Molecular Dynamics Simulation of the pH-Induced Structural

Subscriber access provided by READING UNIV

Article

Molecular Dynamics Simulation of the pH-Induced Structural Transitions in CTAB/NaSal Solution Hui Yan, Zhe Han, Kaiming Li, Guangyong Li, and Xilian Wei Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03715 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Molecular Dynamics Simulation of the pH-Induced Structural Transitions in CTAB/NaSal Solution Hui Yan*, †,∇, Zhe Han‡,∇, Kaiming Li†, Guangyong Li†, and Xilian Wei*,§ †

School of Pharmacy, Liaocheng University, Liaocheng 252059, China

‡

Environmental Engineering Materials, Advanced Materials Institute, Shandong Academy of Sciences, Jinan, 250014, China

§

College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China

Abstract: We performed molecular dynamics simulations to study the pH-induced structural transitions for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal). We obtained rigid cylindrical, spherical, and flexible cylindrical micelles at pH 7, 2, and 0, respectively, which agrees well with the experimental results of Umeasiegbu et al. (Langmuir 2016, 32, 655). With analyzing the different micellar structural properties, including distribution and molecular orientation of CTA+ and Sal- inside the micelle, we found the binding form of the protonated salicylate molecules with CTA+ is different from that of Sal- ions. Due to the protonation of salicylate molecules with reduction in pH, their hydrogen bonding interactions with water molecules strengthened and the electrostatic interactions with CTA+ headgroups weakened. Thus, the repulsion of the CTA+ headgroups led a breakage of the cylindrical micelle into spherical ones. At pH 0, the H-bond strengthened cation-π interactions between salicylate and CTA+ was verified. We concluded that the penetration of salicylate molecules inside the micelle, in complement with the strong association of Cl- ions on the micellar surface play a key role in the formation of flexible cylindrical micelle. This work brings atomic-level insight into the mechanism of pH-induced shape transitions in the CTAB/NaSal systems, which is expected to be helpful to understand the aggregate behavior of cationic surfactant-hydrotrope solution. 1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Wormlike micelles have attracted much attention in fundamental research and industrial applications, due to their striking viscoelastic properties.1 These entangled wormlike micelles can be grown by small spherical micelles under certain conditions, such as increasing the surfactant concentration and addition of oppositely charged surfactants.2-4 In addition, adding inorganic electrolyte (e.g., Cl- and Br-) or aromatic anions (e.g., salicylate) into cationic surfactant (e.g., cetyltrimethylammonium bromide, CTAB) solution can also lead shape transition from spherical micelles to flexible threadlike or wormlike ones.2,5-7 These aggregates formed by cationic surfactants combined with additive salts are very sensitive to the external conditions, such as temperature and pH, because the stable structures of these aggregates are formed by a balance of interactions among the component parts.8-10 Thus, pH control has attracted interest in controlling micellar structure and its rheological behavior.11,12 Recently, Umeasiegbu et al.13 reported a structural transitions phenomenon occurring with variation in pH for the CTAB and sodium salicylate (NaSal) mixture solution. At 323 K, rigid cylindrical micelles in the solution were observed at neutral pH, while they changed to spherical micelles by decreasing pH value to about 2. With further reduction in pH, a re-entrant transition of spherical to flexible cylindrical micelles was observed. The research revealed that a weakening of the electrostatic screening caused by decreasing pH and consequent protonation of Sal-, played an important role in shape transition from cylindrical to spherical micelles. With further decreasing pH, the hydrogen bond strengthened cation-π interactions in complement with the increased Cl- concentration was responsible for the re-entrant transition to flexible cylindrical micelle. 2


Page 2 of 29

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The experimental research is helpful to understand the driving molecular force in the micellar structural transitions. However, further investigations ate still necessary, especially for the microscopic insights into the intermolecular interaction mechanisms within the micelle, which may be difficult to probe experimentally.13 To gain more insight into the micellar shape transitions at a molecular level, computational simulation is considered as an adequate approach which could provide more supplemental information to experimental investigations. Molecular simulations have already contributed to the investigations on the rod-like or wormlike micellar structural and dynamic properties, such as studies on the sphere to rod transitions of various cationic surfactant micelles induced by inorganic (e.g., NaCl and NaBr) and organic aromatic ions.14-20 It is believed that the Cl- or Br- ions are bound to the surfactant headgroups, which effectively screen the electrostatic repulsion among the headgroups. Alternatively, the aromatic ions (e.g. Sal-) penetrate into the micelle, which increases the molecular ordering to maintain the rod shape. However, to our knowledge, computational studies on the pH-sensitive self-assembly systems are scarce. Recently, Morrow et al.21 used continuous constant pH MD (CpHMD) method to study the self-assembly of lauric acids at different pH conditions, which showed a pH-dependent morphology of the aggregates. Liu et al.22 performed coarse-grained (CG) MD simulations to study the pH-dependent morphologies of CTAB and potassium phthalic acid (PPA) mixtures. The authors concluded that the electrostatic screening of the inserted PPA molecules played an important role in the pH-induced self-assembly. The aim of the current work is to validate the mechanisms of the pH-induced structural 3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

transitions proposed by Umeasiegbu et al.13 using atomistic MD simulations. Different from the experimental conditions, three pH values under only one intermediate temperature state (323 K) are considered. The simulations were started from pre-assembled long cylindrical micelles, and the results reproduced the experimental phenomenon. Besides, the detailed explanation on the micellar structure and intermolecular interactions were provided in this study. More importantly, the microscopic mechanism suggested by MD simulations is expected to provide deeper insights into pH-induced morphology transitions. Experimental Section Model Systems. The preassembled cylindrical micelle was prepared according to previous publications.15,18 First, a round slice containing 9 CTA+ molecules was built, in which the principal axis of each CTA+ molecule was aligned along the radial direction of the micelle and the molecules were separated evenly by an angle of 40°. The distance between the terminal methyl group (C16, Figure 1) and the central axis of the micelle was set to 0.3 nm. Then, a neighboring slice was rotated by an angle of 20° around the central axis and placed at 0.5 nm away from the first one. Finally, the two slices with a thickness of 1 nm were duplicated 10 times along the z-direction. The obtained cylindrical micelle consisting of 180 CTA+ molecules was then placed in a simulation box of dimensions 27 × 27 × 10 nm3 with its central axis along the z-direction, which was used for the following simulation systems.

4


Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

C + C16

C7 C5 C3 C1 N C C15 C13 C11 C9 C2 C14 C12 C10 C8 C6 C4 C

(a) O2

O3

H2

C7

O2

O3 C7

H1

H5

O1

C1

H6

H1

C6

C2

C5

C3

H5

H3

C4

C1

H6

O1

C6

C2

C5

C3 C4

H4

H4

(b)

(c)

H3

Figure 1. Structures of (a) CTA+, (b) Sal-, and (c) SA. Table 1. Details of the Simulation Systems: Numbers of Each Component system

CTA+

Sal-

SA

water

Br-

Na+

H3 O+

Cl-

I

180

86

-

237589

180

86

0

0

II

180

9

77

237495

180

9

50

50

III

180

-

86

226673

180

86

4400

4486

IV

180

9

77

237539

180

9

50

50

V

180

-

86

228703

180

86

4400

4486

Based on the experimental conditions, we performed several systems (summarized in Table 1) to study the effect of pH on the microstructure of CTAB/NaSal mixtures. System I was performed to study the behavior of CTAB/NaSal solution at pH = 7 and at 323 K, by inserting 5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

86 Sal- ions outside the micelle randomly. After water molecules were added into the box, corresponding quantities of the simple ions (Na+ and Br-) were added by replacing them for water molecules. The NaSal-to-CTAB molar ratio was 0.8, which is according to the experimental data.13 For the sake of convenience, the molar concentration of CTA+ was set to be around 0.05 M, which is 10 times than the experimental value. Because it could be beyond current computational capabilities, if a larger simulation box was used to meet the experimental concentration. The dimensions along x,y-directions were set as large as 27 nm, which can make sure the micelle do not interact with its periodic replicas. Systems II and III were performed to investigate the aqueous mixtures at two different pH values 2 and 0, respectively. The acid environment was obtained by adding a certain amount of hydronium ions (H3O+) and Cl- ions into the two systems. The p‫ܭ‬௔ value of salicylic acid at 298 K is 2.98 and we ignored the effect of temperature on pH for the sake of simplicity. In Systems II, 77 salicylic acid molecules (SA, Figure 1c) replaced Sal- ions, because at pH 2, 90% of Salions became protonated. While in Systems III at pH 0, all the Sal- ions were changed to SA molecules. To further describe the structural transitions occurring with change in pH for CTAB/NaSal aqueous mixtures, another two systems were employed, named Systems IV and V. The initial configurations of these systems were derived from the final configuration of System I, in which most Sal- ions have been solubilized into the micelle. With the change in solution acidity, 77 and 86 Sal- ions inside the micelle were substituted for SA molecules, yielding the simulation models of Systems IV and V, respectively. Similarly, the corresponding quantity of H3O+, Br-, and Cl- ions were added into the systems by replacing them for water 6


Page 6 of 29

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

molecules. Computational Details. We employed GROMOS96 53a6 force field23 for all the simulations using GROMACS 4.6.3 package.24 The united-atom force field parameters for the molecules, including CTA+, SA, Sal-, and H3O+, were generated from the Automatic Topology Builder (ATB) server.25,26 The ATB molecular IDs for these four molecules are 10929 (CTA+), 28627 (SA), 16917 (Sal-), and 3859 (H3O+). The simple point charge/extend (SPC/E) model27 was used to describe water molecules. All the systems were first minimized using the steepest descent method. After minimization, a 20 ns MD simulation under NPT ensemble at 1 atm and 323 K was carried out for each system. The constant temperature and pressure were maintained by V-rescale thermostat algorithm.28 Periodic boundary conditions were applied in all directions, so that an infinitely long rod-like micelle was considered along the z-direction and the effect of the simulated concentration of CTAB on the micelle length was ignored. In the simulations, bond lengths were constrained using the LINCS algorithm.29 The non-bonded potential truncation was cut at 1.4 nm for the Lennard-Jones interactions. For the long-range electrostatic interactions, the Particle Mesh Ewald (PME) method30 was employed with a cutoff of 1.2 nm. A time step of 2 fs was used during the simulation and the trajectories were saved every 2 fs for further analysis. Results and Discussion Micellar Shape at Each pH Value. We visually observed straight rod-like, spherical, and flexible cylindrical micelles at different pH values from the final simulated configurations of Systems I-III (Figure 2). Most 7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the additive salts (Sal- or SA) have solubilized into the interior region of the micelles. These observations are well coincided with the experimental phenomena,13 which validated the force field parameters used in this work. The stability of these micellar structures was monitored by checking the MD trajectories of the simulated systems. Figure S2 in the Supporting Information shows the configurations at different periods of the simulation. Overall, the initial rod-like micelle exhibits strong fluctuations in its global conformation at early simulation times in these three systems. With more and more Sal- ions solubilized into the interior of the micelle, the fluctuations on the rod-like micelle of System I got gradually weak and disappeared. We note that the micelle remained steady during the last 10 ns simulation from Figure S1a, suggesting a rigid cylindrical micelle formed at pH 7. The stability was also checked by plotting time profiles of the micellar radius (Figure S3), which was defined by the distance from N atoms to the central axis of the cylindrical micelle. It is noted that during the last 10 ns simulation, the radius remained steady around 1.90 nm, which is comparable with previous simulation studies18 and suggests the equilibrium of the simulated system.

8


Page 8 of 29

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. Configurations of CTAB/NaSal mixtures at each pH value. The configuration is shown together with its two periodic images along the z-direction. Snapshots of the configurations from the z-axis view are shown in Figure S1, Supporting Information. Only the CTA+, Sal-, and SA molecules are shown for clarity. Sal- and SA are displayed in red and yellow, respectively. The atom coloring scheme for CTA+ is N, blue and C, cyan.

Figure S2b shows the configurations of System II at pH 2. We observed a breakage occurred on the rod-like micelle at 0.2 ns, even though a certain amount of SA or Salmolecules were solubilized into the surfactant aggregations. After the breakage, the surfactants formed several spherical micelles and they remained stable up to the end of the 10 ns simulations. The aggregation number of the larger spherical micelle is 83, which is consistent with the experimental data of CTA+,31,32 while the smaller micelle consists of 69 9


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

surfactant molecules. The radii of these two micelles (shown in Figure S3) kept steady during the simulation, suggesting equilibrium of the system. The radius of the larger micelle is about 2.45 nm, which are consistent with an experimental data of 2.5-3 nm33 and with another simulation value of 2.53 nm.18 The radius of the spherical micelle is larger than that of the rod-like micelle, because the surfactant molecules are tilted and interdigitated in the rod-like micelle. Like the above two systems, the cylindrical micelle in System III also exhibited strong fluctuations at earlier simulation times (Figure S2c). However, it has not broken until the end of the simulation. The micelle presented wave shape in its global configuration during the last period of the simulation, suggesting a flexible cylindrical micelle formed. To calculate the radius of the flexible cylindrical micelle, we divided the simulation box evenly into 50 slices with a thickness of 0.2 nm along the z-direction, as the central axis of the flexible micelle was not a straight line. In each slice, the center of mass (COM) of the surfactant atoms within the slice was calculated, and the distances from the COM to N atoms in current slice was measured. The averaged micellar radius (Figure S3) shows the flexible micelle keeps steady during the simulation. We note that the radius of the flexible micelle is slightly larger than that of the rigid micelle, which is possibly caused by the fluctuation of surfactant molecules inside the micelle. Because the micellar shape is possibly related to the solubilization of these hydrotrope molecules into the micelle, we counted the absolute number of these molecules inserted into micelles. Figure S4 shows that, the numbers of the additive molecules in systems I and III solubilized into the micelles increased gradually until they reached stable values. In System II, 10


Page 10 of 29

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

the solubilization numbers also kept stable after the spherical micelles formed (after 5ns). By contrasting System I and III, we found that almost all the Sal- ions solubilized into the micelle, while for SA molecules the proportion is only about 65%. It is obvious that Sal- ions prefer to be solubilized into the micelle, due to their stronger affinity for CTA+ headgroups. Detailed Structural Features of the Surfactant-Hydrotrope Mixtures. To investigate the detailed structural properties of the micelle at each pH value, the locations of different species in the micelle were characterized by plotting the number density distribution profiles. Figure 3 shows the distribution profiles of the selected atoms with respect to the central axis of the cylindrical micelle in the neutral solution. For the rigid rod-like micelle, similar results to the previous studies18 were obtained. Briefly, the surfactant headgroups constitute a shell region at the micelle/water interface with a hydrophobic core region inside the micelle, and the Br- counterions are binding on the micellar surface under electrostatic interaction. Figure 3b shows the distribution profiles of different parts in Salions, including atoms O1, O2, O3, and the aromatic ring, as defined in Figure 1. The phenyl groups are located deeply in the hydrophobic region of the micelle, while the COO- groups are pointing out near the micellar headgroup region. Because the COO- groups are hydrophilic and meanwhile they interact with CTA+ headgroups under the electrostatic interactions. Comparing with COO- groups, the hydroxyl groups of Sal- ions are located more deeply inside the headgroup region of the micelle, but both the two kinds of groups are directly in contact with water.

11


Number density / nm -3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Number density / nm -3

Langmuir

Page 12 of 29

35

35

30

30 25

water C1-C16

20

water C1-C16

20

N

15

N

15

BrNa+

10

Cl-

25

10

(a)

BrNa+

5

5

0 2.5

0 2.5

2.0

2.0

1.5

water O1

1.0

O2 O3

0.5 0.0 0

(b) 1

2

H3O+

1.5

water O1

1.0

O2 O3

N benzene

0.5

N benzene

0.0 4 0

3

(c)

r / nm

(d) 1

2

3

r / nm

Figure 3. Number density profiles of components with respect to the central axis of the rigid (panels a and b, System I) and the flexible (panels c and d, System III) cylindrical micelles. Values for N, Br-, Cl-, and Na+ in panels a and c have been multiplied by 10 for clarity.

The density distributions of various species in the flexible cylindrical micelle (System III at pH 0) are shown in Figure 3c. The locations of the headgroup N atoms and the hydrophobic tails are similar to those in the rigid cylindrical micelle. It is noted that the positive ions (Na+ and H+) are located in the bulk solution, which means these ions do not participate in maintaining the shape of the micelle. Besides, the distributions of three O atoms in the protonated salicylate ions are almost overlapped. This means once the Sal- ions were 12


4

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

protonated with decrease in pH, the locations of phenolic hydroxy group move outwards to the micelle/water interface. Thus, the interaction mode between SA molecules and the surfactants is different from that between Sal- ions and CTA+. Before analyzing the interactions between Sal-/SA and the surfactants, we first explored the orientation of the two hydrotropes inside the micelle. The orientation was described by the angle θ between vector C4 to C1 (atoms in Sal- and SA) and vector C3 to N (atoms in CTA+). When calculating the angle, only the neighboring pairs of Sal-/SA and CTA+ molecules were considered. The probability distributions of the angle θ for systems I and III are plotted in Figure 4. The results show that the molecular axis of the ionized Sal- molecules preferred to form an angle of around 30° with CTA+ headgroup. While the angles between SA and CTA+ headgroup in System III have a broad distribution from ~30° to 90°, suggesting that there is no strong preference for the protonated salicylate ions (SA) to form a certain angle with the adjacent surfactants. Figure 4b and 4c show the configurations of the surfactant headgroup interacting with its adjacent Sal- or SA molecules, which were randomly selected from the MD configurations. The Sal- ions interacts with the adjacent CTA+ molecules through electrostatic interactions between carboxyl and ammonium groups, resulting in a strong preference for the molecular orientation inside the micelle. With the COO- groups were protonated, their electrostatic interactions with ammonium groups weakened. We believe that the change in the binding form between the additive salts and surfactants plays an important role in the shape transition of the micelle.

13


Langmuir

0.10 (a)

0.08 Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

0.06 0.04 pH = 7 pH = 0

0.02 0.00

0

20

40

60

80 100 120 140 160 180 θ / degree

Figure 4. (a) Probability distribution of the angle θ between the two vectors defined in the molecular structures. Binding structure of the CTA+ headgroup with Sal- at pH 7 (panel b, System I), and with SA at pH 0 (panel c, System III).

Figure S5 shows the structural properties of spherical micelles formed in in System II. we have selected two larger micelles to plot their density distribution profiles of the selected 14


Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

atoms. The number density of C atoms in the core region is lower, revealing the existence of a cavity in the micellar center. That is why the micellar radius is larger than that of the cylindrical micelle. The locations of the three types of O atoms in Sal- and SA molecules indicates that, the hydroxyl groups of Sal- ions locate deeply inside the micelle, while they move towards the micellar surface when the COO- groups were protonated. Figure S6 shows the distribution of the angle θ between Sal-/SA and surfactant molecules, which is also in accordance with the conclusion obtained from the cylindrical micelle systems. Structural Transitions with Decrease in pH. Figure 5 shows the configurations of Systems IV and V at the beginning and end of the simulations. Likewise, the spherical micelle formed at pH 2, and at pH 0 the flexible cylindrical micelle formed. The configurations at different periods of Systems IV and V are provided in the Supporting Information, Figure S7. We may see that the dynamic variations of the micelles are as same as those in Systems II and III. The radii of the obtained spherical and cylindrical micelles (Figure S8) are comparable with the results obtained from Systems II and III. This suggests that the initial positions of SA or Sal- have no effect on the final simulation results. The difference is the behavior of the hydrotrope molecules (SA and Sal-) inside the micelle. In the first three systems, all the additive molecules were originally placed outside the pre-assembled micelle and a number of these additivities solubilized gradually into the micelle. While in Systems IV and V, the protonated SA molecules were derived from Salions in the end configurations of System I. Figure S9 shows that the numbers of SA and Salinside the micelle remain fairly stable. Take System V for instance, almost all the SA 15


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecules were kept inside the flexible cylindrical micelle during the simulation. While in System III, only about 50 SA molecules solubilized into the micelle from the solution. As discussed above, it is difficult for SA to insert into the micelle, comparing with Sal- ions. But once inserted into the micelle, it is also hard for SA molecules to be disassociated from the micelle.

Figure 5. Configurations of Systems IV and V at the beginning and end of the simulations.

16


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Other results including number density and angle distribution profiles obtained from Systems IV and V are provided in Figures S10–S12. From these results, we could get the same conclusion as that from the first three systems. In summary, the OH- groups of Sal- ions are located more deeply than the COO- groups. But with pH reduced and the subsequent protonation of COO- groups, the locations of OH and COOH groups become almost overlapped. The angle probability distribution profiles obtained from these two systems suggest that Sal- ions prefer to form a certain angle with the neighbored surfactants, while there is no strong preferred orientation for the protonated SA molecules. Again, these results verified the reproducibility of the two modeling methods used in our simulations. Interactions between the Hydrotrope and Water Molecules. From the discussion above, we conclude that the binding form of Sal- ions with the surfactants is different from that of SA, which is probably the main factor in maintaining micellar shape. In what follows, we investigated the intermolecular interactions in different micellar systems to rationalize the structural change with pH. We first investigated the interactions between the aromatic salts and water molecules, by calculating the radial distribution functions (RDFs) between relevant functional groups and water O atoms. Figure 6a shows the RDFs g(‫ݎ‬ைுିைೢೌ೟೐ೝ ) for Sal- and SA molecules at pH = 7 and 0 (Systems I and III), respectively. From the profiles, we can note there are two well-defined hydration shells around the phenolic hydroxyl groups. The intensity of the first peak in System III is stronger than that in System I, suggesting the interaction between OH groups with water molecules was strengthened with decrease in pH. Thus, the OH groups belong to the protonated salicylate counterions locate at an outer position of the micellar surface, while the OH groups 17


Langmuir

of the ionized salicylate ions locate at an inner position of the micelle (Figure 3).

1.0 0.8 pH = 7 pH = 0

g (r)

0.6 0.4 0.2

(a)

0.0

0.2

0.4

0.6

r / nm

1.0 pH = 7 pH = 0

0.8 0.6 g (r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

0.4 0.2 (b)

0.0

0.2

0.4

0.6

0.8

r / nm Figure 6. Radial distribution functions between functional groups of salicylate molecules and water oxygen atoms for Systems I and III: (a) OH groups and (b) COO- and COOH groups.

18


Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 6b shows the interactions between another hydrophilic group of the salicylate molecules, by plotting the RDFs g(‫ݎ‬஼ைைషିைೢೌ೟೐ೝ ) and g(‫ݎ‬஼ைைுିைೢೌ೟೐ೝ ). There are two and three well-defined hydration shells around COO- and COOH groups, respectively. The first peak distance at ~2.7 Å in RDF g(‫ݎ‬஼ைைషିைೢೌ೟೐ೝ ) corresponds to the hydration shell around O atoms in COO- groups, as these oxygen atoms can form H-bonds with water molecules acting as H-bond receptors. While in RDF g൫‫ݎ‬஼ைைுିைೢೌ೟೐ೝ ൯ , the first peak at ~1.7 Å corresponds the hydration shell around H atoms in COOH groups. Since the two kinds of OH groups in SA molecule can act as both H-bond donor and receptor, the hydrophilicity of SA molecules strengthened. Meanwhile, we note that the intensity of the second peak in RDF

g൫‫ݎ‬஼ைைுିைೢೌ೟೐ೝ ൯ becomes weak, indicating that the interaction between O atoms in COOH group with water molecules weakened. The corresponding RDFs for System II at pH 2 are shown in Figure S13. These results are similar to those from Systems I and III. In brief, with the COO- groups were protonated, the hydrophilicity of the adjacent hydroxyl was improved and the OH groups produced outward the micelle-water interface. Meanwhile, the electrostatic interactions weakened, leading to a various angle distribution between SA molecules and surfactants (Figure 4). Subsequently, structural change may occur on the cylindrical micelle. One difference, however, is that the OH groups of Sal- ions also interact strongly with water molecules, as shown in Figure S13a. It could be related to the loose packing of the surfactant headgroups in the spherical micelles, which results in much freedom for molecular motion of SA and Sal- molecules inside the micelle. The much loose packing of the surfactant headgroups in the spherical micelles can be confirmed by the N-N RDF g(‫ݎ‬ேିே ) in Figure 7a. It can be noted that the intensity of the 19


Langmuir

first two peaks in RDF at pH 2 is the weakest. This point is consistent with the observations from previous MD simulations of ammonium surfactants.15,18

8

(a)

7 6

pH = 7 pH = 2 pH = 0

g (r)

5 4 3 2 1 0 0.0

0.5

1.0

1.5

2.0

r / nm 400 (b)

350 300 250 g (r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

200

N-COON-COOH

150 100 50 0

0.5

1.0 r / nm

Figure 7. (a) Radial distribution functions between the nitrogen atoms of the surfactant molecules at each pH value. (b) Radial distribution functions between N atoms and carboxyl 20


Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

groups.

Interactions between the Hydrotrope and Surfactants. The most densely packing of the surfactant headgroups in the cylindrical micelle is related to the inserted Sal- ions, which is the key factor in cylindrical micelle formation. Because the negative charged Sal- ions bind tightly to the positive charged CTA+ headgroups through coulomb interactions. Figure 7b shows the RDFs of COO- and COOH groups around the headgroup N atoms for System I and III, respectively. The corresponding RDFs for System II are shown in Figure S14. There are two evident peaks in the curves, because the carboxyl groups interact with the surfactant headgroups through any one of the two oxygen atoms. The pronounced peak in g(‫ݎ‬ேି஼ைைష ) indicates strong interactions between the headgroups and COO- groups. Due to the strong electrostatic interactions between the COO- groups and headgroups, the Sal- ions bind with surfactants forming a certain angle (Figure 4). But once the COO- groups protonated to COOH at lower pH value, the interactions between SA molecules and surfactants weakened, leading to a change of molecular orientation inside the micelle. Previous studies of CTAC have reported that the threadlike micelle formed at Clconcentration above 3 M in the absence of Sal- ions,13,18 because the high degree of Cl- ions binding to the micellar surface efficiently screened the electrostatic repulsion between CTA+ headgroups. In the presence of NaSal, CTAB molecules can form cylindrical micelles at as low as 1 M HCl solution.13 Krishnamoorti et al. pointed that both the electrostatic shielding from Cl- ions and cation-π interactions between surfactant headgroup and benzene ring 21


Langmuir

contributed the formation of the flexible cylindrical micelle.13 The strong association of Clions on the micellar surface at pH 0 can be described by the density distribution profiles (Figure 3). In addition, Figure S15 shows the configuration of Cl- ions around the flexible cylindrical micelle, indicating a strong association of the inorganic ions with the micellar surface.

(a)

0.5

pH = 7 pH = 0

0.4 Proportion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

0.3 0.2 0.1 0.0 10000

12000

14000

16000

Simulation time/ ps

22


18000

20000

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 8. (a) Proportion of the inserted Sal- or SA molecules forming cation-π conformation with the adjacent CTA+. (b) Cation-π formed between SA and CTA+ inside the flexible cylindrical micelle.

In complement with the electrostatic screening of Cl- ions on the micellar surface, the protonated salicylate molecules inside the micelle can screen the repulsion of the surfactant headgroups to a certain extent. As shown in Figure 7a, the intensity of N-N interactions in flexible cylindrical micelle is weaker than that in rigid cylindrical micelle, but stronger than that in spherical micelle. Unlike the electrostatic interactions, the binding between SA molecules and the surfactants does not make the surfactant headgroups pack much tightly, and the molecular motion the surfactants is much more drastic. However, due to the high concentration of Cl- around the micelle, the cylindrical micelle did not break to form spherical shape and the flexible cylindrical micelle formed. The existence of the cation-π interactions was determined by the conformation formed by surfactants and SA/Sal- molecules. To select the cation-π configurations, two conditions need to be satisfied simultaneously. One is the perpendicular distance from at least one headgroup methyl to planar aromatic ring should be about 4.5 Å to guarantee intermolecular interactions with each other. The other one is the perpendicular to the aromatic plane through the headgroup methyl should cross the aromatic ring to make the methyl groups contact the π system.34,35 We analyzed the cation-π structure in the rigid and flexible cylindrical micelles (i.e. Systems I and III). The proportion of the inserted SA/Sal- molecules forming cation-π conformation with the adjacent surfactants was plotted with time evolution (Figure 8). The 23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

proportion of cation-π structures significantly increases at pH 0, suggesting a stronger cation-π interactions between protonated SA molecules with the surfactant headgroups. On the one hand, the electrostatic interactions between SA molecules with surfactant headgroups weakened. On the other hand, the H-bond interactions between SA and water molecules become stronger. Thus, the SA molecules tend to move toward the micelle/water interface, which enhances the opportunity for the aromatic rings to interact with CTA+ headgroups. The cation-π interactions indeed contribute to the stable micellar structure, but the cation-π structures inside the micelle is still in a small proportion (~ 25%). Thus, the main factors in maintaining micellar shape at a low pH value are the strong association of Cl- ions on the micellar surface and the repulsion screening effect from the inserted SA molecules. Conclusions MD simulations have been performed to study the pH-induced structural changes in aqueous CTAB/NaSal solutions. Two procedures were used in simulating the systems. One is starting the simulations from preassembled cylindrical micelles with the hydrotrope molecules placed outside, and the other is from simulated micelles with the hydrotrope molecules inside the micelles. All the simulated results reproduced the experimental phenomenon. The rigid cylindrical micelle was observed at neutral pH, and the micellar structure changed into spherical micelles at pH 2. With further decrease in pH, the flexible cylindrical micelle was observed. The molecular mechanism of the pH-induced structural transitions was studied based on the simulation results. We found that the ionized salicylate molecules can bind tightly with the CTA+ headgroup through electrostatic interactions, which results in a dense packing of the surfactants and a formation of rigid cylindrical micelle. With 24


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

decrease in pH and the consequent protonation of the Sal- ions, their hydration ability strengthened and the binding with the surfactant weakened. The loose packing of the surfactants led to a breakage of the cylindrical micelle, which then became spherical micelles. With further decrease in pH, the high concentration of Cl- ions on the micellar surface in complement with the inserted SA molecules contributes to formation of the flexible cylindrical micelle. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. Y.); [email protected] (X. W.). ORCID Hui Yan: 0000-0002-9843-0601 Author contributions ∇

These authors contributed equally.

Supporting Information Configurations of the CTAB/NaSal mixtures at each pH value; micellar radii of different structures; numbers of Sal- or SA inserted into the micelle; number density profiles and angular probability distribution obtained from Systems II, IV, and V; RDFs for Systems II; configurations of ions associated onto the surface of the flexible cylindrical micelle. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21203084 and 21473084). References (1) Chu, Z.; Dreiss, C. A.; Feng, Y. Smart wormlike micelles. Chem. Soc. Rev. 2013, 42, 25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7174-7203. (2) Dreiss, C. A. Wormlike micelles: Where do we stand? Recent developments, linear rheology and scattering techniques. Soft Matter 2007, 3, 956-970. (3) Koehler, R. D.; Raghavan, S. R.; Kaler, E. W. Microstructure and dynamics of wormlike micellar solutions formed by mixing cationic and anionic surfactants. J. Phys. Chem. B 2000, 104, 11035-11044. (4) Schubert, B. A.; Kaler, E. W.; Wagner, N. J. The microstructure and rheology of mixed cationic/anionic wormlike micelles. Langmuir 2003, 19, 4079-4089. (5) Gamboa, C.; Rios, H.; Sepulveda, L. Effect of the nature of counterions on the sphere-to-rod transition in cetyltrimethylammonium micelles. J. Phys. Chem. 1989, 93, 5540-5543. (6) Hassan, P. A.; Yakhmi, J. V. Growth of cationic micelles in the presence of organic additives. Langmuir 2000, 16, 7187-7191. (7) Aswal, V. K.; Goyal, P. S. Role of counterion distribution on the structure of micelles in aqueous salt solutions: Small-angle neutron scattering study. Chem. Phys. Lett. 2002, 357, 491-497. (8) Davies, T. S.; Ketner, A. M.; Raghavan, S. R. Self-assembly of surfactant vesicles that transform into viscoelastic wormlike micelles upon heating. J. Am. Chem. Soc. 2006, 128, 6669-6675. (9) Verma, G.; Aswal, V. K.; Hassan, P. pH-responsive self-assembly in an aqueous mixture of surfactant and hydrophobic amino acid mimic. Soft Matter 2009, 5, 2919. (10) González, Y. I.; Kaler, E. W. Cryo-TEM studies of worm-like micellar solutions. Curr. Opin. Colloid Interface Sci. 2005, 10, 256-260. (11) Broz, P.; Driamov, S.; Ziegler, J.; Ben-Haim, N.; Marsch, S.; Meier, W.; Hunziker, P.; Toward intelligent nanosize bioreactors: A pH-switchable, channel-equipped functional polymer nanocontainer, Nano Lett. 2006, 6, 2349-2353. (12) Brinchi, L.; Germani, R.; Di Profio, P.; Marte, L.; Savelli, G.; Oda, R.; Berti, D. Viscoelastic solutions formed by worm-like micelles of amine oxide surfactant. J. Colloid Interface Sci. 2010, 346, 100-106. 26


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(13) Umeasiegbu, C. D.; Balakotaiah, V.; Krishnamoorti, R. pH-Induced re-entrant microstructural transitions in cationic surfactant-hydrotrope mixtures. Langmuir 2016, 32, 655-663. (14) Mohanty, S.; Davis, H. T.; McCormick, A.V. Complementary use of simulations and free energy models for CTAB/NaSal systems, Langmuir 2001, 17, 7160-7171. (15) Maillet, J. B.; Lachet, V.; Coveney, P. V. Large scale molecular dynamics simulations of self-assembly processes in short and long chain cationic surfactants, Phys. Chem. Chem. Phys. 1999, 1, 5277-5290. (16) Marrink, S.J.; Tieleman, D.P.; Mark, A. E. Molecular dynamics simulation of the kinetics of spontaneous micelle formation, J. Phys. Chem. B 2000, 104, 12165-12173. (17) Yakovlev, D. S.; Boek, E. S. Molecular dynamics simulations of mixed cationic/anionic wormlike micelles, Langmuir 2007, 23, 6588-6597. (18) Wang, Z. W.; Larson, R. G. Molecular dynamics simulations of threadlike cetyltrimethylammonium chloride micelles: Effect of sodium chloride and sodium salicylate salts, J. Phys. Chem. B 2009, 113, 13697-13710. (19) Lorenz, C. D.; Hsieh, C. M.; Dreiss, C. A.; Lawrence, M. J. Molecular dynamics simulations of the interfacial and structural properties of dimethyldodecylamine-N-oxide micelles, Langmuir 2011, 27, 546-553. (20) Sangwai, A. V.; Sureshkumar, R. Coarse-grained molecular dynamics simulations of the sphere to rod transition in surfactant micelles. Langmuir 2011, 27, 6628-6638. (21) Morrow, B. H.; Koenig, P. H.; Shen, J. K. Self-assembly and bilayer-micelle transition of fatty acids studied by replica-exchange constant pH molecular dynamics. Langmuir 2013, 29, 14823-14830. (22) Liu, Z. B.; Wang, P; Pei, S.; Liu, B.; Sun, X. L.; Zhang, J. Molecular insights into the pH-induced self-assembly of CTAB/PPA system. Colloids Surf. A 2016, 506, 276-283. (23) Oostenbrink, C.; Soares, T. A.; van der Vegt, N. F. A.; van Gunsteren, W. F. Validation of the 53A6 GROMOS force field, Eur. Biophys. J. 2005, 34, 273-284. (24) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for highly efficient load-balanced, and scalable molecular simulation. J. Chem. Theory 27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Comp. 2008, 4, 435-447. (25) Malde, A. K.; Zuo, L.; Breeze, M.; Stroet, M.; Poger, D.; Nair, P. C.; Oostenbrink, C.; Mark, A. E. An automated force field topology builder (ATB) and repository: version 1.0, J. Chem. Theory Comput. 2011, 7, 4026-4037. (26) Canzar, S.; El-Kebir, M.; Pool, R.; Elbassioni, K.; Mark, A. E.; Geerke, D. P.; Stougie, L.; Klau, G. W. Charge group partitioning in biomolecular simulation, J. Comput. Biol. 2013, 20, 188-198. (27) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The missing term in effective pair potentials, J. Phys. Chem. 1987, 91, 6269-6271. (28) Bussi, G.; Donadio, D.; Parrinello, M. Canonical sampling through velocity rescaling. J. Chem. Phys. 2007, 126, 014101. (29) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. LINCS: A linear constraint solver for molecular simulations, J. Comput. Chem. 1997, 18, 1463-1472. (30) Essman, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A smooth particle-mesh ewald method, J. Chem. Phys. 1995, 103, 8577-8593. (31) Hayter, J. B.; Penfold, J. Determination of micelle structure and charge by neutron small-angle scattering. Colloid Polym. Sci. 1983, 261, 1022-1030. (32) Dorshow, R. B.; Bunton, C. A.; Nicoli, D. F. Comparative study of intermicellar interactions using dynamic light scattering. J. Phys. Chem. 1983, 87, 1409-1416. (33) Imae, T.; Kamiya, R.; Ikeda, S. Formation of spherical and rod-like micelles of cetyltrimethylammonium bromide in aqueous NaBr solutions. J. Colloid Interface Sci. 1985, 108, 215-225. (34) Dougherty, D. A. The cation-π interaction. Accounts Chem. Res. 2013, 46, 885-893. (35) Khandelia, H.; Kaznessis, Y. N. Cation-π interactions stabilize the structure of the antimicrobial peptide indolicidin near membranes: molecular dynamics simulations. J. Phys. Chem. B 2007, 111, 242-250.

28


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table of Contents

29


Molecular Dynamics Simulation of the pH-Induced Structural

Recommend Documents