Geometric Origins of Surfactant Effectiveness in Mixed Solvent Systems

Article pubs.acs.org/JPCC

Geometric Origins of Surfactant Effectiveness in Mixed Solvent Systems Phwey S. Gil and Daniel J. Lacks* Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States ABSTRACT: The surface activity of perfluorooctanoic acid (PFOA) and its hydrocarbon analog octanoic acid (OA) in ethanol−water solutions is studied with a combined experimental and molecular dynamics simulation approach. Experiments show that PFOA is more effective at reducing surface tension in ethanol−water solvents than OA. The surface tension at the CMC is lower with PFOA than with OA, and PFOA is effective at lowering the surface tension in solutions with high ethanol content (up to 60%) while OA is not. We use molecular dynamics simulations to determine the underlying basis for these differences. The tendency of PFOA and OA molecules to partition to the surface at infinite dilution is evaluated using free energy profiles. Features of these free energy profiles at infinite dilution are shown to correlate with the experimental surface tension results. PFOA’s superior surfactant properties are attributed to PFOA having a larger girth than OA, which is largely a consequence of the C−F bond being longer than the C−H bond. We expect the relationship between surfactant girth and effectiveness in reducing surface tension in mixed solvent systems to be general.

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surfactants.1 By studying these two molecules, we are able to examine the effects of fluorinating the carbon tail. We use a combination of experiments and molecular dynamics simulations to investigate the molecular origins of the partitioning force of surfactants in mixed solvent systems. We identify what characteristics of a surfactant molecule cause it to be an effective surfactant in cosolvent systems. By identifying the most important characteristics of a surfactant molecule, it will be possible to select or design better surfactants for cosolvent systems. We hypothesize that the effectiveness of the surfactant in reducing surface tension is related to the driving force pushing surfactant molecules to the surface. Our experimental method determines the magnitude of the decrease in surface tension at the critical micelle concentration and the concentration range of ethanol where each surfactant significantly reduces the surface tension. To study how cosolvent concentration affects the extent to which a surfactant reduces the surface tension, the most straightforward approach would be to simulate a system with the high surfactant concentration that is representative of real surfactant-containing solutions. However, the magnitudes of the changes in surface tension that we are interested in can be small. It is difficult to obtain high precision results in simulations with high surfactant concentration because of the very long equilibration times within the surfactant monolayer and the large fluctuations in the surface tension. Thus, this methodology would be difficult to apply in the present study.

INTRODUCTION Surfactants are amphiphilic molecules, which means that their chemical structure has a polar part and a nonpolar part. This aspect of their chemical structure gives them interesting properties. In aqueous solutions, polar parts are attracted to water, while nonpolar parts are repelled. Because of these attractive and repulsive forces, surfactants partition to the interface of an aqueous solution. The polar part of the molecule would be solvated among the water molecules, but the nonpolar part would reside in the other phase such as a nonpolar liquid (e.g., oil) or vapor (e.g., air) phase. The partitioning effect of surfactants make them useful in various applications. They can decrease the surface tension of liquid surfaces, and they can form micelles in the bulk of the solution, depending on the concentration of surfactant. Surfactants are used as foaming, wetting, or emulsifying agents in industry. However, finding surfactants that are effective in solutions of water and a cosolvent, such as ethanol, remains a challenge. The addition of a cosolvent reduces the repulsive force between the solvent and the surfactant’s nonpolar part. As a result, surfactants are less partitioned to the interface between phases. Therefore, surfactants may become less effective foaming, wetting, and emulsifying agents in aqueous solutions with cosolvents. In this paper, we compare perfluorooctanoic acid (PFOA) and its hydrocarbon analog, octanoic acid (OA), in solutions containing water and ethanol. PFOA and OA are analogs in that they are both surfactants that are eight carbons long and have one carboxylic acid functional group. While octanoic acid has its carbon tail hydrogenated, perfluorooctanoic acid has its carbon tail fluorinated. Fluorinated surfactants are known to be better at decreasing surface tension than regular hydrocarbon © XXXX American Chemical Society

Received: October 1, 2015 Revised: November 11, 2015

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DOI: 10.1021/acs.jpcc.5b09603 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

The surfactants are simulated at infinite dilution by including a single surfactant molecule in a slab of solvent. NVT simulations containing approximately 13500 atoms inside a box of size 6 × 6 × 15 nm3, periodic in all directions, are simulated for 20 million steps with a time-step of 2 fs (40 ns total). The liquid slabs are 3−4 nm thick and oriented with the surfaces perpendicular to the Z-axis. The position of the surfactant molecule is defined as the position of the carbon atom on the carboxylic acid functional group; this position is tracked as a function of time, and is denoted as Z(t). To obtain the free energy profile, the following procedure is used. The probability that a surfactant molecule is at a position Z is related to the free energy, G(Z) by

Our computational method involves simulating a surfactant in the infinite dilution limit, coupled with the hypothesis that the behavior in the infinite dilution limit can be predictive of the ability of the surfactant to decrease the surface tension.2 Simulations at the infinite dilution limit offer several advantages. First, high precision results can be easily obtained. Second, more physical insight is obtained as we can predict at what concentration of the cosolvent the surfactant ceases to partition to the surface. With this methodology, we are able to correlate the experimental and simulation results, and more importantly, we are able to determine at the atomic level why PFOA is more effective in ethanol−water solutions than OA.

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METHODS Experimental Methods. Surface tension is measured using the Wilhelmy plate method with the K100 Krüss Tensiometer.3 The Wilhelmy method offers the advantage that it is independent of liquid density.4,5 During the experiments, the temperature ranged from 20 to 23 °C, which was small enough that it had no significant impact on the experiment results. Deionized water is obtained from the Barnstead and Millipore water purification systems in series. 100% ethanol is obtained from Decon Laboratories. 96% PFOA and 99.8% PFOA are obtained from Sigma-Aldrich. All solutions are prepared measuring concentration in percentage by mass (wt %). The procedure for measuring the surface tension is as follows. First, the ethanol−water solvent is prepared by mixing ethanol and water to the desired concentration in ethanol wt %. Then, a specified amount of surfactant is added to the solution to yield the desired concentration of surfactant. The surface tension of this solution is measured. Next, the solution is diluted with the prepared ethanol−water solvent and another measurement is taken. The surface tension is measured at various surfactant concentrations through the series of dilution steps. 96% PFOA was used for the experiments. Experiments done in pure water produced similar results with both the 96% and 99.8% purity PFOA samples. Computational Methods. Molecular dynamics is a computer simulation method that calculates the physical movement of atoms in a system. The atoms interact with each other while following Newton’s equations of motion. The forces between each atom and the potential energy of the system are defined by molecular mechanics force fields. The trajectory, or the simulated movement, of the atoms can be analyzed to yield macroscopic properties about the system. For all molecular dynamics simulations, the Optimized Potentials for Liquid Systems All Atom (OPLSAA) force fields are used for ethanol, OA, and PFOA.6 The SPC/E force field is used for water molecules,7 because it was shown to be able to calculate the surface tension of water better than the TIP3P, TIP4P, and the SPC force fields.8 The simple three-site model would cost less computational time than the four-site models. The simulations are fully atomistic, which is necessary for comparing the subtle differences between PFOA and OA. Temperature is held constant at 300 K using the velocityrescaling thermostat with a time constant of 0.1 ps.9 Coulombic interactions are calculated using the Particle Mesh-Ewald (PME) method with a cutoff distance of 1 nm for the realspace sum.10 Non-Coulombic interactions are directly calculated with a cutoff distance of 2 nm. Bond constraints are used to keep bond lengths constant; tests without the constraints showed that using bond constraints did not affect results.

prob(Z) = C × e−G(Z)/ kT

(1)

where C is a normalizing constant. This equation can be rearranged to yield G(Z) = −kT ln[prob(Z)] + C′

(2)

where G(Z) is the free energy profile and C′ is another constant. Z(t) is obtained from molecular dynamics trajectories. By making a histogram of the Z(t) results, the probability function prob(Z) is obtained, from which G(Z) is calculated. At lower concentrations of ethanol, the surfactant molecule partitions to the surface so strongly that regions within the bulk cannot be sampled adequately for the probability function. In this case, umbrella sampling is utilized to better sample the configurations within the bulk.11 A false energy bias, Uumb(Z) is introduced into the MD simulations: Uumb(Z) =

1 k umb(Z − Zo)2 2

(3)

The effect of this bias potential is then removed in the free energy calculation: G(Z) = −kT[prob(Z)] − Uumb(Z) + C′

(4)

This method can pull the molecule into a region near a desired position Zo, where the probability of finding the molecule would otherwise be extremely low, thus, enabling effective sampling near Zo. A holistic sample is generated from the results of a set of simulations with different Zo using the selfconsistent histogram method.12,13 These simulations are carried out with the harmonic force constant k = 100 kJ/mol·nm2. To pinpoint the molecular characteristic of PFOA that is responsible for its enhanced surface activity relative to OA, the following method is used to study the effects of specific molecular features. The terms in the potential energy of the surfactant (e.g., bond stretching, angle bending, Coulombic, etc.) are sequentially changed from OA-like to PFOA-like. The method we use is not the free energy perturbation method; rather we compare the free energies obtained with the method described above for these molecules with “hybrid potentials”. Through sequential changes in the terms that govern the bonded (bond stretching, angle bending, and dihedral) and nonbonded (Lennard-Jones and Coulombic) potentials, the effect of each term on the surface activity of the surfactant is identified. The total potential energy of the molecule is the sum of all potentials: Vtotal = Vbond + Vangle + Vdihedral + VLJ + Ve B

(5)


Article


ethanol) of the surface tension curve is highly negative but it becomes less negative as ethanol concentration increases. The surface tension data obtained for ethanol−water mixtures is in good agreement with literature.16,17 We next address the surface tension of ethanol−water solutions with added PFOA or OA. Example results for surface tension as a function of surfactant concentration are displayed in Figure 2. As surfactant concentration increases, the surface tension decreases. However, above a certain concentration of surfactant the surface tension becomes essentially independent of surfactant concentration. This concentration is the critical micelle concentration (CMC), above which micelles form in the solution. At high ethanol concentration, for example, 50 wt % ethanol, the surface tension continues to decrease slightly beyond the CMC-like point with additional PFOA. Similar observations have been made with surface tensions of binary alcohol−water solutions, where there was a marked changes in slope but there was a continued decrease in surface tension beyond the CMC-like point.18 We also note that the CMC increases with increasing ethanol content, in agreement with previous studies.19 The concentration of ethanol has very strong influence on the surface tension at the CMC as shown in Figure 3A. PFOA reduces the surface tension more strongly than OA. To compare the reduction in surface tension more clearly, the relative decrease in surface tension, from the surfactant-free value to the value at the CMC, is plotted in Figure 3B. The magnitude of the reduction in surface tension decreases as the ethanol concentration increases. PFOA remains effective at reducing the surface tension to significantly higher concentrations of ethanol than OA. Molecular Dynamics Simulation Results. We address the case of the surfactant at infinite dilution, such that the surfactant molecule interacts with only water and ethanol molecules. Our previous work demonstrated that at high concentrations of ethanol, OA ceases to partition to the surface.2 This result underlies the experimental observation that OA does not reduce surface tension of ethanol−water solutions with higher concentrations of ethanol. The molecular dynamics simulations are carried out to investigate the driving force partitioning OA and PFOA to the surface. The driving force corresponds to the difference in the free energy of the surfactant at the surface versus in the bulk. A surfactant has higher free energy in the bulk than at the surface, which is why it preferentially partitions to the surface. We determine the relative free energy profiles using the techniques described in the Methods section, which take as input the probability distribution of the position of the surfactant relative to the surface. For each set of conditions, five independent trials of 40 ns each were carried out, and the results were averaged. Figure 4 shows the position of a surfactant molecule as a function of time in two cases (the position of the surfactant molecule defined as the position of its carboxylic acid carbon). For PFOA surfactant in a solution of 75 wt % ethanol, the surfactant adequately samples the bulk, allowing us to directly calculate the free energy profile from this simulation. The data in Figure 4A lead to the probability distribution shown in Figure 5A. For PFOA in a solution of 70 wt % ethanol, the sampling was inadequate throughout the bulk, and umbrella sampling was required to obtain the free energy profile. The probability distributions obtained in the separate umbrella sampling simulations are shown in Figure 5B, with Zo = 0, 0.5, 1.0, 1.5, and 2.0 nm (relative to the center of the slab of liquid).

The bond stretching potential between two atoms is represented as a harmonic potential, Vbond: bonds

1 bond ki (ri − rio)2 2

∑

Vbond =

i

(6)

The angle bending potential formed by three adjacent atoms, Vangle, is also represented as a harmonic potential: angles

Vangle =

1 θ ki (θi − θio)2 2

∑ i

(7)

The dihedral potential energy function is dihedrals

Vdihedral =

5

∑ ∑ Cn[cos(φ − 180)]n i

n=0

(8)

The two nonbonded interactions, Lennard-Jones interactions and Coulombic interactions, are implemented as pairs

VLJ =

⎡

⎤ σ 12 σ6 ⎥ − 12 rij6 ⎥⎦ ⎣ rij

∑ 4ε⎢⎢ i,j

(9)

and pairs

Ve =

∑ i,j

1 qiqj 4πεo εr rij

(10)

Gromacs 4.5.6 is the molecular dynamics simulation package used in this study.14 Visual Molecular Dynamics (VMD) is used for visualization of the simulations.15

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RESULTS Experimental Results. The amphiphilic chemical structure of surfactants cause them to partition to the surface of an aqueous solution, which decreases the surface tension (γ). The ability of PFOA and OA to decrease surface tension in ethanol−water solutions is determined as a function of ethanol concentration. First we show the surface tension measurements of ethanol− water mixtures as a function of ethanol concentration in Figure 1. As the ethanol−water composition changes from pure water (γ = 72 mN/m) to pure ethanol (γ = 21 mN/m), the surface tension decreases nonlinearly. The initial slope (at 0 wt %

Figure 1. Surface tension of the ethanol−water solution as a function of ethanol wt %. The experimental data (▲) is compared with literature data: (−) Vazquez and (- -) Kahlweit.14,15 C


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Figure 2. Surface tension as a function of PFOA concentration in (A) 0 wt % ethanol plus PFOA and (B) 50 wt % ethanol plus PFOA. Generally, above the CMC the surface tension does not change with additional surfactant, as shown in (A). In (B), a relatively small surface tension decrease beyond the CMC was observed for solutions with 50 wt % ethanol. Note that plot (B) does not have a logarithmic x-axis.

magnitude of the free energy when the surfactant is in the bulk, near Z = 0, relative to the magnitude of the free energy when the surfactant is at the surface is defined as ΔGbulk/kT. The value of ΔGbulk/kT as a function of ethanol concentration is plotted in Figure 7. The results show that the driving force for partitioning to the surface is greater for PFOA than for OA. ΔGbulk/kT decreases as ethanol concentration increases, eventually becoming less than 1, in which case random thermal fluctuations (kT) dominate. The surfactant does not effectively partition to the surface at this stage. ΔGbulk/kT decreases below 1 at lower ethanol concentrations for OA than for PFOA. The simulation results in Figure 7 show that (1) PFOA partitions to the surface more than OA and that (2) PFOA is able to partition to the surface at higher ethanol concentrations than OA. Similarly, the experimental results shown in Figure 3B demonstrate that (1) PFOA reduces surface tension more than OA and that (2) PFOA is able to reduce the surface tension at higher ethanol concentrations than OA. We now describe results that address why PFOA is a more effective surfactant than OA at high ethanol concentration. Additional molecular dynamics simulations were designed to identify the molecular characteristics of PFOA that gave it higher ΔGbulk/kT than OA at high ethanol concentration. The parameters that describe a single component of the energy (Vbond, VLJ, Vangle, Vdihedral, Ve) are changed from OA-like to PFOA-like (or vice versa). Table 1 displays the sequence of these changes. After each change of an energy component, simulations are carried out to determine the free energy profile.

Figure 3. (A) Experimental surface tension at the CMC for various concentrations of ethanol−water solvents. (B) shows the data presented in (A) normalized to clearly show the effectiveness of each surfactant to decrease surface tension. Our experimental surface tension values of ethanol−water are plotted as open circles (o). OA and PFOA in ethanol−water at the CMC are shown as red squares and blue circles, respectively.

With the effects of the bias removed, an overall unbiased probability distribution is obtained as shown in Figure 5C. Umbrella sampling is required for PFOA in 0 wt % to 70 wt % ethanol simulations, and for OA in 0 wt % to 60 wt % ethanol simulations. The free energy profiles for the surfactant molecule as a function of position in the liquid is obtained from these probability distributions. The free energy profiles for the two example cases are shown in Figure 6. The difference in the

Figure 4. (A) Trajectory of PFOA at 75 wt % ethanol. The molecule travels to the bulk region frequently. (B) Trajectory of PFOA at 70 wt % ethanol. The molecule poorly samples the bulk region and requires umbrella sampling to obtain the free energy profile. D


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Figure 5. (A) Probability distribution for PFOA at 75 wt % ethanol. (B) Separate probability distribution of umbrella sampling simulations of PFOA at 70 wt % ethanol, with Zo = 0, 0.5, 1.0, 1.5, and 2.0 nm. (C) Overall probability distribution obtained from umbrella sampling results (B) after biases are removed.

Figure 6. (A) Free energy profile obtained without umbrella sampling for PFOA at 75 wt % ethanol. (B) Free energy profile obtained with umbrella sampling for PFOA at 70 wt % ethanol.

due to the near cancellation of the VLJ(σ) and VLJ(ε) contributions. Thus, the different behaviors of PFOA and OA are controlled by Vbond. The physical significance of the change in Vbond and VLJ(σ) component of the energy is that these potentials directly affect the girth of the molecule. C−F bond lengths are longer than the C−H bond lengths and fluorine atoms are significantly larger than hydrogen atoms, leading to the PFOA molecule being wider. Our simulations show that the greater girth causes the PFOA molecule to be more strongly partitioned to the surface than OA. Surprisingly, factors such as partial atomic charges, which are part of Coulombic interactions Ve, contribute little to the partitioning force.

Figure 7. Free energy in the bulk relative to that at the surface, for PFOA (blue circle) and OA (red square). Below ΔGbulk/kT = 1 (green dashed lines), random thermal fluctuations dominate.

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DISCUSSION AND CONCLUSION Octanoic acid and perfluorooctanoic acid are analogs of each other. Their chemical structures are very similar in that they are both eight carbons long and have a carboxylic acid functional group. The difference is that PFOA has its carbon tail fluorinated, whereas OA has its carbon tail hydrogenated. Because of this molecular difference, PFOA and OA behave differently in ethanol−water solvents. We have suggested that the driving force of a surfactant to the surface in the infinite dilution limit can predict the ability of the surfactant to decrease the surface tension of the liquid.2 In this work we bring further evidence to this idea with results for PFOA. The molecular dynamics simulations showed that PFOA partitions to the surface at higher concentration of

These simulations were carried out at 70 wt % ethanol, where PFOA significantly partitions to the surface, but OA does not. Figure 8 presents results for ΔGbulk/kT as the components of the energy change from OA-like to PFOA-like and vice versa. The results show that the most significant changes in ΔGbulk/ kT occur with the change of Vbond, VLJ(σ), and VLJ(ε); all other components of the energy have less significant effects on ΔGbulk/kT. The effects of Vbond and VLJ(σ) cause ΔGbulk/kT to be larger for PFOA than OA, while the effect of VLJ(ε) is in the opposite direction. These effects of changing Vbond, VLJ(σ), and VLJ(ε) occurs both as the molecule is changed from OA-like to PFOA-like as well as from PFOA-like to OA-like. The net effect of changing VLJ on the surfactant behavior is relatively small, E


Article

The Journal of Physical Chemistry C Table 1. Molecular Topologies of the Surfactant Used in the Sequence of Simulations are Shown Belowc Vtotal OA

OA OA OA V bond + VLJ(σ OA , εOA ) + V angle + V dihedral + V eOA

+ Vbond

PFOA OA OA Vbond + VLJ(σ OA , εOA ) + V angle + V dihedral + V eOA

+ VLJ diameter

PFOA OA OA Vbond + VLJ(σ PFOA , εOA ) + V angle + V dihedral + V eOA

+ VLJ energy

PFOA OA OA Vbond + VLJ(σ PFOA , ε PFOA ) + V angle + V dihedral + V eOA

+ Vangle

PFOA PFOA OA Vbond + VLJ(σ PFOA , ε PFOA ) + Vangle + V dihedral + V eOA

+ Vdihedral

PFOA PFOA PFOA Vbond + VLJ(σ PFOA , ε PFOA ) + Vangle + Vdihedral + V eOA

+ Ve

PFOA PFOA PFOA Vbond + VLJ(σ PFOA , ε PFOA ) + Vangle + Vdihedral + VePFOA

+ massa


PFOA


+ Vbond

OA PFOA PFOA V bond + VLJ(σ PFOA , ε PFOA ) + Vangle + Vdihedral + VePFOA

+ VLJ

OA PFOA PFOA PFOA V bond + VLJ(σ OA , ε PFOA ) + Vangle + Vangle + Vdihedral + VePFOA

+ VLJ energy

OA PFOA PFOA PFOA V bond + VLJ(σ OA , εOA ) + Vangle + Vangle + Vdihedral + VePFOA

+ Vangle

OA OA PFOA V bond + VLJ(σ OA , εOA ) + V angle + Vdihedral + VePFOA

+ Vdihedral

OA OA OA V bond + VLJ(σ OA , εOA ) + V angle + V dihedral + VePFOA

+ Ve


+ massb


The final swap results in a PFOA molecule. bThis final swap results in an OA molecule. cIn the first sequence beginning with OA, the OA parameters were swapped with PFOA parameters one by one: Vbond, VLJ(σ), VLJ(ε), Vangle, Vdihedral, Ve, and mass. In the second sequence, PFOA parameters are swapped with OA parameters one at a time. a

Figure 8. Free energy in the bulk relative to at the surface in the sequential change of terms in the potential energy of the surfactant from (A) OA to PFOA and (B) PFOA to OA.

minimize the disruption, surfactants partition to the surface where the disruption is minimal.20 This phenomenon extends to surfactants in ethanol−water solutions, where the density of hydrogen bonds decrease with increasing ethanol concentration. While water can form two hydrogen bonds per molecule, ethanol can only form one hydrogen bond per molecule. Thus, water can form a hydrogen bonded network while ethanol cannot, and the hydrogen bonded network is less dense when there is a significant amount of ethanol in the solution. Since the hydrophobic force

ethanol compared to OA, which coincides quantitatively with the experimental result that PFOA maintains its ability to decrease surface tension to higher concentration of ethanol than OA. We also address the specific characteristics of the fluorinated carbon tail that cause PFOA to be a more effective surfactant at higher ethanol concentrations than OA. Surfactants partition to the surface of water because they have hydrophobic portions in the molecule. In the bulk solution, these hydrophobic portions disrupt hydrogen bonding between water molecules. To F


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(10) Darden, T.; Perera, L.; Li, L.; Pederson, L. New Tricks for Modelers from the Crystallography Toolkit: the Particle Mesh Ewald Algorithm and Its Use in Nucleic Acid Simulations. Structure 1999, 7, R55−R60. (11) Torrie, G. M.; Valleau, J. P. Nonphysical Sampling Distributions in Monte Carlo Free-Energy Estimation: Umbrella Sampling. J. Comput. Phys. 1977, 23, 187−199. (12) Ferrenberg, A. M.; Swendsen, R. H. Optimized Monte Carlo Data Analysis. Phys. Rev. Lett. 1989, 63, 1195. (13) Li, P. C.; Makarov, D. E. Ubiquitin-Like Protein Domains Show High Resistance to Mechanical Unfolding Similar to that of the I27 Domain in Titin: Evidence from Simulations. J. Phys. Chem. B 2004, 108, 745−749. (14) Hess, B.; Kutzner, C.; Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435−447. (15) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33−38. (16) Kahlweit, M.; Busse, G.; Jen, J. Adsorption of Amphiphiles at Water/Air Interfaces. J. Phys. Chem. 1991, 95, 5580−5586. (17) Vázquez, G.; Alvarez, E.; Navaza, J. Surface Tension of Alcohol + Water from 20 to 50 °C. J. Chem. Eng. Data 1995, 40, 611−614. (18) Zana, R. Aqueous Surfactant-Alcohol Systems: A Review. Adv. Colloid Interface Sci. 1995, 57, 1−64. (19) Flockhart, B. D. The Critical Micelle Concentration of Sodium Dodecyl Sulfate in Ethanol-Water Mixtures. J. Colloid Sci. 1957, 12, 557−565. (20) Chandler, D. Interfaces and the Driving Force of Hydrophobic Assembly. Nature 2005, 437, 640−647. (21) Dalvi, V. H.; Rossky, P. J. Molecular Origins of Fluorocarbon Hydrophobicity. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 13603− 13607.

is due to the disruption of hydrogen bonds, the hydrophobic force decreases with increasing ethanol concentration. When the girth of the hydrophobic portion becomes larger, the surfactant disrupts more hydrogen bonds. We find that the girth of the hydrophobic part of a surfactant is the critical factor that determines how strongly the surfactant will partition to the surface and thus decrease the surface tension. Other molecular characteristics, such as charge distribution, contribute less to the surfactant partitioning force. Dalvi et al. also concluded that the hydrophobic force depends mostly on the size of the molecule, while studying alkanes and analogous perfluoroalkanes.21 In their work, they also examined the effects of the individual components of the energy. The main differences between their work and ours are that (1) they calculated the free energy change associated with solvating a single gaseous molecule into water; (2) they used thermodynamic integration to determine the free energy of solvation; and (3) they addressed behavior in pure water, while we address behavior in ethanol−water solvents as a function of ethanol concentration. Our method is more directly applicable to evaluating the driving force that pushes a surfactant molecule to the surface. Although our methods are different from Dalvi et al., the fundamental conclusions are congruent. Both studies find that the greater hydrophobicity in fluorocarbons relative to hydrocarbons is a geometric effect, due to the greater girth of the fluorocarbons.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant No. CBET-1159327, and the simulations were carried out using the computational resources of the Ohio Supercomputing Center.

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REFERENCES

(1) Abe, M. Synthesis and Applications of Surfactants Containing Fluorine. Curr. Opin. Colloid Interface Sci. 1999, 4, 354−356. (2) Htet, A.; Gil, P.; Lacks, D. Surface Activity of Octanoic Acid in Ethanol-Water Solutions From Molecular Simulation and Experiment. J. Chem. Phys. 2015, 142, 084702. (3) Pallas, N. R.; Pethica, B. A. The Surface Tension of Water. Colloids Surf. 1989, 36, 369−372. (4) Rolo, L. I.; Caço, A. I.; Queimada, A. J.; Marrucho, I. M.; Coutinho, J. A. P. Surface Tension of Heptane, Decane, Hexadecane, Eicosane, and Some of Their Binary Mixtures. J. Chem. Eng. Data 2002, 47, 1442−1445. (5) Rusanov, A. I.; Prokhorov, V. A. Interfacial Tensiometry; Elsevier: Amsterdam, Netherlands, 1996. (6) Jorgenson, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225−11236. (7) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269−6271. (8) Vega, C.; de Miguel, E. Surface Tension of the Most Popular Models of Water by Using the Test-Area Simulation Method. J. Chem. Phys. 2007, 126, 154707. (9) Bussi, G.; Parrinello, M. Stochastic Thermostats: Comparison of Local and Global Schemes. Comput. Phys. Commun. 2008, 179, 26−29. G


Geometric Origins of Surfactant Effectiveness in Mixed Solvent Systems

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