Interactions of Sulfobetaine Zwitterionic Surfactants with Water on

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Interactions of sulfobetaine zwitterionic surfactants with water on water surface Amirhossein Mafi, Dan Hu, and Keng C. Chou Langmuir, Just Accepted Manuscript • Publication Date (Web): 02 Oct 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Interactions of sulfobetaine zwitterionic surfactants with water on water surface Amirhossein Mafi1,2, Dan Hu1, and Keng C. Chou1*

1

Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada

2

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

* EMAIL ADDRESS: [email protected]

Abstract Graphic

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ABSTRACT We carried out a combined study using surface tension, phase-sensitive frequency generation (SFG) vibrational spectroscopy, and MD simulations to investigate the industriallyrelevant

zwitterionic

surfactant

N-dodecyl-N,

N-dimethyl-3-ammonio-1-propanesulfonate

(DDAPS) on water surface. The SFG Im(χ(2)) spectra showed that the interaction between DDAPS and water was different from those between biologically-relevant zwitterionic phospholipids and water. While zwitterionic phospholipids were found to be anionic-like and flipped water molecules with their OHs pointing toward the air, DDAPS oriented water molecules with their OHs mostly pointing toward the liquid water. We built a new force field for the MD simulation which produced the correct surface tension of water with various DDPAS coverage. The MD simulation showed that the head groups of DDPAS were nearly parallel to the water surface. When the surface coverage of DDPAS was increased, the averaged tilting angle of DDPAS’s tails decreased but it had little effect on the orientation of the head group. The sulfobetaine zwitterionic surfactant was found to be more cationic-like because the positively charged group was more capable of orienting interfacial water. .

KEYWORDS. sum frequency generation vibrational spectroscopy, surface tension, molecular dynamics simulation, hydrogen bond,


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Introduction Zwitterionic (or amphoteric) surfactants, which have two distinct and opposite charges in their head groups, are of great interest because of many unique properties, such as their water solubility, biodegradability, biosafety, and temperature stability.1, 2, 3, 4 They have been widely used in a variety of consumer and industrial products, which underlie many aspects of our daily lives. A range of methods has been developed for producing zwitterionic surfactants, many of which contain a positively-charged quaternary ammonium ion and a negatively-charged group, such as sulfonate (R-SO3-). The interaction between zwitterionic surfactants and water is particularly interesting because the surfactants, carrying both positive and negative charges, induce a complex behavior of water molecules. The structures of water surfaces in the presence of ziwitterionic surfactants are not fully understood. It was observed that zwitterionic molecules enhanced the ordering of surface water molecules.5,

6, 7

Many previous studies on the interaction between water and ziwitterionic

surfactants were focused on phospholipids. Sovago et al. studied the ziwitterionic lipids dipalmitoyl phosphatidylethanolamine (DPPE) and dipalmitoyl phosphatidylcholine (DPPC) on water surface using sum frequency generation (SFG) vibrational spectroscopy.8 With a numerical maximum entropy phase retrieval algorithm,9,

10

Sovago et al. concluded that the averaged

orientation of water dipoles pointed toward the bulk.8 Sovago et al. proposed that water in contact with the net neutral zwitterionic lipids DPPC and DPPE were oriented in the same fashion as those in contact with the anionic surfactants because there was a layer of water situated above the phosphate group with their OHs pointing down, which had more contribution to the SFG signal than those underneath the head group. A contradictory result was reported by Chen et al. using phase-sensitive SFG showing that the imaginary second-order nonlinear


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susceptibility Im(χ(2)) was positive over the entire OH stretch region indicating that the water molecules were oriented with their OHs pointing up in the presence of DPPE and DPPC.11 Another SFG study by Mondal et al. on zwitterionic phospholipid, palmitoyl-2-oleoyl-snglycero-3-phosphocholine (POPC), at air/water interface also showed a positive Im(χ(2)) spectrum over the entire OH stretch region.12 Mondal et al. also concluded that the negatively charged phosphate group was more capable of orienting interfacial water than the positively charged choline group, i.e., the zwitterionic phospholipids were anionic-like. Relatively fewer studies have been carried out on industrially-relevant zwitterionic surfactants, such as sulfobetaine, which has some fundamental structural differences compared to phospholipids. Surface tension has been the most widely used measurement to study the properties of sulfobetaine on water interfaces.13,

14, 15

However, surface tension provides little

molecular-level information on their interaction with water molecules. Previously the micelle formation and aggregation of sulfobetaine have been studied by molecular dynamics (MD) simulation.16,

17

However, MD simulation of sulfobetaine on water surface that has a correct

surface tension prediction has not been reported.18, 19 To gain a better insight into the interaction between water and sulfobetaine zwitterionic surfactants, we carried out a combined study using surface tension, phase-sensitive SFG, and MD simulations on N-dodecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) at air/water surface. Here we also report a new force field which correctly simulates the surface tension of DDAPS on water surface. We found that the Im(χ(2)) spectrum of DDAPS/water interface exhibited both positive and negative peaks, which is significantly different from those of ziwitterionic phospholipids. In contract to the anionic-like phospholipids, we found that the


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sulfobetaine zwitterionic surfactant was more cationic-like because the positively charged group was more capable of orienting interfacial water.

Material and Methods Material and Sample Preparation. DDAPS (> 99.5%) was purchased from SigmaAldrich. Water with a resistivity > 18.2 MΩ·cm was obtained from a Millipore system. DDPAS solutions were freshly prepared before experiments. All experiments were performed at 20 ± 0.5 °C and 1 atmosphere pressure. Phase-sensitive SFG setup. A femtosecond Ti-sapphire laser (120 fs, 800 nm, 1 kHz, and 2 mJ/pulse) was used to pump an optical parametric amplifier for generating a femtosecond IR beam. The broad-band IR beam and a narrow-band picosecond 800 nm beam were aligned collinearly.20, 21, 22 The incident angle was 60°. A reference SFG was generated by focusing the IR and the picosecond 800 nm beams into a 50-µm thick quartz crystal. The IR, 800 nm, and reference SFG beams were then focused again on the sample. The reference SFG and the SFG generated at the sample went through a time-delay, a polarizer, a band pass filter, a lens, and a monochromator, and then the interference pattern was recorded by a camera. The polarization combination used in this study was SSP (s-polarized SFG, s-polarized 800 nm and p-polarized IR). The energy of the 800 nm and IR beam were ~10 µJ/pulse and ~3 µJ/pulse, respectively. Each spectrum presented in the paper was acquired over a period of 20 min. MD simulation. Molecular dynamics simulations were carried out using GROMACS 5.1.2 23, 24, 25

in the canonical ensemble. We built an all-atom type force field by combining the TEAM

(Transferable, Extensible, Accurate, and Modular) force field26 with the Generalized Amber Force Fields (GAFF)27 to describe the behavior of DDAPS on water. The tail and the sulfonate


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atoms were parameterized using the TEAM force field, which has been designed in a way that the parameters can be transferred to the other molecules with the same functional groups. Some intra-molecular and inter-molecular interactions in DDAPS, which were not defined by the TEAM force field (i.e. N-C), were obtained from the GAFF force field using ACPYPE28. The TEAM and GAFF force fields are compatible as they both are designed for the same potential energies:

U=

 kr

∑  2 (r - r

0

bonds

k   k  ) 2  + ∑  θ (θ - θ 0 ) 2  + ∑ ∑  n (1 + cos(nφ - φ0 ))  +  angles  2  torsions n  2 

    4ε ( σ ij )12 - ( σ ij ) 6   +  q i q j ∑∑ ij  rij    4πε 0 rij i j ≠i   rij 

    

(1)

where kr, kθ, and kn are constants, r0 is the equilibrium length of bond, is the equilibrium angle between two bonds, is the dihedral angle, σ is the van der Waals diameter, ε is the well depth, εo is the permittivity of the free space, and q is the partial charge. The partial charges of atoms in the head group were obtained by using Gaussian 0929 B3LYP30, 31/6-31g(d) with the Mullikan atomic charge method. The partial charges of the other atoms were given by the TEAM force field. The parameters are presented in the Supporting Information. The flexible SPC/E model was used to describe the water molecules.32 The simulation box dimensions were 3.6 × 3.6 × 32 nm3. As shown in Figure 2a, a slab with thickness of 7 nm was filled with 3017 water molecules with vacuum at the both ends of the box. Various number of surfactants, as shown in Table 1, were randomly distributed using PACKMOL33 on each side of the water surface with their head groups pointing toward the water. The steepest descent energy minimization was conducted to prepare the system for the MD simulation. The system temperature was maintained at 293 K using the V-rescale thermostat34 with the temperature constant equal to 0.1 ps. All bonds including water’s OH bonds were constrained by the P-LINCS35 algorithm with a LINCS


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order of 8. The Lennard-Jones interaction was truncated with a cut-off radius of 1.6 nm. Unlikeatom interactions were computed using the standard Lorentz-Berthelot combination rules.36,

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Periodic boundary conditions were applied to all three directions. The particle mesh Ewald (PME) algorithm38 with a real cut-off radius of 1.6 nm and a grid spacing of 0.16 nm was used to calculate the long-range columbic interactions. Each simulation was carried out for a time period of 40 ns with a step of 2 fs when integrating the equations of motion. The system required 10 ns to reach an equilibrium state. Therefore, data in the first 10 ns were discarded, and all results presented in the current study were based on the later 30 ns. The visualizations were produced using VMD 1.9.1.39 To correlate the measured surface tension with the MD simulation, the surface coverage of the surfactant for each concentration of DDAPS in Table 1 was determined by the Gibbs adsorption equation

Γ=−

A=

1  dγ    RT  d ln c T

1 N AΓ

(2)

(3)

where c is the surfactant concentration, R is the gas constant, T is the absolute temperature, γ is the surface tension, Γ is the surface excess concentration, NA is the Avogadro’s number, and A is the area occupied by one surfactant molecule. Eq. (2) was solved numerically by the forward difference method which requires a known value to find the next data point. When the slope ( dγ ) in Eq. (2) approaches 0, Eq. (2) fails to predict the surface coverage ( A → ∞ ). Therefore, d ln c the Gibbs adsorption equation is applicable only in the range of ~2.5×10-5 - 2.5×10-3 M where the slope is significantly larger than 0 in Figure 1a. We started with a concentration of 2.5×10-5


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M and a surface tension of 71.85 mN/m to estimate the surface coverage of DDPAS at a higher concentration. The calculated surface coverage is presented in Table 1. The surface tension in the simulated system was calculated by the Kirkwood-Buff method:40

γ=

1 2

Lz

∫ (P

zz

− 0.5(Pxx + Pyy ))dz

(4)

0

where Pxx and Pyy are the components of the pressure tensor parallel to the surface, and Pzz is the component perpendicular to the surface. To calculate the hydrogen bonds, geometric criteria were used based on 1) the distance between the O atom of acceptor and the O atoms of donors and 2) alignment of H between both O of donors and acceptors.41 Two molecules were assumed to be hydrogen bonded if the following conditions were satisfied RO −OW < RO −OW

θO

W

− H W ...O

cutoff

> θ cutoff

(6)

where the cutoff distance ( Rcutoff ) was derived from the first minimum of radial distribution function. The cutoff distance ( Rcutoff ) was 0.32 nm between a surfactant O atom and a water O atom (OW) and 0.33 nm between two OWs. The cutoff angle ( θ cutoff ) for a OW − HW ---O hydrogen bond was 140 degrees.41

Results and Discussion Figure 1a shows the surface tension of water vs. the concentration of DDAPS with the corresponding SFG spectra in the CH and OH regions shown in Figure 1b and 1c, respectively. When the DDAPS concentration is less than 10-5 M, the surface tension of water has a relatively


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small change. At 10-5 M, no significant CH peaks were observed (magenta curve in Figure 1b), and the OH spectrum of water (magenta curve in Figure 1c) was very similar to that of pure water (cyan curve in Figure 1c) indicating DDAPS had little surface activity at 10-5 M. Above 2.5×10-5 M, the surface tension decreased when the DDPAS concentration increased. Once the concentration reached the critical micelle concentration (CMC), ~2×10-3 M, micelles formed in the bulk water, and further increase in the surfactant concentration did not further decrease the surface tension of water.

Figure 1. (a) Surface tension of water with various DDAPS concentrations. Im(χ(2)) spectra of air/water interfaces in the CH (b) and OH (c) regions with DDAPS at 0 M (cyan), 1×10-5 M (magenta), 2.5×10-5 M (orange), 1×10-4 M (blue), and 2.6×10-3 M (red). The Im(χ(2)) spectra have a lower signal-to-noise ratio toward both ends of the spectra because the broadband IR laser has a Gaussian spectral profile.

The interpretation of the SFG spectrum of pure water surface (cyan curve in Figure 1c) has been controversial. The spectrum shows a smaller positive OH band near 3100 cm-1 and a larger


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negative OH band near 3450 cm-1. For the OH symmetric stretch, the Im(χ(2)) can be positive or negative depending on the sign of the OH projection with respect to the surface normal: a positive peak indicates water molecules with the hydrogen pointing toward the air (up), and a negative peak indicates the OHs pointing toward the liquid water (down).21, 42, 43 The OH stretch mode of water in the gas phase is around 3756 cm−1. The frequency is red-shifted in the liquid phase because of hydrogen bonds. The SFG band near 3450 cm-1 is generally accepted as the OH stretch mode from water molecules which are weakly hydrogen-bonded, but the origin of the low-frequency peak near 3100 cm-1 has been controversial. Tian et al. proposed that “ice-like” tetrahedrally bonded water molecules had the dominating contribution to the 3100 cm-1 band.44 On the other hand, Nihonyanagi et al. reported that the 3100 cm-1 band came from surface water dimers, which generated a vertical induced dipole pointing toward the air,45 rather than tetrahedrally coordinated water molecules. Nevertheless, the MD simulations carried out by Pieniazek et al. using a three-body-interaction model showed that the positive peak at the lower frequency was a result of cancellation between the positive contributions from four-hydrogenbonded molecules and the negative contribution from those molecules with one or two broken hydrogen bonds.46 Despite the uncertainty in the origin of the 3100 cm-1 band, it is theoretically correct that a larger SFG peak indicate a better ordering of water. Figure 1c suggests that the organization of water molecules near the sulfobetaine zwitterionic surfactants is fundamentally different from those near zwitterionic phospholipids. While zwitterionic phospholipids, such as DPPE and DPPC, are anionic-like and produce positive Im(χ(2)) over the entire OH stretch region,11, 12 the presence of DDAPS only moderately enhances both the positive OH peaks. The SFG spectra suggest that, in contrast to anionic-like


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zwitterionic phospholipids, which flip the orientation of water molecules, DDAPS enhances the ordering of water without significantly flipping the orientation of water. MD simulations were carried out to obtain a more detailed structure of water at the interface. A snapshot of the equilibrium state in the simulation is shown in Figure 2a. Since a new force field was used (details described in the Material and Methods section and the Supporting Information), the accuracy of the force field was verified by comparing the calculated surface tension to the measured surface tension at various concentrations of DDAPS. The results are summarized in Table 1. A reasonably good agreement between the measured and simulated surface tension values was achieved in the range of 1×10-4 - 2.6×10-3 M, in which the Gibbs adsorption equation (Eq. 2 and 3) could be applied as described in the Material and Methods Section, Table 1. Measured and simulated surface tension of water in the presence of DDAPS. DDAPS concentration (M) 1×10-4 3×10-4 2.6×10-3 Calculated surface coverage

0.85

0.63

0.48

30

41

54

66.3 ± 0.2

59.2 ± 0.2

41.7 ± 0.2

65.3 ± 0.9

61.6 ± 1.3

47.0 ± 4.6

(nm2/molecules) Number of surfactants in the simulation (both surfaces) Measured surface tension (mN/m) Simulated surface tension (mN/m)


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Figure 2b shows the density profiles of water with various concentrations of DDAPS. The density profiles can be fitted with an error function47

ρ ( z) =

ρ L + ρV 2

−

ρ L − ρV 2

 z − z0  erf    2d 

(5)

where ρ L is the liquid density, ρ V is the vapor density, z 0 is the Gibbs dividing surface, and d is a thickness parameter. The so called "10-90" thickness te is defined as the distance along the surface normal over which the density changes from 10% to 90% of the bulk density. Using Eq. (5), it can be shown that te = 2.56d. Our simulations show that the "10-90" thickness for pure water is 0.35 nm, which is in good agreement with the values reported previously using the SPC/E water model.47,

48

The presence of surfactants significantly perturbs the structure of

surface water as the thickness of the "10-90" layer increases by ~3 times from 0.35 nm (pure water) to a value between 0.95 nm and 1.10 nm.


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Figure 2. (a) Simulation box with a dimension of 3.6 × 3.6 × 32 nm3 filled with 3017 water and 54 DDAPS molecules. Color codes for atom types: white (hydrogen), red (oxygen), cyan (carbon), blue (nitrogen), yellow (sulfur), and magenta (water). (b) Water density as a function of depth z. (c) Orientation factor of water's dipole vs. z. The insert shows the definition of the orientation angles of the dipole moment and the OH bond with respect to the surface normal (z axis). (d) Density weighted orientation factor. The statistical error of is ~0.06 (standard deviation).


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To demonstrate the effect of DDAPS on the orientation of water molecules, Figure 2c shows the averaged orientation factor of water's dipole as a function of depth, where θ µ is the angle between the dipole moment of water and the surface normal (z-axis), as shown in the inset of Figure 2c. The triangular bracket denotes an average over time and all water molecules located at the same depth within a layer with a thickness of 0.2 nm. On the pure water surface (cyan curve in Figure 2c), water molecules have their averaged dipole pointing up in the low density region and pointing down in the higher density region. The density-weighted plot in Figure 2d is a better description of the depth dependent orientation factor with their relative population. For pure water in Figure 2d (cyan), water with their dipole pointing down dominates, which is consistent with previous studies showing water molecules at the surface have their OH pointing down to maximize the number of hydrogen bonds.45 Overall, in the presence of the zwitterionic surfactant, the thickness of the non-isotropic layer ( ≠ 0) increases compared to that of pure water. It is interesting that in Figure 2d the maximum value of the densityweighted orientation factor ρ occurs at a DDAPS concentration below the CMC. This is qualitatively consistent with the SFG spectra in Figure 1c showing the OH spectrum with the highest DDAPS concentration does not have the highest SFG peak intensity, which suggests that a larger number of DDAPS on the surface will disturb the ordering of water molecules. Our studies also suggests that DDAPS is cationic-like in contrast to DDAO zwitterionic surfactant12 or zwitterionic lipids which show anioinic-like behaviors.11,

12, 49

As explained

below, the positively-charged cationic group of DDPAS has a higher impact on the orientation of water molecules because the positive charge of the head group is distributed on a larger number of atoms, in comparison to the negative charge. Therefore, a larger number of water molecules can interact with the positively charged atoms in the head group.


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Figure 3a-b show the angle distribution of water’s OH bonds in the ‘10-90’ layer with and without DDAPS. Figure 3a indicates that the non-hydrogen-bonded OHs (red curve) in the pure water peak at θOH ~ 10° and 170° suggesting that the non-hydrogen-bonded OHs are more likely to align normal to the surface. On the other hand, the hydrogen-bonded OHs (blue curve in Figure 3a) show a peak at θOH ~ 85° indicating that hydrogen-bonded OHs tend to orient parallel to the water surface. Figure 3b shows that DDAPS significantly reduces the number of OHs parallel to the water surface. The water OHs forming hydrogen bonds with the sulfonate groups are mostly pointing upward (green curve in Figure 3b). This is consistent with the SFG spectra in Figure 1c showing DDAPS also enhances the positive OH peak. Overall, the presence of DDPAS on water surface reduces the number of OHs parallel to the surface; hence it enhances both the positive and negative peaks in Figure 1.


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Figure 3. Orientation distributions of water’s OH bonds in the ‘10-90” layer without (a) and with (b) DDAPS (2.6×10-3 M). The blue curves are OHs forming hydrogen bonds with water, the red curves are OHs not forming hydrogen bonds, the black curves are the total OHs, and the green curve is OHs forming hydrogen bonds with the sulfonate groups of DDAPS.

The orientation of DDPAS on water surface was investigated by plotting the angle distribution of the vectors defined from the N atom to the C atom in the methyl group ( NC1) and from the N atom to the S atom ( NS). The labeling of atoms is shown in Figure 4a. Figure 4b shows the angle distribution of NC1 and NS with respect to the surface normal ( = 0° ). The tails

NC1) on average point toward the air ( < 90°), and the tilting angle decreases when the surface ( coverage of DDPAS increases. Interestingly, the majority of the head groups ( NS) are nearly parallel to the water surface, and an increase in the surface coverage of DDPAS has little effect on the orientation of the head group. In this geometry, both the positive segment (from C12 to CT2) and the negative segment (from CT3 to SO3) of the head group (detailed partial charges given in the Supporting Information) have nearly equal chance of interacting with water molecules. However, because the size of the positive segment is significantly larger than that of the negative segment, a larger number of water molecules can interact with the positively charged atoms in the head group, which makes DDPAS appear more cationic.


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) Figure 4. (a) Labeling of atom in DDAPS. (b) Distributions of the titling angle for the tail (NC1 ) groups with DDAPS concentration at 1×10-4 M (blue), 3×10-4 M (green), and and the head (NS is the vector from the N atom to the C1 atom, and 2.6×10-3 M (red). NC1 NS is the vector from the N atom to the S atom.

Conclusions We carried out a combined study using surface tension, phase-sensitive SFG, and MD simulations to investigate DDAPS on water surface. The Im(χ(2)) spectra showed that the presence of DDAPS enhanced the ordering of surface water molecules. We built a new force field for the MD simulation and produced the correct surface tension of water with various DDPAS coverages. MD simulations showed the head groups of DDPAS were nearly parallel to the water surface, and an increase in the surface coverage had little effect on the orientation of


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the head group. The sulfobetaine zwitterionic surfactant was cationic-like because the positively charged group was more capable of orienting interfacial water.

Supporting Information. Partial charges and parameters used in the MD simulation.

ACKNOWLEDGMENT This work was financially supported by the Natural Sciences and Engineering Research Council of Canada, and the Canada Foundation for Innovation. A. Mafi was partially supported by the Four Year Doctoral Fellowship Program at the University of British Columbia. This research was enabled in part by support provided by WestGrid and Compute Canada Calcul Canada.

REFERENCE 1. Doussin, S.; Birlirakis, N.; Georgin, D.; Taran, F.; Berthault, P. Novel Zwitterionic Reverse Micelles for Encapsulation of Proteins in Low-Viscosity Media. Chem-Eur J. 2006, 12 (15), 4170-4175. 2. El-Aila, H. J. Y. Effect of Urea and Salt on Micelle Formation of Zwitterionic Surfactants. J. Surfactants Deterg. 2005, 8 (2), 165-168. 3. Zajac, J.; Chorro, C.; Lindheimer, M.; Partyka, S. Thermodynamics of Micellization and Adsorption of Zwitterionic Surfactants in Aqueous Media. Langmuir 1997, 13 (6), 14861495. 4. Zhai, L. M.; Tan, X. J.; Li, T.; Chen, Y. J.; Huang, X. R. Influence of Salt and Polymer on the Critical Vesicle Concentration in Aqueous Mixture of Zwitterionic/Anionic Surfactants. Colloid Surface A 2006, 276 (1-3), 28-33.


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Interactions of Sulfobetaine Zwitterionic Surfactants with Water on

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