Effect of Surfactant Shape on Solvophobicity and Surface Activity in Alcohol-Water Systems

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1 Effect of Surfactant Shape on Solvophobicity and Surface Activity in Alcohol-Water Systems Phwey S. Gil, Daniel J. Lacks Department of Chemical and Biomolecular Engineering Case Western Reserve University Cleveland, OH Abstract Here we study the relationship between a surfactant s molecular shape and its tendency to partition to the interface in ethanol-water mixtures. In general, finding surfactants that are effective in alcohol-water mixtures is more challenging than finding ones that are effective in pure water. This is because the solvophobic effect that partitions surfactants from bulk solution to the interface becomes weaker as ethanol concentration increases. We use experiments and molecular dynamics to observe the effects of increasing surfactant tail length or width. The results show that increasing surfactant tail length causes the surfactant to partition to the surface better in low ethanol concentrations, but not at high ethanol concentrations. In comparison, increasing surfactant tail width causes the surfactant to partition to the surface better at higher concentrations of ethanol. We examine the liquid structure to elucidate the mechanisms that weaken the partitioning effect as ethanol concentration increases. Ethanol-water mixtures are nanoscopically heterogeneous with protic and aprotic regions in the bulk solution. We see that the surfactant tail is most likely to be solvated in the aprotic regions where it perturbs fewer hydrogen bonds. Introduction Surfactants are molecules that decrease the surface tension between two distinct phases (e.g. water and air or water and oil). They are amphiphilic molecules that have a polar head group and a non-polar tail group. 1

2 The standard picture of how surfactants decrease surface tension is that water attracts the hydrophilic head group while repelling the hydrophobic tail group, partitioning surfactants to the interface between distinct phases. Then the surfactants decrease the surface tension by disrupting attractive forces at the interface. In water, surfactant surface activity is directly related to the hydrophobic effect. Studying surfactants in alcohol-water presents an interesting challenge. Most surfactants are less surface active when there is a high concentration of alcohol in solution. From a practical standpoint, we can deduce that the surfactant tail is less repelled by alcohol molecules compared to water molecules, hence the reduced surfactant surface activity. Certain classes of surfactants maintain their effectiveness in both water and alcohol-water systems. For example, fluorosurfactants and silicone surfactants are more effective at lowering surface tension of alcohol-water than hydrocarbon surfactants. 1 3 Therefore, it is our interest to discover molecular-level characteristics of these surfactants that enable them to be effective in alcohol-water systems. In particular, we are interested in the relationship between solute geometry and solute surface activity. We concluded in our previous work that geometric factors are accountable for the augmented surface activity of fluorosurfactants over hydrocarbon surfactants in alcohol-water environments. 2 We investigated perfluorooctanoic acid (PFOA) and octanoic acid (OA). The two surfactants have analogous structures, and differ only in that the hydrogen atoms along the carbon chain in OA are replaced by fluorine atoms in PFOA. Experiments show that PFOA can reduce surface tension at high ethanol concentrations where OA is unable to reduce the surface tension. Molecular dynamics simulations elucidate these experimental results, and show that PFOA has greater surface activity due to the carbonfluorine bond being longer than the carbon-hydrogen bond. Similar conclusions are also reported by Dalvi et al, 4 who compared the free energy of hydration of alkanes and fluoroalkanes and found that fluoroalkanes have higher free energy of hydration than alkanes due to the larger width of molecules. It is simply this size effect, rather than effects associated with atomic charges or other interatomic forces, that leads to the differences in surfactant surface activity. 2

3 Zygmunt et al. also studied the hydration free energy of perfluoroalcohols and hydrocarbon analogs in water. 5 They found that fluorinating methylene groups adjacent to the alcohol functional group increased the hydration free energy more than fluorinating methylene groups further down the carbon tail. They concluded that the fluorination sterically affects the alcohol group and decreases the average number of hydrogen bonds that the alcohol group can form. The novelty of the present work is the isolation of the separate effects on surfactant activity coming from molecule length and width. To address the effects of molecular length, we examine analogous surfactants that differ only in the length of the hydrocarbon chain. The molecules studied in the simulations are shown in Fig. 1a: butyric acid (which we denote C4), octanoic acid (C8), dodecanoic acid (C12) and hexadodecanoic acid (C16). In our experiments, we study C4 and C8. To address the effects of molecular width, we exploit the capability of molecular simulations to address model molecules in which we can vary one parameter at will to determine the consequences of this one parameter. Note that the isolation of the effect of molecular width is not possible in experiments, as a modification to make the molecule wider, such as methylation or fluorination, will also lead to changes in the intermolecular van der Waals and Coulombic forces that could also have consequences on surface activity. We study modifications of the C8 molecule shown in Fig. 1b, where the carbon-hydrogen bond length has been decreased to 0.9Å (C8 ), increased to 1.332Å (C8 + ), and increased to 1.5 Å (C8 ++ ); in addition we study a modification of C8 molecule where the hydrogen atoms have been replaced with methyl groups (C8m). We also consider how the solvent structure influences the surfactant surface activity. The structure of alcohol-water mixtures is nontrivial. Although ethanol-water solutions are fully miscible, it is known that the excess entropy is largely negative. 6 Experiments using NMR, fluorescence probe, and Raman spectroscopy techniques have explained the negative excess entropy by demonstrating that alcohols selfassociate in aqueous solutions. 7 9 A combination of neutron diffraction and molecular dynamics studies on concentrated methanol-water mixtures revealed that most water molecules hydrogen bond with each other in clusters and strings. 10 More recently, molecular dynamics studies have shown that hydrogen 3

4 bonded networks in ethanol-water mixtures form rings, and can be fully percolating at low ethanol concentrations but non-percolating at high ethanol concentrations. 14,15 Our goal in this work is to relate surfactant surface activity in ethanol-water mixtures to these complex solvent structural effects. Methods Molecular dynamics is a computer simulation method for studying the physical motions of atoms and molecules in a system. The trajectory of the particles is numerically solved for following Newtonian laws of motion. Molecular dynamics force fields define the potential energies of the particles and the forces between the particles. The trajectory of the system is analyzed to determine microscopic and macroscopic properties of the system. Here we use molecular dynamics to simulate linear surfactants in ethanol-water solutions at the liquidvapor interface. To create the liquid-vapor interface, we model the ethanol-water liquid phase in a slab geometry. Periodic boundary conditions apply in all directions. In the z-direction, the lattice parameter is set to a large value so that the molecules cannot fill up the entire cell. Instead, the molecules form a slab that is infinite in the x- and y-dimensions, but finite in the z-direction. This slab creates top and bottom surfaces oriented perpendicular to the z-direction. The large value of the lattice parameter in the z- direction makes interactions between periodic images of the slab negligible. The simulations contain approximately 23,500 atoms inside a simulation cell of size 6 x 6 x 15 nm 3 (the precise number of atoms depends on the composition of the system). This system results in liquid slabs that are about 6 nm thick, which is twice the length of the longest surfactant molecule studied. The simulations are carried out with an NVT ensemble. Temperature is maintained at 300 K using a velocity-rescaling thermostat with time constant of 0.1 ps. 16 The simulations are run for 20 million steps with a time-step of 2 fs for a total of 40 ns. The OPLS-AA force field is used for the ethanol and the surfactant molecules, except the C8m molecule in which the methyl group is modeled with the OPLS-UA 4

5 force field. 17 The SPC/E force field is used for water molecules; 18 previous work has shown that the SPC/E force field better models the surface tension of water better than the TIP3P, TIP4P, and SPC force fields. 19 The Particle Mesh-Ewald method is used to evaluate the Coulombic interactions, with a cutoff distance of 1 nm for the real-space sum. 20 Non-Coulombic interactions are summed to a cutoff distance of 2 nm. Bond constraints are used to keep bond lengths constant, as our previous work showed that the use of bond constraints did not affect results for the surface activity of surfactants. 2 Gromacs software is used for the molecular dynamics simulations, and Avogadro and Visual Molecular Dynamics software are used for visualization We obtain the free energy profile of surfactants with respect to the surface of liquids, at the infinite dilution limit. A single surfactant molecule is inserted to the slab of ethanol-water. The molecular dynamics trajectory is used to determine the probability that the surfactant is at position, denoted, where the position of the surfactant molecule is defined as the position of the carboxylic acid carbon. The center of the liquid slab is taken to be 0, and there are surfaces at both positive and negative due to the symmetry of the system. We average the results from positive and negative to obtain better statistics. The free energy profile, denoted, can be determined, in principle, from, (1) where is Boltzmann s constant, is temperature, and is a constant. In practice, Eq. 1 can be used directly only in cases where the surfactant naturally samples all relevant values of during the trajectory; we find this case occurs at high ethanol concentrations. When the surfactant does not sample all values of z during the simulation, the direct use of Eq. 1 is not possible. This situation occurs at low ethanol concentrations, where the surfactant remains at the surface and does not sample states within the bulk. In this case we use umbrella sampling 24,25 to introduce a bias to force the molecule into the bulk, and the effects of the bias are removed in the analysis stage. For each 5

6 state point five simulations are carried out where the system is biased to different positions with respect to the liquid surface, and a force constant of 100 kj/mol nm 2 is used for the biasing potential. The umbrella sampling methodology is described more fully in our previous works. 2,26 In addition to the simulations described above, we carry out bulk simulations (i.e. without surfaces) to determine the radial distribution functions (RDF) of the surfactants in solution. The NPT ensemble is used for these simulations. Berendsen isotropic pressure coupling is used with a reference pressure of 1 atm and a time constant of 2 ps. 27 The number of atoms in these simulations is approximately 23500, corresponding to simulation cell sizes of approximately 6 x 6 x 6 nm 3. We also carry out experiments to measure the surface tension of water-ethanol solutions with C4, C8 and PFOA surfactants. The Wilhelmy plate method is utilized, which has the advantage that the measurement is independent of solution density. 28,29 The Kruss K100 Tensiometer is used to take the measurements. The surface tension as a function of surfactant concentration is obtained as follows. First, water and ethanol are mixed to a desired concentration. Then, a large amount of surfactant (2-4 g) is added to 5 g of the solution. After mixing thoroughly, the surface tension is measured at room temperature (20-23 C). The solution is diluted with more solvent (ethanol and water) and another measurement is made. A series of dilution steps is repeated to obtain the surface tension of the mixture over the desired range of surfactant concentrations. The critical micelle concentration is found as the surfactant concentration above which there is little change in the surface tension with increasing surfactant concentration. Results Experimental results We carried out surface tension experiments to determine the effectiveness of C4, C8 and PFOA surfactants in reducing the surface tension of ethanol-water mixtures as a function of ethanol concentration. Fig. 2a shows results for the surface tension for water-ethanol mixtures in the absence of 6

7 surfactant, as well as with surfactant at the critical micelle concentration. Fig. 2b shows the reduction in surface tension due to the addition of surfactant, as a function of ethanol concentration. We address chain length effects by comparing results for C4 and C8. The results demonstrate that there is no significant difference in the surfactant effectiveness between C4 and C8. In contrast, our previous experiments showed that PFOA is more effective at reducing the surface tension at high ethanol concentrations, and our analysis showed this effect is a geometric effect due to the width of PFOA being greater than that of C8. 2 Simulation results surfactant free energy We use molecular dynamics to calculate the free energy difference between the bulk and the interface. This quantity allows us to compare the surface activity of the surfactants using simulations. First, we show in Fig. 3 examples of results for the free energy of the surfactant as a function of its position with respect to the liquid surface. The results in 0 wt% ethanol required umbrella sampling, but the results in 70 wt% ethanol were obtained from unbiased molecular dynamics simulations. The minimum free energy of the surfactant is at the interface. The free energy profiles are normalized to the minimum free energy at the surface, around 2.5 nm. We define the free energy difference between the bulk and the interface as G. Then, / describes the ratio of the magnitude of the free energy driving force pushing the surfactant to the interface relative to the thermal energy. When the thermal energy is greater than the free energy driving force (i.e. / 1), the free energy driving force becomes insignificant. In this case the molecule lacks substantial affinity for the interface, and thus the surfactant would not act to reduce the surface tension. A comparison of Fig. 3a and Fig. 3b shows that / depends strongly on ethanol concentration. We calculated free energy profiles for the surfactants in various ethanol concentrations. Fig. 4 shows / as a function of ethanol concentration for each surfactant. Fig. 4a focuses on the effects of surfactant length, while Fig. 4b focuses on the effects of surfactant width. 7

8 We first examine the effects of chain length, by comparing the behavior of C4, C8, C12 and C16. In pure water, increasing the length of the surfactant from C4 to C16 increases the magnitude of / rather significantly (see Fig. 4a). The nonlinearity with increasing chain length could be due to statistical uncertainty in the simulation. As ethanol concentration increases, the value of / for all of these surfactants decreases, and at 70 wt% ethanol where / <1 regardless of chain length. The nonlinear behavior with increasing ethanol concentration is likely due to the changes in the liquid structure (addressed below). At 70 wt% ethanol the value of / is smallest for C4, and it increases with chain length to C8; however, further increases in chain length do not have a significant effect on /. We conducted a two-sample t-test for C4, C8, C12, and C16 surfactants and found statistically significant difference between C4 and other surfactants (95% confidence interval of [-1.867, ]). The differences in / between C8, C12, and C16 were not found to be statistically significant. Thus, our results show that increasing the chain length is not a viable strategy to design more effective surfactants at high ethanol concentrations. On the other hand, increasing surfactant width enhances the surface activity at high ethanol concentrations. As described above, we examine three artificial analogs of C8, where the C-H bonds have been artificially shortened or lengthened (C8, C8 +, C8 ++ ). Our results (Fig. 4b) show that the magnitude of / is a strong function of molecule width at high ethanol concentration. Although C8 + and C8 ++ have smaller molecular volume and surface area than C16, they nonetheless have greater values of / at high ethanol concentrations, as it is only the width and not the length that is relevant to surfactant activity. To study the effect of molecular width without artificially changing a molecular parameter, we examine an analog of C8 where the surfactant tail H atoms have been replaced by methyl groups (C8m). / for C8m is greater than that of C8, as expected because of its greater width. We note that the value of / for C8m is less than or equal to that for C8 + and C8 ++, even though C8m is significantly wider. C8m is wider than C8, but it also has stronger non-bonded attraction; i.e., the Lennard-Jones ε 8

9 parameter is larger for a methyl group than for a hydrogen atom. We have previously shown that increased non-bonded attraction leads to decreased / at high ethanol content. Thus this energetic effect cancels some of the width effect in C8m, causing the value of / for C8m to be slightly less than or equal to that of C8 + and C8 ++. Simulation results liquid structure We analyze the liquid structure in order to better understand the surfactant free energy results presented above. First, we address the structure of water-ethanol mixtures in the absence of surfactant. Although water and alcohol are fully miscible, in mixtures water and alcohol molecules have the tendency to cluster at the molecular scale, as supported by thermodynamic, experimental, and simulation evidence. 6,7,9,10,30 Fig. 5a shows radial distribution functions (RDF) for water molecules in 38 wt% and 70 wt% ethanol. The first peak is larger in 70 wt% ethanol than in 38 wt% ethanol. Similar differences with concentration are observed in the ethanol-ethanol and ethanol-water RDFs, also shown in Fig. 5a. These results show that increasing the ethanol concentration increases the short range order in the system. We plot the ratio of the water-water and ethanol-water RDFs in Fig. 5b. For distances less than approximately 0.8 nm, the water-water RDF is greater than the water-ethanol RDF, which implies that there are waterrich clusters with diameters of approximately 1.5 nm. Ethanol molecules are found to be generally oriented such that the oxygen end faces the water-rich regions. This result is evident in Fig. 5a (right-most plots), where the oxygen head of ethanol is closer to the water regions than the carbon tail. This result shows the strong preference of polar moieties to interact with each other. Thus, we demonstrate that in ethanol-water mixture there exist protic and aprotic regions, where the protic region includes hydrogen bonds and the aprotic region does not include hydrogen bonds. The protic regions are composed of water molecules, with heads of ethanol molecules at the boundaries. 9

10 To illustrate, we show a snapshot of the molecules in a bulk ethanol-water mixture in Fig. 6a. The protic and aprotic regions discussed above are evident i.e., there are water-rich clusters and the oxygen atoms of the ethanol molecules are found at the borders of these regions. Fig. 6b shows that hydrogen bonds are heterogeneously distributed through the system, as the network of hydrogen bonds extends only through the protic regions. In comparison, as shown in a snapshot of pure water in Fig. 6b, pure water has homogeneously distributed hydrogen bonds. The heterogeneous solvent structure will affect the configuration of the surfactant in the solution. Fig. 7a shows the RDFs between the surfactant tail atoms and the solvent molecules. The first peak (r=0.5 nm) corresponds to interactions with ethanol carbon atoms, and the second peak (r=0.6 nm) to interactions with ethanol oxygens; note that interactions with water molecules show no peak (only depletion). This data implies that the surfactant tail is surrounded by ethanol molecules oriented such that their oxygen end points away from the surfactant tail. Fig. 7b shows the RDFs between the surfactant head group and the solvent molecules. At r=0.25 nm there is a tall peak for interactions with water oxygens and a short peak for interactions with ethanol oxygens. This result shows that the surfactant head group forms hydrogen bonds with the polar components in the solvent, with a stronger preference to hydrogen bond to water than to ethanol. This stronger preference occurs even at high ethanol concentration. The depletion of water molecules is due to the ethanol molecules that surround the surfactant tail (discussed above). From the RDF results we conclude that the surfactant tail is surrounded mainly by ethanol molecules oriented with their carbon tail towards the surfactant, and that the surfactant head is surrounded mainly by water molecules but also by some ethanol molecules oriented with their oxygen head towards the surfactant. Fig. 8 shows snapshots from the simulation, in which this structure is evident. While this structure is in some ways reminiscent of the suggestion long ago of clathrate or iceberg structures surrounding hydrophobic solutes in water, 31 we emphasize that the structure we identify is non-rigid and transient, 32 and all types of atoms are significantly sampled in the region adjacent to the surfactant (see Fig. 7). 10

11 Discussion The hydrophobic effect in pure water has been widely studied, and our results can be analyzed in this context. The hydrophobic effect can have entropic or enthalpic origins, depending on the situation. The entropic mechanism is identified when the free energy of hydration, ΔG f, increases with increasing temperature; the enthalpic mechanism is identified when ΔG f decreases with increasing temperature. For macroscopic interfaces between hydrophobic and water phases, and for large hydrophobic solutes (> 1 nm) in water, the enthalpic mechanism is observed. For small solutes (< 1 nm), the entropic mechanism occurs at lower temperatures and the enthalpic mechanism occurs at higher temperatures. The molecular level origins of both the enthalpic and entropic effects involve hydrogen bonding in the water. 33 For macroscopic interfaces and around large solutes, the water molecules near the interface cannot maintain their full complement of two hydrogen bonds per molecule, and the dangling hydrogen bonds which create an enthalpic penalty for the hydration. For small solutes, in contrast, the water molecules can maintain their full complement of hydrogen bonds while wrapping around the solute. In fact, Raman spectroscopy shows that hydrogen bonds around a small solute are stronger than hydrogen bonds in bulk water, 34 and molecular dynamics shows that water molecules around a small solute are more highly ordered (though still dynamic) than in the bulk. 35 While the hydrogen bonds are stronger and more ordered near the small solute, the presence of the solute reduces the number of possible configurations of water, which creates an entropic penalty for the hydration. For non-spherical molecules, such as the surfactants addressed here, the smallest length scale is the most relevant. Single-molecule force-spectroscopy studies of long polymers, which varied the polymer width by changing the monomer pendant groups, showed that increased molecular polymer causes a decrease in the temperature at which the hydrophobic mechanism changes from entropic to enthalpic. 36 Molecular dynamics simulations showed that the structural order of the water molecules surrounding a solute is essentially independent of the length of the solute. 35 The width, rather than the length, matters most 11

12 because the water molecules can most readily maintain its hydrogen bonded network around the width of the molecule. Our results in alcohol-water systems concur with these ideas for the hydrophobic effect in pure water. As discussed above, we find that molecular width, rather than molecular length, is the important factor for making a surfactant effective in alcohol-water systems. This observation is understandable in terms of these previous ideas, in that it is only the solute width that controls whether the solvent molecules can maintain a complete hydrogen bonded network around the solute. The presence of alcohol changes the liquid structure, which in turn affects the magnitude of the hydrophobic effect; these changes can also be understood in terms of the above ideas derived in the context of water. In pure water, hydrogen bonds have hydrogen bond networks that percolate throughout the system. 37 But adding ethanol creates nanometer-sized clusters of water and ethanol molecules. The non-polar regions, which are devoid of hydrogen bonds, do not occur in pure water see Fig. 6b in comparison to Fig. 6d. Previous works show that high ethanol concentrations cause the network of hydrogen bonded water molecules to no longer percolate throughout the system, and leads to discrete water clusters that get smaller with more ethanol in the system. 14,15 We have shown above that the surfactant tail is surrounded mostly by the hydrocarbon tails of ethanol molecules. Thus the surfactant tail will reside in the non-polar regions of the liquid, where it does not perturb the hydrogen bonding network of this liquid. Since the hydrophobic effect is due to solute perturbation of the hydrogen bonding of the solvent, the fact that the surfactant can reside in the liquid without significantly perturbing the hydrogen bonded structure makes the magnitude of hydrophobic effect smaller as ethanol concentration increases. 12

13 Conclusion We investigated the effect of surfactant tail geometry on the molecule s solvophobicity in ethanol-water systems. We calculated the free energy difference between the surfactant molecule at the surface and in the bulk, which determines the propensity of the surfactant molecule to reside at the surface. Longer tail groups increase the free energy difference significantly at low ethanol concentrations, but not significantly at high ethanol concentrations. Wider tail groups increase the free energy difference at all ethanol concentrations. By analyzing radial distribution functions, we determined that ethanol causes the solvent to organize into protic and aprotic regions, which enables surfactant molecules to perturb fewer hydrogen bonds while residing in the bulk liquid, which in turn explains why the surface activity of surfactants decreases as ethanol concentration increases. Acknowledgements This work was supported by the National Science Foundation Grant No. CBET , and the simulations were carried out using the computational resources of the Ohio Supercomputing Center. 13

14 A B Figure 1. Molecular structures of the surfactants addressed in this work. (A) From top to bottom: C4, C8, C12, and C16 surfactants. (B) From left to right: C8, C8, C8 +, C8 ++, where the C-H bond length is arbitrarily varied to determine the effect of this particular parameter on surfactant behavior; at far right, C8m surfactant in which the pendant H atoms are replaced by methyl groups. 14

15 A B Figure 2. Experimental surface tension values. (A) surface tension at the CMC vs ethanol concentration. (B) normalized surface tension. Ethanol-water with no surfactant (Black ); C4 (Pink ); C8 (Blue ); PFOA (Red ). The error bars show 95% confidence intervals. 15

16 A B Figure 3. Free energy profiles of octanoic acid in (A) pure water and (B) 70 wt% ethanol solution. 16

17 A B Figure 4. / versus ethanol concentration for the different surfactants. (A) Surfactants with varying molecular length. (B) Surfactants with varying molecular width. The inset in B shows / versus C-H bond length at 70 wt% ethanol. C4 (Pink ); C8 (Blue ); C12 (Teal ); C16 (Orange ); C8 + (Red ); C8m (Purple. The red dotted line (--) denotes / =1. 17

18 A B C Figure 5. Radial distribution functions for water oxygen atoms (O w ) and ethanol oxygen atoms (O E ). Top row is at 38 wt% ethanol, and bottom row is at 70 wt% ethanol. (A) From left to right: RDF for O w -O w ; O E -O E ; O E -O w. The red dotted line is for ethanol methyl carbon to water oxygen interactions, (CH 3 ) E -O w. (B) Ratio of RDFs for O w -O w to O E -O w. (C) Ratio of RDFs for O E -O E to O W -O E 18

19 A B C D Figure 6. Snapshot of molecule arrangements. (A, B) bulk solution of 70 wt% ethanol. (C, D) bulk solution of pure water. Blue atoms are water, gray atoms are ethanol tail groups, and red atoms are ethanol head groups. Water-water hydrogen bonds are blue, water-alcohol hydrogen bonds are black, and alcohol-alcohol hydrogen bonds are red. 19

20 A B Figure 7. Radial distribution functions for the C8 surfactant molecule, in bulk liquid. Top row is at 38 wt% ethanol, and bottom row is at 70 wt% ethanol. (A) Results for the surfactant tail. RDF from surfactant carbon atoms to water s oxygen atoms (Black Line); RDF from surfactant carbon atoms to ethanol carbon atoms (Red Line); RDF from surfactant carbon atoms to ethanol oxygen atoms (Blue line). (B) Results for the surfactant head. RDF from surfactant oxygen atoms to water s oxygen atoms (Black line); RDF from surfactant oxygen atoms to ethanol carbon atoms (Red Line); RDF from surfactant oxygen atoms to ethanol oxygen atoms (Blue Line). 20

21 A B Figure 8. Snapshots from the trajectory. (A) Molecules around the surfactant tail; the tail is highlighted green. (B) Molecules around the surfactant head; the surfactant tail is highlighted in white. The carboxylic acid carbon is highlighted green, the oxygen atoms orange, and the hydrogen atoms white. The ethanol molecules are shown in blue, and water molecules in red. The oxygen atoms of ethanol are highlighted purple to show the orientation of these molecules. The hydrogen bond is shown with a red dotted line. 21

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