Understanding the Structure of Hydrophobic Surfactants at the Air

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Understanding the Structure of Hydrophobic Surfactant at Air/Water Interface from Molecular Level Li Zhang, Zhi Pei Liu, Tao Ren, Pan Wu, Jia-Wei Shen, Wei Zhang, and Xin Ping Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5030586 • Publication Date (Web): 30 Oct 2014 Downloaded from http://pubs.acs.org on November 5, 2014

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Understanding the Structure of Hydrophobic Surfactant at Air/Water Interface from Molecular Level Li Zhang∗1, Zhipei Liu1, Tao Ren1, Pan Wu1, Jia-Wei Shen2, Wei Zhang1, Xinping Wang1

1. Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, XiaSha Higher Education Zone, Hangzhou, 310018, China. 2. School of Medicine, Hangzhou Normal University, Hangzhou 310036, PR China

Abstract Understanding the behavior of fluorocarbon surfactants at the air/water interface is crucial for many applications such as lubricants, paints, cosmetics and fire-fighting foams. In this study, molecular dynamics (MD) simulations were employed to investigate the microscopic properties of nonionic fluorocarbon surfactants at the air/water interface. Several properties including the distribution of head groups, the distribution probability of tilt angle between hydrophobic tails with respect to the xy plane and order parameter of surfactants were computed to probe the structure of hydrophobic surfactants at air/water interface. The effects of monomer structure on interfacial phenomena of nonionic surfactants were investigated as well. It is observed that the structure of fluorocarbon surfactants at air/water interface are more ordered than that of hydrocarbons, which is dominated by the van der Waals interaction between surfactants and water molecules. However, replacing one or two CF2 with CH2 group does not significantly influence the interfacial structure, suggesting that hydrocarbons may be promising alternatives to perfluorinated surfactants.

1. Introduction ∗

Corresponding author, Fax: +86-571-86843600 E-mail address: [email protected]

ACS Paragon Plus Environment

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The adsorption of surfactant at the air/water interface plays an important role in many technological and industrial applications, such as mineral flotation, corrosion inhibition, dispersion of solids, oil recovery and so on.1 Fluorocarbon surfactants show great potential of lowering considerably the surface tension of aqueous solutions, high chemical and thermal stability owing to their excellent properties. Therefore, fluorocarbon surfactants offer a wide range of application in the fields of high-performance lubricants, paints, cosmetics, and fire-fighting foams.2 Since most applications of surfactant are governed by processes at the air interface, understanding the arrangement of monolayer and how the organization of surfactants influences the interfacial properties is necessary. Recently, some experimental methods have been widely employed to investigate the structure of surfactants at air/water interface. For example, Tyrode et.al 3

used the surface-sensitive technique vibrational sum frequency spectroscopy (VSFS) to detect the

adsorption layer of penta(ethyleneoxide) n-dodecyl ether (C12E5) at the air/liquid interface. The surfactant molecules were found to first adsorb on the air/water interface with their hydrocarbon tails preferentially orientated close to the surface plane. They also used this method to show that the ammonium perfluorononanoate (APFN) molecules lie essentially flat on the air/water interface at low surface coverage.4 Analysis of the spectra showed that the averaged orientation of both the terminal CF3 group and carboxylate head group was constant over a broad range of surface densities, and no evidence of gauche defects could be detected in the fluorocarbon chain. Besides, Neutron Reflection (NR) has also been widely employed to investigate the detailed structure of the monolayers formed at the air/water interface5,6. The structural studies of partially fluorinated cationic surfactants (CnF2n+1CmH2mN+(CH3)3Br -) at air/water interface show a decrease in molecular orientation with increasing fluorination.5 The mean tilt angle away from the surface


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normal varies from 55°(for C16TAB) to 25°(for C8F17C6H12N(CH3)Br). Partial structure factor analysis of the reflectivity data for 1,2-di-O-octadecyl-rac-glyceryl-3-

(ω

-methoxydodecakis(ethylene glycol)) (2C18E12) show that the distributions of alkyl chains and head groups in the surfactant become width concomitant as the surface pressure increasing6. Until now most works focus on the structure of nonionic hydrocarbon surfactants or ionic fluorocarbon/hydrocarbon surfactants. It is found that the structure of hydrocarbon at air/water interface is quite different from that for fluorocarbon. In the past few years, some studies of fluorocarbon/hydrocarbon surfactants and their mixtures were carried out7, 8. Shin et.al9 revealed that the adsorption behavior of fluorinated surfactants F(CF2)11COOH and F(CF2)10CH2COOH at their liquid/air interface are similar to each other, and the packing structure as well as the breakup of the homogeneous ordered monolayer change into ordered islands with the same collective tilt in both system. While the surface tension for CF3 (CF2)5(CH2)8N+ (CH3)3Br-(fC6hC8TAB) and CF3 (CF2)7(CH2)6N+(CH3)3Br- (fC8hC6TAB) are 24.0mN/m, 20.2mN/m, respectively5. The surface tension increases correspondingly with the contents of fluorinated groups decreasing. For the hydrocarbon surfactants, the surface tension decreases to 25.5 mN/m when the carbon atom numbers increases to 1210. This may cause by the different structure of fluorocarbon/hydrocarbon surfactants forming at the air-liquid surface. It was reported that perfluoroalkyl surfactant prefer forming stable crystalline structure. Liquid-like state can always be detected in hydrocarbon system, and the hexagonal monolayer structure of fluorosurfactant has been reported in some previous studies11-13. However, it is still not clear that which part of hydrophobic groups influences the structure of surfactant at air/water interface strongly.


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It has been confirmed that molecular dynamics (MD) simulation is an effective tool to study the structural and dynamical properties of surfactant14. During the simulation, the structure of surfactants at air/water interface could be revealed quantitatively rather than qualitatively. The arrangement of monolayer and how their organization influences the interfacial properties can also be revealed from molecular level. For example, Stephenson et.al 15 studied a much lower degree of ordering for the perfluoroalkyl side chains in each bolaamphiphile than in the "long chain" anionic fluorosurfactant. The analysis of penetration parameter shows the density profiles and hydration data, and it suggests why each poly(fluorooxetane) is capable of reducing surface tension significantly. Chanda et.al reported16 that the polar head groups of monododecyl hexaethylene glycol (C12E6) monolayer preferred to tilt more toward the aqueous interface due to the strong interaction between surfactants and water molecules. It is also found that the adsorbed monolayer could be strongly influenced by the translational and rotational mobility of water molecules confined in the interfacial layer. As literatures mentioned above, the structure of surfactants at air/water interface is closely related to their monomer structure. It is found that the fluorocarbon surfactants show better efficiency and effectiveness in the reduction of surface tension than hydrocarbon surfactants17. Nevertheless, we have no idea about which factor influences the interfacial structure of fluorocarbon/hydrocarbon surfactants strongly. So far there is not yet a definitive model relating the monomer structure of surfactants to the surface tension of their solutions in water. The relationship between the interfacial structure of surfactants and its monomer structure was poorly understood. Can we find some parameters which can be employed to reveal the surface tension of surfactant directly? With these questions, the structure properties of nonionic fluorocarbon/hydrocarbon


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surfactants at air/water interface was investigated through MD simulations in this paper, and the relationship between the structure of surfactants at air/water interface and its surface tension was also explored.

2. Simulation details In

this

work,

two

hydrophobic

surfactants,

oligo(ethyleneoxide)-2-pefluorooctyl

(C8F17(OCH2CH2)2OH,8F2EO) and oligo(ethyleneoxide)-2-octyl (C2H5(CH2)6(OCH2CH2)2OH, 8H2EO) were employed as model surfactant, since the minimum surface tension is attained with C8F17 for a series of bolaamphiphilic poly(fluorooxetane) surfactants with different tail lengths, the hydrocarbon surfactant with similar structure is also employed for comparison. A system with two monolayers on opposite sides of a water slab is constructed, as shown in Scheme 1. The total length of the unit box in the z dimension (water slab plus surfactant monolayers plus vacuum spaces) is 400 Å. The water slab is 80Ǻ thick, which is sufficient to isolate two monolayers. The initial configuration in all simulated surfactant systems are similar (Scheme 2). Initially, two monolayers are set up in the x-y plane with the z axis perpendicular to the air/water interface. The head groups of surfactant (polyethylene oxide) point into the water slab, and the tail groups point out toward the vacuum spaces, which are above or below the water slab Unfortunately, the surface area per molecule of 8F2EO and 8H2EO cannot be found in previous

work.

It

has

been

reported

that

the

surface

area

per

molecule

of

CH3(CH2)7(OCH2CH2)10OH (8C10EO) and CF3(CF2)7SO3-N+(CH2CH3)4(PFOS) are 58Ǻ2 and 35Ǻ2, respectively17, 18. The surface areas of pentakis(oxyethylene) dodecyl ether (C12E5) at critical micelle concentration are 50Ǻ2,10. Some literature10, 18 reported that the surface area of nonionic surfactant is larger than that of ionic surfactants. The average surface area per surfactant molecules


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is set to be 44Ǻ2, i.e. 81 surfactant molecules are distributed at each side of the water/air interfaces on average. In order to investigate the structural and dynamical properties of 8H2EO and 8F2EO systems at the same conditions, the identical surface area are set. The short name and molecular formula for all the surfactant systems with different fluorinated group contents investigated in this work are shown in Table 1. Different concentrations for 8F2EO systems were also constructed to investigate the effect of starting conditions on the final structures. The distribution probabilities of tilt angle and order parameter for surfactants system with different concentrations are shown in Figure S1 in supporting information; it is found that the final structure is influenced little when the concentration is larger. In this paper, all molecular dynamics (MD) simulations were performed with the program NAMD version 2.6.19 The CHARMM force filed20 was used for the surfactants, and the TIP3P model was employed to describe water molecules.21 The partial charge on CF3 and CF2 groups was taken from Rossky’s works22, and the charge value on C and F atoms in CF3 groups is +0.51 and -0.17. As for CF2 group, they are +0.24 and -0.12, respectively. The partial charges of CH2/CH3 and ethylene glycol (EO) groups were taken from Yang’s work.23 The Particle Mesh Ewald (PME) summation technique was used to calculate the long-range electrostatic interactions.24 Lennard-Jones (LJ) pair potentials were evaluated within a cutoff 1.2 nm. The cross interaction parameters were obtained from the Lorentz-Berthelot rules.25 Periodic boundary conditions were employed for all xyz directions, and the MD simulations were carried out using an NVT ensemble. The temperature was controlled at 298K with Nose-Hoover thermostat, and the temperature is set to be 2fs. Each system was simulated for 4ns, since the conformation will not change much after 4ns. The trajectories of the last 1 ns were used

for data analysis.


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3. Results and Discussion 3.1 Interfacial structure of perfluoroalkyl /hydrocarbon surfactants Figure 1 shows the mass density profiles of different segments perpendicular to the air/water interface at equilibrium. Simplify, T、J and H are used to represent the carbon atom in tail groups (CF3/CH3 ), the carbon atom in the joint group(CF2/CH2) and the oxygen atoms in hydroxyl head group, respectively. As shown in Figure 1, the density profiles of head、tail and joint groups increase to a maximum then decrease to zero, the peak positions of density profile for CF3/CH3 group show that the tail groups (CF3/CH3) preferentially point to the air. Although the density profiles for 8F2EO and 8H2EO surfactants are similar to each other, some differences are still observed from Figure 1: the peak of the density profile for CF3 groups is slightly higher and sharper than that for CH3 groups. This phenomenon may be caused by the different arrangements between fluorocarbon and hydrocarbon tails. The perfluoroalkyl chains favor stable crystalline order even for short chains due to the rigidity of amphiphiles, 26 while the hydrocarbon surfactants are usually inclined to form a liquid-like layer on the surface.5

To explore the orientation of the whole surfactant chains relative to the interface, especially for the hydrophobic chains, the distribution probability of tilt angle θ between the hydrophobic chain vector with respect to the xy plane were calculated and plotted in Figure 2, where the hydrophobic chain vector is defined from the oxygen atom in hydroxyl group to the carbon atom in CF3 (CH3) groups. To confirm model and parameter used in the simulation, the tilt angle distribution for C2H5(CH2)10(OCH2CH2)2OH (12H2EO) is calculated and compared with experimental data. The main tilt angle is ~36 degree (as shown in Figure 6a), which is consistent with the data analysis form NR. 10


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As shown in Figure 2, the main distribution of tilt angle for 8H2EO and 8F2EO systems are around 35° and 50°, respectively. It suggests that the hydrophobic chains of 8F2EO tilt more toward the air/water interface. The shape of tilt angle distribution curve of 8F2EO is more compact than that for 8H2EO system, and the relative wide tilt angle distribution indicates disorder arrangement in hydrocarbon system, which can be attributed to the unique properties of fluorine atoms. As we know, the diameter of fluorine atom is relatively larger than hydrogen atom, and it is also more electronegative with smaller polarizability and higher ionization potential. Thus, fluorocarbon chains are more bulky than those of hydrocarbons (the mean volumes of CF2 and CF3 are 38Å3, 92Å3, whereas CH2 and CH3 are 27Å3, 54Å3). Therefore, F-chains are more rigid and difficult to rotate intramolecularly. Besides, the low polarizability of fluorine atom results in weaker VDW interactions between fluorinated chains.11 Owing to the above mentioned two reasons, F-chains often show helical conformation, it has been reported that the concentration of gauche defects in the CF3(CF2)10COOH monolayer is only 0.11%.27 The radial distribution functions (RDFs) between carbon atoms in CH3/CF3 groups were shown in Figure 3. Three peaks at 5.5 Å, 11 Å, and 15.5 Å could be observed in the RDFs for 8F2EO system, only one peak shows at 4.5Å in 8H2EO system. This agrees with Broniatowski’s result, since they have reported that the cross-section areas for fluorocarbon and hydrocarbon atoms are 28.3 Å2, 18.9 Å2, respectively,28,29 i.e. the cross-section radius of carbon atoms in CF3 and CH3 group are 5.31 Å and 4.34 Å. Their values are close to the first peak position shown in Figure 3. It is also found that the height of g(r) CF3-CF3 is larger than that for 8H2EO system. Both the position and height of RDFs reveal that the fluorocarbon system are more ordered than the hydrocarbon one, which is consistent with the relatively sharp distribution of tilt angle in 8F2EO system shown in


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Figure 2. It is also supported by the snapshots in Figure 4b, which shows the anisotropic interface monolayer for 8F2EO, while an isotropic interface monolayer could be observed in 8H2EO system. The different orientations of surfactant chains may be attributed to the rigid chain of 8F2EO surfactant. The hexagonal monolayer structure of fluorosurfactant has been reported by both molecular simulations and experimental methods in previous studies.11, 12, 13 Both the distribution probability of tilt angle and RDFs suggest different structural organizations in the two hydrophobic surfactant systems, but neither of them is direct evidence. In this section, the order parameter (Sm）was introduced to investigate the arrangement of all the surfactant chains directly. It is calculated by following equation:

Sm =

1 1 3 cos2 α − 1 = 3 li ⋅ lj 2 − 1 2 2

(1)

Where α represents the angle between vectors i and j as a function of the distance r separating the midpoint vector, and its schematic is shown in Figure4a. Herein, vectors i and j are defined as the vectors connecting the joint carbon atom (CH2 and CF2) to the carbon atom in the tail group (CH3 or CF3) for different chains. The order parameter has been employed by Xu et al. 8 Figure 4b presents the order parameters for the 8F2EO and 8H2EO surfactants, and the snapshots of the surfactants viewed from top were also inset in Figure 4b. Sm changes from~ 0.1 to ~0.55, indicating different arrangement of surfactant chains in 8F2EO and 8H2EO systems. Small value suggests the isotropic orientation of surfactant chains, or in other words, the surfactant chains adsorbed at the air/water interface with different orientations. For 8F2EO system, the order parameter fluctuates around 0.55, much larger than that for 8H2EO (~0.1), which reveals an anisotropic orientation of the perfluoroalkyl surfactant chains. Nevertheless, the hydrocarbon chains are more disordered, and this is confirmed by the snapshots inserted in Figure 4. It is also consistent


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with other reported experiment results12, in which, they have reported that perfluoroalkyl surfactants prefer forming stable crystalline structure, whereas liquid-like state can always be observed in hydrocarbon system. Some study30 suggested that the ratio between trans-conformation and straight ones would be high when the order parameter value is larger than 0.5. On the contrary, more gauche states would be found in the hydrocarbon system (8H2EO).

3.2. Influence of electrostatic and van der Waals parameters on interfacial structure Both CF3 and CH3 groups show hydrophobic properties. However, the surface tension of perfluoroalkyl surfactant is lower than that for hydrocarbons with the equivalent chain length. Why is such big difference found in surface tension and orientation when we replace CF2/CF3 groups with CH2/CH3 groups? To explain this phenomenon and identify the main factor influencing the arrangement of surfactants, in this section, we exchange the LJ parameters and charges of atoms in CH2/CH3 groups with those in CF2/CF3 groups. Firstly, the charge of each atom in 8F2EO was fixed, and then the LJ parameters of F and C atoms were replaced with those of H and C atoms in CH3 groups. Such a system was denoted as 8F2EO-VDW-modified. Correspondingly, the system with fixed van der Waals (VDW) parameters of CF2/CF3 groups and exchanged charges of the C and F atoms with those of hydrocarbon atoms is named as 8F2EO-Charge-modified. All the detailed information for LJ parameter and electrostatic charge in the original and modified systems are listed in Table S1 of Supporting Information (SI). The similar method has been adopted by Rossky et al.31 to investigate the difference in water solvation between hydrocarbon and perfluorinated analogues. The distribution probabilities of tilt angle in original and modified systems were shown in Figure 5a.When we exchange LJ parameters of F and C atoms with those of H and C atoms in CH3


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groups, the tilt angle distribution is widened, and its shape is similar to that for hydrocarbon system. On the contrary, if keeping the LJ parameters of CF2 and CF3 constant and changing their charges to those of hydrocarbon, the main peak in the tilt angle distribution probability curve becomes smaller, i.e. the surfactant chains are more vertical to the water interface. The distribution probabilities of tilt angles in two modified systems demonstrate that the VDW interaction between hydrophobic groups and water molecules strongly influence the arrangement of surfactant chains. In other words, the interfacial structure is dominated by VDW interaction rather than the electrostatic interaction. The order parameters for modified systems were also calculated and shown in Figure 5b, and the result is consistent with the tilt angle distribution. When we change the VDW parameters, the order parameters become small, and its value is close to zero and similar to that for 8H2EO, indicating the disorder of surfactant chains in 8F2EO-VDW-modified system. On the other hand, the order parameter of 8F2EO-Charge-modified system is also ~ 0.5 and close to that of 8F2EO, suggesting the dominant role of VDW interaction, which determines the orientation and arrangement of surfactant chains. The snapshots of original and modified systems were also shown in Figure 5b, and it is found that all the surfactant chains with VDW parameters of CF3 are well-ordered and prefer tilting to the same orientation. On the contrary, the surfactant chains in the systems with VDW parameters of CH3 are more disordered at the air/water interface. Figure 5c shows the RDFs between carbon atoms in tail groups. Three sharper peaks was observed in the 8F2EO and its charge-modified systems, while only one peak was present in 8H2EO and 8F2EO-VDW-modified systems. The main peak in 8F2EO-Charge-modified is higher than that for 8F2EO, which suggests more compact surfactant layer in the charge modified system. Regarding the VDW modified system, the shape of RDF curve is wide, and the first peak is much


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lower than 8F2EO system. This also indicates the relative disordered orientation in 8F2EO-VDW-modified, which is consistent with the tilt angle distribution and order parameter. In summary, both the distribution probability of tilt angle and the order parameter suggest the order and arrangement of surfactant chains are determined by the VDW parameter of hydrophobic groups. Such conclusion is also supported by Rossky’s work22, in which they have revealed that higher hydrophobicity of fluorocarbon than hydrocarbon across geometries is determined by their size.

3.3. Influence of fluorine content on Interfacial structure As discussed above, the surfactants with low surface tension exhibits higher order parameter. On the contrary, the lower the order parameter is, the higher the surface tension. To confirm this viewpoint, the distribution of tilt angle and order parameter for C2H5(CH2)n(OCH2CH2)2OH (nH2EO) were calculated and shown in Figure 6. Since it is reported that the surface tensions of hydrocarbon surfactants solutions decreases with the carbon number increasing.33 From Figure 6, the distribution of tilt angle (Figure 6a) and order parameter value (Figure 6b) become sharper and larger when the carbon number (n) increase, which suggests the arrangement of surfactants become ordered with the increase of the carbon number. In other words, the changing trend of the surface tension is contrary to that for the order parameter. If we compare the interfacial structure of hydrocarbon surfactant with that for fluorocarbon one, the order parameter of 8F2EO is ~0,5, it is larger than that for 12C2EO, i.e., the surface tension of 8F2EO solution should smaller than that for 12C2EO (25.5 mN/m),

10

it is consistent with previous study.2 In addition, the surface tension for

nonionic and ionic fluorocarbon surfactants with similar monomer structure is compared to their structure information; these results are shown in the supporting information. Similar conclusion


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could be revealed: when the monomer structure is similar, the lower the order parameter, the higher the surface tension. Fluorocarbon surfactants are widely utilized in many applications due to their outstanding performance. However, due to their toxicity and environment unfriendliness, some perfluorinated surfactants, such as Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), have been forbidden by certain European and other countries.

33

Therefore, it is urgent to find a

substitution with less fluorine content. In this section, surfactants with different fluorine contents (the ratios of CF2: CH2 groups were 7:1, 5:3, 3:5, and 2:6, respectively) were modeled. Figure 7a shows the distribution probabilities of tilt angle in surfactants with different fluorine contents. The peak becomes wider with the decrease of fluorine content, i.e., the arrangement of surfactant chains becomes more and more disordered as the content of fluorinated groups decreases. The interfacial structure of 7F1H2EO surfactant is similar to that of 8F2EO when only one CH2 spacer is introduced to perfluorinated chain. This point was also confirmed by the order parameter. Sm is not evidently changed by replacing one CF2 group with CH2 group. The snapshots in Figure 7b also manifests that the surfactant chains of 7F1H2EO tend to tilt toward the same direction. The interfacial structure of 7F1H2EO is similar to that of 8F2EO, which may be good substitution of 8F2EO. The order parameter decreases with the increased substitutions of CF2 groups. It can be attributed to the increase of gauche defects. Collazo et al.11 reported that the concentration of gauche defects in the CH3(CH2)10COOH monolayer is much higher (2.16%) than in that of the CF3(CF2)10COOH monolayer (0.11%). They also verified that the average concentration of gauche configurations decreased from 3.5% to 0.5% when changing the surfactant from C7F15C4H6COOH to C10F21CH2COOH12. When more and more CF2 groups were replaced with CH2 groups, the


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increased number of gauche configurations would be formed in the surfactant system, and the structure of surfactant at air/water interface become disordered. In conclusion, the disordered arrangement of surfactant chains would be hindered by introducing fluorinated chains and facilitated by a hydrocarbon spacer due to the gauche configuration mainly resulted from hydrocarbon groups. The RDFs between carbon atoms in tail groups (CH3 and CF3) with different fluorine contents were calculated and illustrated in Figure 7c. The peak heights and positions of 7F1H2EO are identical to 8F2EO; the arrangement of surfactant at the air/water interface is not significantly affected when only one CH2 spacer was introduced to the perfluorinated surfactant. The main peak decreases when replacing more CH2 spacers in perfluorinated surfactant chain. The positions of the first peak for all partially fluorinated surfactant and 8F2EO are 5.5Å, which reveals the distances between nearest surfactants in fluorinated surfactant systems are the same. Similar results and conclusion were also obtained by Krafft et al. through compression isotherms and grazing-incidence X-ray diffraction (GXID).12 They also reported that the higher chain fluorine content preferentially formed more compact monolayers. In summary, the structure of fluorinated surfactant at air/water interface is closely related to the fluorine content. The interfacial structure is not significantly influenced by replacing one CF2 with CH2 group. 7F1H2EO is a good substitution of 8F2EO. The distribution of tilt angle and the order parameters would be narrower and smaller with the decrease of fluorine content. It is caused by the increase of gauche configurations, which is closely related to hydrocarbon groups. The structure of fluorocarbon surfactants at the air/water interface becomes ordered by introducing more fluorinated groups; on the contrary, hydrocarbon spacer enhances the disorder of interfacial surfactants.


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Conclusion The adsorption behavior and the structure of nonionic fluorocarbon/hydrocarbon surfactants at air/water interface were investigated by molecular dynamics simulations. The density profiles for fluorocarbon and hydrocarbon surfactants are similar, and their tail groups are inclined to point toward the air phase with similar their interfacial layers. The distribution probability of tilt angle between the hydrophobic chains with respect to the interface and the order parameter of hydrophobic chains indicate the different orientations of surfactant chains in two systems. An anisotropic orientation was inspected in the air/water interface of fluorocarbon surfactants, while the hydrocarbon chains present more disorder. In other words, perfluorinated surfactants lean to form stable crystalline structure. As for hydrocarbon system, liquid-like state was always observed from molecular level. To explore the mechanism for disorders caused by CH2/CH3 groups, we exchanged the LJ parameters and charge information in CH2/CH3 groups with those in CF2/CF3 groups. Both the tilt angle and order parameter suggest that the arrangement of surfactant chains is dominated by the VDW parameter of hydrophobic groups. When the monomer structure is similar to each other, the changing trend of the surface tension is contrary to that for order parameter. The surfactants with low surface tension exhibits higher order parameter value. On the contrary, the lower the order parameter is, the higher the surface tension. Calculation of order parameters for surfactants could be used to evaluate its capacity in the reduction of surface tension; it will provide some insight into surfactant design and synthesis. The structure of fluorinated surfactant at air/water interface is closely related to the content of


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fluorinated groups. By replacing one CF2 group in 8F2EO with CH2 group, the interfacial structure was not significantly influenced. 7F1H2EO may be a good substitution of 8F2EO. Due to the increase of gauche configurations caused by hydrocarbon groups, the tilt angle distribution and the order parameter become narrower and smaller with the decrease of fluorine content. The structure of fluorocarbon surfactants at the air/water interface becomes ordered by introducing more fluorinated groups; on the contrary, hydrocarbon spacer enhances the disorder of interfacial surfactants. This work provides molecular insights into the microstructures of fluorocarbon surfactants at air/water interface and may inspire more investigations on the design of surfactants in future.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 20904048) and the Natural Science Foundation of Zhejiang province (Grant No LY13B030008, LY13B040004 and LY14B030008). It was also supported by Science foundation of Zhejiang Sci-Tech University (No 0813825-Y).

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Table of Contents Understanding the Structure of Hydrophobic Surfactant at Air/Water Interface from Molecular Level Li Zhang∗1, Zhipei Liu1, Tao Ren1, Pan Wu1, Jia-Wei Shen2, Wei Zhang1, Xinping Wang1

1. Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, XiaSha Higher Education Zone, Hangzhou, 310018, China. ∗

Corresponding author, Fax: +86-571-86843600 E-mail address: [email protected]


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2. School of Medicine, Hangzhou Normal University, Hangzhou 310036, PR China

The structure of hydrophobic surfactant at air/water interface is closely related the monomer structure, an anisotropic orientation could be detected in the air/water interface of perfluoroalkyl surfactant, while the hydrocarbon chains present more disorder.


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Scheme 1. Snapshot of the initial configuration of the surfactant 8H2EO system. Coloring is as follows: red point for water molecules, the oxygen、carbon and hydrogen atoms in surfactant are represented by red, green and white sphere, respectively.

Table 1. Molecular formula and short name of surfactants studied in this work Molecular formula

Short name

C8F17(OCH2CH2)2OH

8F2EO

C8H17(OCH2CH2)2OH

8H2EO

C2F5(CH2)6(OCH2CH2)2OH

2F6H2EO


3F5H2EO


5F3H2EO


6F2H2EO

C7F15CH2(OCH2CH2)2OH

7F1H2EO


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1200

8H2EO-water 8H2EO-T 8H2EO-J 8H2EO-H 8F2EO-water 8F2EO-T 8F2EO-J 8F2EO-H

1000

800 3

Density(kg/m )

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600

400

200

0 -100

-50

0

50

100

Z axis (Å)

Figure 1. Mass density profiles of oxygen atoms in the hydrophilic head (H)、joint carbon atoms （J)、carbon atoms in the tail groups(T) and water normal to the monolayer interface in perfluoroalkyl (8F2EO)and hydrocarbon(8H2EO) surfactant systems.


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Figure 2. The distribution probabilities of tilt angle (θ) between the tail vector in 8F2EO (red dash dot line) and 8H2EO (black solid line) systems with respect to the normal to the interface.

Figure 3. Radial distribution functions between carbon atoms in CH3 or CF3 groups of 8F2EO (red dash dot line) and 8H2EO (black solid line) systems


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(a)

Figure 4.

(a) The schematic of tilt angle α between surfactant chain and its neighbor,(b) the order parameters for surfactants 8F2EO (circle) and 8H2EO (square)


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Figure 5. (a) The distribution probabilities of tilt angle between the tail vector and xy plane (b) the order parameters (c) Radial distribution functions between carbon atoms in CH3/CF3 groups for the original systems 8H2EO (black solid line), 8F2EO (red dash dot line) and modified systems 8F2EO-Charge-modified (red dot line), 8F2EO-VDW-modified (black dash line).


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Figure 6. (a) The distribution probabilities of tilt angle between the hydrocarbon tail and xy plane (b) the order parameters for the 12H2EO, 8H2EO and 6H2EO surfactants


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Figure 7. (a)The distribution probabilities of tilt angle between the tail vector and xy plane (b) the order parameters (c) Radial distribution functions between carbon atoms in CH3/CF3 groups for surfactants with different fluorine content


Understanding the Structure of Hydrophobic Surfactants at the Air

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