamer) inside the micelle during the simulation time depending on the PAH concentration in the solubilization systems. Furthermore, the micelle properties (i.e., size, shape, micelle internal structure, alkyl chain conformation and orientation, and micelle internal dynamics) are found to be nearly unaffected by the solubilized PAHs, which is irrespective of the properties and concentrations of PAHs.

1. INTRODUCTION Surfactants, stemming from their ability to self-assemble into mesoscopic aggregates, such as micelles, vesicles, and bilayers, have long been of importance as solubilizing agents in the detergent industry, medical areas, and environmental remediation.1 In particular, surfactant-enhanced solubilization of hydrophobic organic compounds such as polycyclic aromatic hydrocarbons (PAHs) has been widely studied for the selection of optimum surfactant for soil and water remediation.2,3 Solubilization power is closely related with solubilization characteristics of solutes in surfactant micelles, especially under cosolubilization conditions. Guha et al.4 reported that, when naphthalene (NAP, Figure 1b) and phenanthrene (one benzene ring greater than NAP) cosolubilized, the former could enhance the solubility of the latter in the Triton X-100 (TX100) surfactant solutions. As such, Masrat et al.5 found that in sodium dodecyl sulfate (SDS) micelles the solubility of pyrene (PYR, Figure 1c) was increased in the presence of NAP. The enhancing effect is assumed to arise from the reduction in interfacial tension in the micelle−water interface caused by NAP solubilized in the micelle shell region.4 Consequently, detection of the locus of solutes in the micelle is regarded as an © 2016 American Chemical Society

important way for investigating enhanced solubilization characteristics. Experimental approaches used to study solubilization sites of PAHs in micelles include NMR,6 UV−visible spectroscopy,7 and fluorescence measurements.8 For instance, Bernardez6 has shown with 1H NMR that NAP could distribute in both the shell and core regions of micelles formed by TX100 surfactant. Recently, we have also found that NAP and PYR could partition in both the shell and core regions of the SDS micelles by means of 2D NOESY NMR observations (manuscript in preparation). The locus of NAP in micelle shell is presumably rooted in the formation of hydrogen bonds (HBs) between NAP and water.6 Because the surfactant micelle is very small, usually on the scale of a few nanometers, and coexists with the solvent, it is challenging for experimental approaches to capture a direct microscopic view of partition of solutes inside the micelle. To tackle this problem, molecular dynamics (MD) simulations Received: January 18, 2016 Revised: March 19, 2016 Published: April 6, 2016 3645

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Figure 1. Physicochemical properties and heavy atom sequences of (a) SDS, (b) naphthalene (NAP), and (c) pyrene (PYR). Selected physicochemical properties of NAP and PYR are listed in panels b and c, respectively.23 V means the NAP/PYR volumes in the simulations, which is computed with Voronoi approach with the homemade computational tool, trjVoronoi.28,29 formed by 60 SDS monomers. The micelle aggregation number, 60, was chosen according to previous fluorescence quenching experiment30 and MD simulations.9,12,21,31 In the case of the numbers of PAHs used for solubilization, they were estimated from our enhanced solubilization experiments, in which solubility of NAP and PYR was 0.015 and 0.0080 mol L−1 in 0.10 mol L−1 SDS solutions (manuscript in preparation). Details of simulations performed in this study are listed in Table S2. 2.2. Simulation Details. All MD simulations were performed using the GROMACS 5.0.4 package.32,33 The initial conformation of the SDS micelle was obtained by building a preassembled spherical aggregate with 60 SDS monomers using Packmol.34 The prepacked micelle was then centered into a cubic box of 9.95 nm on each side. For each solubilization system, the corresponding numbers of NAP and PYR molecules were introduced randomly around the micelle surface at 1.2 nm of the micelle surface. Finally, water (modeled with the CHARMM TIP3P water model35) and sodium ions36 were added to solvate and neutralize each system, respectively. The initial conformation of each system was first minimized with the steepest descent algorithm and an energy tolerance lower than 1000 kJ mol−1 nm−1. The energy-minimized systems were then equilibrated in the NVT (T = 300 K) and NPT (T = 300 K and P = 1.015 bar) ensembles for 400 ps and 1 ns, respectively. In the NVT and NPT stages, the temperature and pressure were controlled by the v-rescale37 thermostat (τT = 0.1 ps) and Berendsen38 barostat (τP = 2.0 ps). During these periods the SDS molecules were allowed to relax, whereas the PAH and water were kept fixed. Finally, the production runs were carried out at T = 300 K and P = 1.015 bar with the temperature and pressure controlled by the v-rescale thermostat37 (τT = 0.1 ps) and the Parrinello−Rahman39,40 barostat (τP = 3.0 ps). Periodic boundary conditions were used, and a time step of 2 fs for integrating the equation of motions was used with the neighbor list updated every 10 fs. The Particle Mesh Ewald method41 was used to compute the long-range electrostatic interactions with a cutoff of 1.2 nm. The Lennard-Jones interactions were treated with a switch potential with rvdw‑switch = 1.0 nm and rvdw = 1.2 nm. All bonds involving hydrogens were constrained with the P-LINCS algorithm.42 The pure SDS system was run for 60 ns, while the others were done at least 81 ns, depending on the time at which all PAHs molecules solubilized into the micelle. The NAP/PYR solubilization time was estimated roughly by computing the radius of gyration, Rg, of the micelle−PAH complex and corresponds to the time when the micelle−PAH complex Rg is stabilized. As shown in Figure 2, the Rg of the micelle−PAH complex of Systems II−VIII reached stable values at ∼1.65 nm after 0.3, 19.5, 14.0, 54.3, 0.2, 29.0, and 24.5 ns, respectively, indicating that the NAP/PYR solubilization time depends on PAH concentration.

have proven their usefulness, in particular, to study the timedependent structural and dynamical properties of surfactant solutions (see, for instance, refs 9 and 10). Several MD papers have reported the solubilization process of PYR within the membrane and surfactant aggregates.11−14 Yan et al.,12 for instance, observed that two PYR molecules could distribute dynamically in the shell and core regions of an SDS micelle and form dimers occasionally. These simulations, however, were done merely with one/two pyrene molecules in the solubilization systems, far from saturation according to the enhanced-solubilization experiments. Hence the solubilization characteristics as a function of PAH concentration remain obscure. Moreover, although the pure SDS micelle has been widely investigated through MD simulations,9,10,15−21 little is known regarding the effect of the increase in solubilized PAHs on the SDS micelle structural properties. We present results from atomistic MD simulations of SDS micelles with and without PAHs. We chose NAP and PYR as the model PAHs due not only to their common existence in the environment,22 but also to their distinct physicochemical properties.23 Thus, the aims of our investigation are as follows: (i) providing detailed descriptions of the distribution and orientation of PAHs in the micelle; (ii) gaining insights into the dynamics of PAHs in the micelle; and (iii) finally studying the influence of PAH concentration on micelle structure and dynamics. Results in this study will also further our understanding of the solute solubilization process within micelles.

2. SIMULATION DETAILS 2.1. Model Systems. NAP and PYR were modeled with the CGenFF24 with the atomic partial charges for NAP directly taken from the CHARMM force field distribution. In the case of PYR, the atomic charges were obtained with the ParamChem tool (see Table S1 in the Supporting Information).25,26 In addition, ParamChem can generate a penalty score for each assigned parameter to judge the quality of the parameters. The penalty scores obtained for PYR were all zero, indicating that this molecule is well supported by the CGenFF force field. In the case of the charges for SDS, we took them from the work of MacKerell17 with the alkyl chain bonded and nonbonded parameters from Klauda et al.27 Selective physicochemical properties and the corresponding heavy atom sequences of SDS, NAP, and PYR are presented in Figure 1. To investigate the effect of PAH concentration on solubilization characteristics, we inserted various numbers of NAP (1, 2, 5, and 10) and PYR (1, 2 and 5) into a box containing water and an SDS micelle 3646

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every 2 ps for subsequent analysis with different GROMACS tools and homemade programs.

3. RESULTS AND DISCUSSION 3.1. Distribution and Orientation of NAP and PYR in the Micelle. 3.1.1. Distribution of NAP and PYR in the SDS Micelle. At first, to get a picture of the solubilization sites of NAP and PYR in the micelle, we computed the radial density profiles (RDPs) of relevant SDS subunits (i.e., SDS sulfate group, C1, C6, and C12 alkyl carbon atoms) and the NAP/ PYR center of mass (COM) with respect to the micelle COM for the pure SDS and PAH−micelle solubilization systems (Figures 3a−c). Before analyzing the RDP curves of NAP/PYR, we first turn our attention to the RDP obtained for the pure micelle (Figure 3a). In the case of the SDS headgroup, the RDP shows a strong peak at the distance of ∼2.0 nm from the micelle COM, close to the micelle effective radius Rm (∼2.14 nm, see Section 3.3.1). The wide (∼0.8 nm) overlap of the water RDP with the RDPs of sulfate, C1 and C6 atoms indicates that water could not only solvate the SDS headgroup but also penetrate to a limited extent into the micelle hydrophobic region. The RDP of the SDS C1 atom shows maxima at ∼1.8 nm, indicating that this atom mainly located in the micelle surface. From the RDP of C6 atom, we can conclude that this atom stays inside the micelle hydrophobic region at distance (∼1.2 nm) slightly larger than half of the micelle COM and surface distance. Finally, concerning the terminal carbon atom of the SDS alkyl chain (C12), the widest distribution of the RDP (0 to 1.8 nm)

Figure 2. Radius of gyration of pure SDS micelle and the micelle− PAH complexes as a function of the simulation time. NAP and PYR are abbreviations for naphthalene and pyrene, respectively. The numbers before NAP or PYR in the legends mean the number of PAH molecules inserted into an SDS micelle, which corresponds to Systems II−VIII (Table S2). Moreover, Figure 2 also shows that the sizes of the micelle−PAH complexes do not change significantly with the solubilized PAHs whatever their concentrations are (see Section 3.3) and are close to the SDS pure micelle size. Finally, to have enough sampling to compute simulation results, we continued the production runs for 80 ns after all PAHs partitioned into the micelle and collected the data

Figure 3. Radial density profiles of (a) subunits of SDS, (b) naphthalene (NAP), and (c) pyrene (PYR) with respect to the micelle center of mass in different simulation systems. Headgroup, C1, C6, and C12 represent the subunits belonging to the SDS molecule, as depicted in Figure 1a. (a) Results computed from Systems I (solid line), V (dashed line), and VIII (dotted line). (b,c) Meanings of the legends are the same as those described in Figure 2. 3647

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Langmuir suggests a wider localization of this atom inside the micelle and also at the surface of the micelle. The RDP curves obtained for the SDS pure micelle are comparable to those obtained in previous works.9,10,21,43−45 Then, we turn our attention to the changes of the micelle internal structure induced by solubilized NAP and PYR. The corresponding RDPs (Figure 3a, dashed and dotted lines) show that even at the highest PAH concentration the RDPs of the SDS headgroup and alkyl chain carbon atoms have similar shapes and are located at the same distance from the micelle COM as those for the pure micelle. These results indicate that solubilization of the two PAHs have a negligible effect on the micelle internal structure (see Section 3.3.1). Similar results were obtained, for instance, by Yan et al.,12 who found with MD simulations that solubilization of two PYR molecules into the SDS micelle has negligible influence on the micelle internal structure. Concerning specifically the RDPs of NAP as a function of its concentration (Figure 3b), we could see a peak at ∼1.5 nm with in amplitude that increase with NAP concentration indicating a preferential localization of NAP in the micelle shell region as a function of the NAP concentration. The increase in NAP RDP near the micelle COM also suggests that NAP could reside in the micelle core. In the case of PYR solubilization systems (Figure 3c), the RDPs also show a peak at ∼1.5 nm that also increase with the PYR concentration, indicating that PYR, similar to NAP, can reside in the micelle shell region; however, in 1 PYR solubilization system, no peak of PYR RDP is observed near the micelle COM compared with that of 1 NAP solubilization system. This suggests that it is relatively rare for PYR to enter into the innermost core region of the micelle under low concentration condition. Only with increasing PYR concentration is the peak of PYR RDP observed near the micelle COM. Theoretically, PYR should be easier than NAP to reside near the micelle hydrophobic core due to its larger log Kow (Figures 1b,c); however, meanwhile, PYR has larger molecular volume than NAP (Figures 1b,c), which could contribute to the relative difficulty for this molecule to spontaneously enter into the micelle innermost core region. Moreover under high solute concentration, the repulsion between PYR molecules could favor PYR to move deeper into the micelle. In addition, comparison of the three RDP plots also suggests that NAP and PYR can share some contact with water (see Section 3.2.2 for detail analysis). The results are in line with our experimental work,46,47 in which NAP and PYR were demonstrated to be capable of situating in the core and shell regions of the SDS micelle using 1H NMR chemical shift measurements. Moreover, through comparison of the 1H NMR paramagnetic relaxation enhancement of the hydrocarbons and the SDS alkyl chain segments, Wasylishen et al.48 also observed the distribution of NAP in both regions of the micelle. Despite the usefulness to examine the micelle internal structure changes with PAH concentrations, the RDPs do not provide enough information about the preferred solubilization sites of two PAHs, in particular, along the surfactant alkyl chain. Therefore, we also compute the probability distribution (Figure 4) that a given SDS heavy atom is closest to the NAP and PYR C5 atoms, which are adjacent to the PAH COM (Figure 1b,c). The probability distributions were determined by first calculating the minimum distances between any SDS heavy atom and the NAP/PYR C5 atoms. Then, a tally of the closest heavy atom in the SDS molecule and the C5 atom in the PAH molecule was kept. This information was then normalized. We

Figure 4. Normalized probability distribution of the closest SDS heavy atoms to C5 atoms of naphthalene (NAP) and pyrene (PYR), which are adjacent to the center of mass of the two PAH molecules, as depicted in Figure 1. The meanings of the legends are the same as those described in Figure 2.

notice that the probability distributions are similar whatever the PAH concentrations are but slightly differ with the PAH types. Figure 4 shows that both the C5 atoms of NAP and PYR are unable to contact with the sulfur atom of SDS due to the steric and electrostatic energy barriers stemming from the three oxygen atoms surrounding the sulfur.31 Furthermore, the two molecules obtain higher probability to close to the middle and terminal carbon atoms than the oxygen and front carbon atoms of SDS. The difference is that NAP exhibits relatively higher probability to be close to the oxygen and front and terminal carbon atoms of SDS than PYR (showing a higher probability to contact with the middle of the SDS alkyl chain). The higher probability of NAP to contact with C12 than PYR implies that, apart from contacting with C12 in the surface and shell region of the micelle, NAP has more frequencies to contact with C12 in the micelle inner core region. Instead, PYR may be relatively rare to contact with C12 near micelle COM, especially under 1 PYR condition, due to its larger molecular volume (Figure 1b,c), resulting in its preference to stay in the middle region of the SDS tail. This may further explain why we found a low RDP of PYR near micelle innermost core in the 1 PYR solubilization system (Figure 3c). 3.1.2. Movement of NAP and PYR Inside the Micelle. As mentioned in the previous section, NAP and PYR are both able to stay close to any atoms of SDS alkyl chain except the sulfur atom. We presume that this may arise from the movement of the two PAHs in the micelle. To illustrate this, we compute the COM distance between PAH molecule and the micelle, dCOMs, as shown in Figure 5. Figure 5 shows that for the SDS system with 1 NAP/PYR the distance between the micelle and PAH COMs varies a lot during the simulation time. Similar results were also observed in the systems with higher PAH concentrations (data not shown). This means that NAP and PYR could move significantly inside the micelle whatever their concentrations are. Moreover, from Table 1, we could see that in all solubilization systems both NAP and PYR gain a relatively higher percentage of simulation time (75−80 and 78−84%, respectively) to stay in the micelle shell, suggesting that this is their preference localization in the micelle. The probability distribution of the distance between PAH and micelle COM, as well, indicates that NAP/PYR has a higher probability to stay in 3648

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Furthermore, to examine the orientation of NAP/PYR in the micelle, we computed the angles between the normal vector of the PAH plane and the vector formed by the COMs of PAH and micelle and converted them to a normalized angle probability distribution. As shown in Figure S3, the aforementioned angle exhibits a wide distribution (20−160°) in all PAH−micelle systems, meaning that the solubilized PAHs exhibit broad orientation during their residence in the micelle. 3.2. PAH−PAH and PAH−Water Interaction. 3.2.1. PAH−PAH Interaction. At first, we examined the possibility of forming PAH dimers between two any PAH molecules in the micelle by computing the distance distribution between the COMs of NAP/PYR. As shown in Figure S4, the COM distances between two NAPs/PYRs show high probability to be within the range of 1.3 to 2.3 nm. We find that in the distance range, SDS molecules could intercalate between two PAH molecules (Figure S5a). Furthermore, the PYR COM distance exhibits a small peak at the distance of 0.7 nm (Figure S4); however, we find that 0.7 nm is still larger to define a cutoff distance for the formation of dimer due to the fact that some SDS molecules could also intercalate between two PYRs under this circumstance (Figure S5b). Moreover, according to ref 51 and 52, the dimer distances for NAP/PYR are ≤0.5 nm. Hence, a cutoff distance of 0.5 nm is selected for defining the formation of dimer between two NAPs/PYRs. Then, we computed the dimer formation time through dividing the contacts when the COM distances between two PAH molecules are within 0.5 nm by their whole contacts. We find that, irrespective of PAH concentration, NAP and PYR form dimers for ∼0.2 and ∼0.4% of simulation time in our simulations, respectively. As shown in Figure S6, the instantaneous COM distances between two NAP/PYR molecules in Systems III and VII change so frequently that contribute to the short dimer formation time. These phenomena also exist in higher PAH concentration systems (data not shown). Occasional formation of PYR dimer with a cutoff distance of 0.42 nm in an SDS micelle was observed by Yan et al.,12 in which the GROMOS 45a3 force field was chosen to simulate the solubilization process. Furthermore, we examined the dimer configurations when two NAP or PYR molecules form a dimer by computing the angle θ, defined as the angle between the normal vectors n1⃗ and n⃗2 of the molecular planes of two PAH monomers (Figure 6f). We used the same criterions to define the configurations as those described by Gladich et al.53 Specifically, the parallel displaced (PD) and perpendicular (T-shaped, T) arrangements of two PAH monomers corresponds to θ = 0/180° and θ = 90°, respectively. The other θ values can be characterized as the tilted T-shape (TT). Exemplified snapshots of dimer configurations are shown in Figure 6. Moreover, distribution of dimer angle θ as a function of dimer distance was plotted, as shown in Figure S7. Interestingly, we find that the distribution of the θ values for NAP broadens as a function of NAP concentration, whereas that for PYR is relatively stable (0−40° and 140−180°). NAP could form the PD, TT, and T dimer

Figure 5. Distance between the center of mass (COM) of naphthalene (NAP)/pyrene (PYR) and the micelle as a function of time computed from Systems II and VI. On the basis of the water RDP (Figure 3a), the blue dashed line, located at ∼1.2 nm, is used to define the micelle core (below the line) and shell (above the line) regions.

the shell (Figure S1). The observation is consistent with that observed by Marqusee and Dill.49 They proposed that most solutes, like arenes, should prefer the shell region of a spherical micelle due to the greater volume available there. Hence, the entropic preference of NAP/PYR to occupy the larger volume of the micelle shell region may be one of the reasons contributing to their preferable locus in the shell. Moreover, the formation of HB between NAP and water molecules (discussed in Section 3.2.2), along with the larger molecular volume of PYR, may facilitate NAP/PYR to stay in the shell. To highlight the movement of PAHs inside the micelle, we depicted in Figure S2a−l the localizations of the PAHs at different simulation points of Systems II, V, VI, and VIII. According to Figure S2a−d, we can see that during the simulation NAP can sometimes locate in the inner core of the micelle (i.e., adjacent to SDS C12 atom in the micelle core) and then after a few nanoseconds move to the middle (i.e., close to SDS C6 atom in the middle region of the micelle) and the surface (i.e., close to the headgroup oxygen or the extruded SDS C12 atom). This movement occurs on PYR (Figure S2e− h) as well. Moreover, the phenomena exist not only at low PAH concentration but also under saturated PAH concentrations. Figure S2i−l indicates that under high PAH concentration conditions the marked NAP/PYR located in core region could move toward the micelle surface, as do the other NAP/PYR molecules (snapshots not shown) and vice versa. Movement of PAHs inside micelles was also observed by Cang et al.50 Through measuring the fluorescence lifetime of 2-ethylnaphthalene (2EN) in several cationic micelles such as dodecyl-trimethylammonium chloride, they found that 2EN could exchange between the shell and core regions of the micelle on a nanosecond time scale.

Table 1. Time Percentage for NAP/PYR To Stay in Micelle Shell and Core in Each Systema

a

system

II

III

IV

V

VI

VII

VIII

core (%) shell (%)

25 75

22 ± 0 78 ± 0

20 ± 4 80 ± 4

22 ± 3 78 ± 3

16 84

22 ± 2 78 ± 2

21 ± 6 79 ± 6

Definition of the micelle shell and core is according to that described in Figure 5. 3649

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Figure 6. (a−e) Examples showing the possible NAP (purple)/PYR (green) dimer configurations extracted from Systems IV and VIII. (f) Definition of the angle θ used to describe the dimer configuration.

configurations, whereas PYR forms only the first two dimer configurations. This may stem from the more benzene rings of PYR that could facilitate its stacking to form parallel structures.54 Finally, we also find that at high PAH concentration (i.e., Systems IV, V, and VIII) NAP/PYR could form other type of clusters (i.e., trimer, tetramer, pentamer, hexamer, heptamer, and octamer) besides monomer and dimer. To examine this, we computed the number of clusters formed by all of the PAH molecules as a function of time in Systems IV, V, and VIII, and present the time percentage for each number of cluster to occur during the simulation (Figure 7). We considered that a cluster is formed when the distances between the NAP COM (delimited by the C3, C4, C5, C6, C7, and C10 atoms) or the PYR COM (delimited by the C5, C6, C9, C10, and C11 atoms) are I3) of each SDS micelle by excluding the NAP/PYR molecules (described in Section 7 of the Supporting Information). The mean MOI ratios (I1/I2, I1/I3, and I2/I3) and α values of each micelle are reported in Table S3. The I1/I2, I1/I3, I2/I3, and α values of the pure SDS micelle (System I) are equal to 1.10 ± 0.05, 1.32 ± 0.12, 1.20 ± 0.13 and 0.12 ± 0.04, respectively, close to the values obtained for the micelles with solubilized PAHs (1.10 ± 0.05, 1.33 ± 0.12, 1.21 ± 0.13, and 0.12 ± 0.04) indicating that the micelles are slightly ellipsoidal and their shapes are not affected by the solubilized PAHs. The maximum MOI ratio for System I, I1/I3, is within the range of the reported values (1.03 to 1.69).9,17,18,21,60 Moreover, we find that the micelle semiaxis lengths in all simulation systems are similar (a ≈ 2.10 nm, b ≈ 1.84 nm, and c ≈ 1.61 nm) with an oblate-like shape. The ellipsoidal shape of the SDS micelle obtained here is consistent with that reported in other studies.9,10,12,21 SDS Alkyl Chain Conformation and Orientation. In Section 3.1, we have shown that NAP/PYR molecules could localize inside the micelle near the SDS alkyl chain carbons. Hence it would be interesting to examine if their localizations affect the SDS alkyl chain conformation and their orientations inside the micelle. In this purpose, we computed the percentages of the trans conformation of the SDS alkyl chain CCCC dihedrals, ptrans, for the micelles binding with PAHs and compared them with the value obtained for the pure micelle. In all PAH solubilization systems we find a ptrans value of ∼73% similar to the value computed for the pure SDS micelle (∼73%) and slightly lower than the previous MD report (∼81%),17 indicating that the PAH solubilization also has no effect on SDS alkyl chain conformation whatever the types and concentrations of PAHs. Furthermore, we investigated the SDS alkyl chain orientation inside the micelle as a function of the NAP/PYR concentration by computing the normalized angle distribution P(θ) formed by the vector that joins C1 (the headgroup carbon) to C12 (the terminal carbon) and the vector from C1 to the micelle COM, as presented in Figure S10. When the angle θ is zero, the chain is aligned perfectly along the micelle radial axis. In all of the systems (including the pure SDS micelle) the computed P(θ) shows a strong peak at ∼25°, indicating that the alkyl chains are not perfectly aligned to the micelle radial axis whatever the PAH types and their concentrations (Figure S10). Allen et al.31 also found that binding of testosterone propionate does not affect the SDS alkyl chain conformation. 3.3.2. Effect of Solubilized PAHs on the Micelle Internal Dynamics. The internal dynamics of the SDS alkyl chain as a function of PAH concentration were also examined by computing the amplitudes of displacement (ADs) for the different carbon atoms along the SDS alkyl chain. The way to compute this is described in Section 9 of the Supporting

Information. From Figure S11, it is obvious that the SDS alkyl chain carbons exhibit different motion behaviors. The asymmetric U-shaped distribution indicates that the first (i.e., C1) and the last (i.e., C12) carbons of SDS have larger displacements than the others. The ADs of carbon atoms in pure micelle (System I) resemble those observed by Aoun et al.43 for the ADs of hydrogen atoms attached to SDS alkyl chain carbons with MD simulations. By computing the translational diffusion coefficients of each SDS alkyl carbon atom in a pure SDS micelle, Palazzesi et al.10 also found similar asymmetric Ushaped distribution. The addition of PAH molecules into the micelle appears to have little influence on the ADs of carbon atoms of SDS chain.

4. CONCLUSIONS We performed explicit molecular dynamics simulations to investigate the solubilization of NAP and PYR into an SDS micelle with different PAH concentrations. The radial density profiles of PAH COM with respect to micelle COM indicated that NAP as well as PYR could reside in the micelle shell and core region, which may arise from the movement of the two PAHs inside the micelle. In addition, both NAP and PYR prefer to reside in the micelle shell. This may be induced by the more available volume of micelle shell, the formation of HB between NAP and water, and the larger molecular volume of PYR. Meanwhile, PYR as well as NAP can form sporadically clusters of two to eight NAP/PYR molecules depending on the PAH concentration in the systems; however, these events rarely occur. By examining several micelle properties (size, shape, internal structure, SDS alkyl chain conformation and orientation, and micelle internal dynamics), we show that the solubilization of PAHs whatever their concentrations have no significant effect on the micelle structures and alkyl chain conformation. This study provides systematic examinations of the solubilization characteristics of PAHs within a model ionic micelle commonly used in environmental remediation. To extend this study, we plan to examine other types of PAHs and their mixtures in different types of micelles to gain a large overview of solubilization characteristics that dictate the synergistic/inhibitive phenomena during PAH cosolubilization process observed in experiments.

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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.6b00182. Pyrene atomic partial charges, atom types, summary of the simulated systems, as well as additional tables and figures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +33 0649377060. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X. J. Liang acknowledges the China Scholarship Council (No. 201406150028) and the CEA for the scholarship. This study was supported by the granted access to the HPC resources of 3652

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CCRT/CINES made by GENCI (Grand Equipement National de Calcul Intensif) under the allocations (t2015077426), the Science and Technology Planning Project of Guangdong Province (2014A020217002), and the Natural Science Foundation of China (No. 41330639).

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DOI: 10.1021/acs.langmuir.6b00182 Langmuir 2016, 32, 3645−3654

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DOI: 10.1021/acs.langmuir.6b00182 Langmuir 2016, 32, 3645−3654