Interfacial Water Features at Air−Water Interfaces as Influenced byCharged SurfactantsVu N. T. Truong,† Xuming Wang,† Liem X. Dang,‡ and Jan D. Miller*,†

†Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, 135 South 1460 East, Rm 412,Salt Lake City, Utah 84112, United States‡Physical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, UnitedStates

ABSTRACT: The features of interfacial water at air−waterinterfaces of anionic sodium dodecyl sulfate (SDS) andcationic dodecyl amine hydrochloride (DDA) solutions wereexamined by combining sum frequency generation (SFG)vibrational spectroscopy measurements and molecular dynam-ics simulations (MDS). The SFG spectra revealed thatinterfacial water molecules for SDS solutions were highlyordered compared with those for DDA solutions. To elucidatethis observation, in addition to agreement with the literature in regards to the interfacial electric field at the interfaces, weinvestigated the features of interfacial water molecules with respect to their network and their interaction with surfactant headgroups. Our simulation analysis results revealed a higher number density, more strongly connected hydrogen bonding, and moreorderly oriented interfacial water molecules at the interface of the SDS solutions as compared to the DDA solutions. The goal ofthis research is to identify significant features of interfacial water for our improved understanding of such interfacialphenomena.

■ INTRODUCTION

Understanding the structure of interfacial water at chargedinterfaces has been considered to be of critical importance inmany industrial applications, such as in biology, colloidscience, electrochemistry, surfactant chemistry, and so on.The features and properties of water molecules have continuedto attract many research scholars in various fields in recentdecades. One of the very powerful experimental techniques tostudy the structure of interfacial waters is sum frequencygeneration (SFG) vibrational spectroscopy.1−7 Several well-known research groups have developed more advancedtechniques to study interfacial water, such as the phase-sensitive SFG by the Shen research group8,9 and theheterodyne-detected SFG by the Tahara researchgroup.2,10−12 Interestingly, Tahara et al. could determine theabsolute orientation of water molecules at the chargedinterfaces by the measurement of the complex second-ordersusceptibility χ(2). The positive Im χ(2) of the OH bands for theanionic sodium dodecyl sulfate (SDS) solution indicated thatwater molecules were oriented with their hydrogen pointingtoward the negatively charged sulfate head group, whereas thenegative Im χ(2) of the OH bands for the cetyltrimethylammonium bromide (CTAB) solution indicated that watermolecules were oriented with their hydrogen pointing awayfrom the positively charged head group.10 In recent years,Geiger and his research group developed a model to examinehow the SFG spectra may be affected by the presence ofadsorptive−dispersive interactions from a charged interface. Asreported, the features of interfacial water, as well as the

properties of charged interfaces, still remain unanswered issuesto be discovered.3

Anionic sodium dodecyl sulfate (SDS) and cationic dodecylammonium hydrochloride (DDA), well-known surfactants,have been widely used as detergents, as well as collectors in themineral flotation process. A number of research studies havebeen conducted on the adsorption behavior of SDS and DDAsurfactants, as well as the properties of surfactant sol-utions.13−16 Based on sum frequency vibrational spectroscopyexperiments, Richmond and co-workers have providedimportant information about how the molecular structures ofSDS and DDA alter the orientation of water molecules at theair−water interface.17 Nguyen et al. studied different effects ofsingle and binary surfactant systems of SDS and DDA on theiradsorption, packing, and water structures at the air−waterinterface.18 On the other hand, due to the dramatic increase incomputational resources, computer simulations allow for abetter understanding of the structural features and the behaviorof surfactants at the molecular level. Schweighofer and co-workers explored a molecular picture of SDS surfactantmolecules at the water−vapor and water−carbon tetrachlorideinterfaces. The molecular structure of the SDS surfactant wasreported for the head group and hydrocarbon chain locations,dihedral distribution, and head group−water radial distributionfunction.19 Recent developments in technology have provided

Received: February 7, 2019Published: February 15, 2019

Article

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promising tools that are being used to gain more under-standing about molecular interaction at air−water interfaces.Although a number of experimental and theoretical reports

confirm the interaction between surfactant and watermolecules at the interfaces, very limited research on thefundamental understanding of interfacial water at air−waterinterfaces at the molecular level has been reported. In thisstudy, by combining sum frequency generation (SFG)vibrational spectroscopy experiments with molecular dynamicssimulation (MDS) analysis, we investigated the features ofinterfacial water at the air−water interface of two typicalsurfactant solutions, specifically SDS and DDA solutions atdifferent concentrations.

■ MATERIALS AND METHODS

Materials. Sodium dodecyl sulfate (SDS) with purityhigher than 99% was purchased from Sigma-Aldrich. Dodecylammonium hydrochloride (DDA) was received from ACROSOrganics (99%). Potassium hydroxide (KOH) was purchasedfrom Mallinckodt (98%) and used as received. Purifieddeionized water with a resistivity of 18.2 MΩ·cm was obtainedfrom a Milli-Q system and used to prepare the surfactantsolutions in all experiments.In the SFG air−water interface experiments, the surfactant

solutions of appropriate concentrations and pH condition wereeach contained in a 25 mL glass cell. To clean the glass cellproperly, the cleaning was conducted with acetone andmethanol, followed by washing with deionized water, andthen drying with high-purity nitrogen gas.Sum Frequency Generation Vibrational Spectrosco-

py. An EKSPLA, Ltd. sum frequency generation spectrometerwas used for all SFG experiments. The laser system wasdescribed in a previous publication from our group.20 Theincident angles of the visible and IR beams were set as 60 and66° at the air−water interface, respectively. The angle of thereflected SFG beam was taken at 65°. The spectra werecollected in ssp polarization conditions (s, polarized sumfrequency; s, polarized visible; and p, polarized infrared) andwere normalized to the visible and IR energies. Each data pointwas collected at 4 cm−1 increments and is the average of 30laser shots.The SFG measurements of surfactant solutions at the air−

water interfaces were conducted at room temperature (∼23°C) to minimize evaporation. The experiments were designedto obtain the interfacial water structure in the region of 3000−3600 cm−1, as well as the hydrocarbon nonpolar groups ofadsorbed SDS and DDA at the solution surface in the range of2800−3000 cm−1.Molecular Dynamics Simulation. The Amber 14

program was used for the MDS study of SDS and DDA atthe air−water interfaces.21,22 The simulation system for theair−water interface consisted of 25 molecules of either SDS orDDA surfactant and 4322 water molecules in a primary waterbox of 4.0 × 4.0 × 8.0 nm3. A high coverage of surfactant at theair−water interface was examined for the investigation ofinterfacial water structures for a solution having a highsurfactant concentration.The simulations were employed in the periodic boundary

condition with a box size of 4.0 × 4.0 × 20.0 nm3 to providevacuum space between the two water slabs. Snapshots of theinitial simulation states for high coverage of SDS and DDAsolutions are shown in Figure 1.

In all simulation studies, the SPC/E water model23 was usedand the water molecules were kept internally rigid by using theSHAKE algorithm. The SDS and DDA molecules wereconstructed by Gaussview and followed by structuraloptimization in Gaussian before organization at the air−water interfaces. The general Amber force fields (GAFF)24 andAmber ff99SB force fields25 were applied for the SDS andDDA molecules. Some intermolecular potential parameters arepresented in Table 1.The air−water interface system was simulated in the NVT

ensemble where the number of particles (N), the simulationbox volume (V), and the temperature (T) were kept constant.

Figure 1. Snapshots of initial simulation states at the air−waterinterfaces; on the left are SDS molecules and on the right are DDAmolecules. The color code for atoms is as follows: red, O; white, H;cyan, C; yellow, S; blue, Na; purple, N; green, Cl.

Table 1. Intermolecular Potential Parameters for SDS,DDA, and SPC/E Water Molecules

atom type σ (Å) ε (kcal/mol) q

SDS c3 3.8160 0.1094 −0.1805hc 2.9740 0.0157 0.0423os 3.3674 0.1700 −0.4807s6 2.0000 0.2500 −0.0326o 3.3224 0.2100 −0.2162Na+ 2.5400 0.1000 0.6344

DDA c3 3.8160 0.1094 −0.2752hc 2.9740 0.0157 0.0603n4 3.6480 0.1700 −0.7652hn 1.2000 0.0157 0.3530Cl− 3.8960 0.2650 −1.0173

SPC/E OW 3.5534 0.1553 −0.8476HW 0.0000 0.0000 0.4238

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The temperature was kept at 298.0 K using an Andersonthermostat. The energy of the system was initially minimizedand equilibrated for 500 ps before the simulation. Simulationswere carried out with a 2 fs time step for a total time of 1.0 ns.The van der Waals and electrostatic interactions were cut off at9 Å, and the long-range electrostatic interactions werecalculated using Particle Mesh Ewald, a method available inthe Amber program.To analyze the MDS results of interfacial water molecules,

water number density profiles, hydrogen bonding, and waterdipole orientation were computed from the trajectorycoordinators of the water molecules. Fortran 90 languagewas used for coding the programs of interfacial water analysis.The information about interfacial water molecules at the air−water interface in the presence of anionic SDS and cationicDDA surfactants provides a better understanding of themolecular features and behavior of interfacial water molecules.

■ RESULTS AND DISCUSSIONSFG Analysis of Interfacial Water. The adsorption

features of anionic sodium dodecyl sulfate (SDS) and cationicdodecyl ammonium chloride (DDA) surfactants at the air−water interface were studied at different bulk concentrationsbelow the critical micelle concentration (CMC). Four solutionconcentrations of anionic SDS surfactant including 2 × 10−4, 5× 10−4, 1 × 10−3, and 5 × 10−3 M were investigated and theconcentrations of 5 × 10−4, 1 × 10−3, 5 × 10−3, and 1 × 10−2

M were studied for the cationic DDA surfactant. The SFGspectra collected at the air−water interface at neutral pH areshown in Figures 2 and 3 for SDS and DDA solutions,

respectively. Of particular interest in this study is the interfacialwater structure with respect to three typical characteristicpeaks for O−H vibrational modes including 3200, 3450, and3700 cm−1 as reported in previous SFG studies.16,26,27 OurSFG results of the interfacial water structure show twosignificant peaks at 3200 and 3450 cm−1 for the anionic SDSsolution, which are less prominent for the cationic DDAsolution. A strong enhancement of O−H vibrational signals isnoticed in Figure 2 with an increase in SDS concentration. Inparticular, the 3200 cm−1 peak, which represents the O−H

stretching of strongly hydrogen-bonded water molecules,dominates in comparison to the 3450 cm−1 peak ofasymmetrically bonded water molecules. It is evident that theinterfacial water molecules strongly interact with the sulfatehead groups at the air−water interface for the anionic SDSsolution. In addition, for the more concentrated SDS solution,a strong enhancement of O−H vibration in the region of3000−3600 cm−1 is found and suggests that interfacial watermolecules become highly ordered at the surface. This finding isin good agreement with other research work.17 Theobservation of the O−H stretching mode was elucidated bythe large electrostatic field at the interfaces. When increasingthe concentration of the SDS solution, the interface potentialwas found to increase both from theory and from experimentas reported in the literature.13,28,29 It was demonstrated thatthe interfacial water molecules were aligned by the electrostaticfield in the double layer region.17

Quite the opposite was found for the SFG spectra of theDDA solution. In contrast to the SDS results, the SFG spectraof O−H vibrational peaks in the water region (3000−3600cm−1) decrease with an increase in the concentration of thecationic DDA surfactant. Further, it can be seen in Figure 3that the peaks of the C−H vibrations for the hydrocarbonchain of the DDA molecules are very sharp and strongcompared to the spectra for the O−H vibrations of theinterfacial water molecules. These results indicate that theinteraction between interfacial water molecules and cationicammonium head groups is not as strong as in the case of theanionic SDS surfactant. As a result, the interfacial waternetwork does not play a significant role at the interface of theDDA surfactant solution. Furthermore, at the highest DDAconcentration studied (1 × 10−2 M), very sharp and high-intensity peaks of CH3- symmetric stretching and CH3- Fermiresonance at 2880 and 2940 cm−1, respectively, together withan N−H symmetric stretching peak of 3300 cm−1, wereobserved, but with very weak interfacial water signals. Thisconfirms that the surfactant molecules form a strong and densepacking structure at the surface and interfacial water moleculesare not easily accommodated at the interfaces. A similarobservation has been reported in the literature.27 Nguyen et al.studied the interfacial water structure in cetyltrimethyl

Figure 2. SFG spectra under SSP polarization conditions for the air−water interface of SDS solutions.

Figure 3. SFG spectra under SSP polarization conditions for the air−water interface of DDA solutions.

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ammonium bromide (CTAB) solution and found that atconcentrations above 0.1 mM, increasing the concentrationlowered the intensity of the O−H stretching peaks. Undersimilar conditions, most of our experiments at higherconcentrations exhibited a similar trend. The decrease in theintensity of O−H stretching modes was attributed to asignificant decrease of Debye screening length, leading to thereduced dipole moment of water molecules. In this study, inaddition to the strong agreement of the interfacial electric fieldexplained from the literature, we also investigated the featuresof interfacial water molecules with respect to their network andtheir interaction with the surfactant head groups for futureunderstanding at the molecular level.To elucidate the interfacial phenomena for SDS and DDA,

first the head group sizes of anionic SDS and cationic DDAmolecules were considered. The SDS and DDA moleculeswere evaluated using Gaussview 09 software. The geometryoptimization was executed and followed by a single-pointenergy calculation using the density functional theory (DFT)method in the Gaussian simulation program. Figure 4

illustrates the optimized SDS and DDA molecules. The bonddistance between the sulfur atom and the oxygen atoms in theSDS molecule and the bond distance between the nitrogenatom and the hydrogen atoms in the DDA molecule werecalculated and are listed in Table 2. Results show that the bond

distances of elemental atoms in the SDS group are significantlygreater than in the DDA group. In other words, the anionicSDS head group has a larger size than the cationic DDA headgroup. Therefore, at the air−water interface, the sulfate groupprovides more available space for the interfacial watermolecules and, together with the charging effect, theinteraction between the anionic sulfate group and theinterfacial water molecules is strongly enhanced.In addition, it was considered that the molecular packing of

the surfactant molecules at the air−water interface shouldimpact the population of interfacial water molecules. It wasreported that the molecular areas of anionic SDS and cationicDDA at critical micelle concentration (CMC) concentrations(8 mM for SDS and 14 mM for DDA) are 45 and 35 Å2,respectively.17,30 This means that for the same conditions, thenumber of cationic DDA molecules at the interface are moredensely packed than the anionic SDS molecules. Figure 5illustrates the molecular packing of the SDS and DDAmolecules at the air−water interface. Regardless of theorientation of the hydrocarbon chain for either SDS or

DDA, the hypothesis is proposed here that because of thedenser molecular packing of the DDA molecules, less watermolecules are present at the interface. Therefore, SFG signalsof O−H stretches decrease with an increase of DDA bulkconcentration. The proposed explanation is also supported bythe extremely sharp and strong intensity of CH3- symmetricstretching and the CH3- Fermi resonance peaks at highconcentrations of DDA, as shown in Figure 3. Furthermore,the MDS snapshots of the SDS and DDA interfacial surfactantmolecules after 1 ns of simulation time, presented in Figure 6,

are in good agreement with the hypothesis. It can be seen thatSDS molecules at the surface occupy a greater surface area permolecule, whereas DDA molecules occupy less surface area.On further consideration, the average distance between twoneighboring SDS head groups was computed and comparedwith the average distance between two neighboring DDA head

Figure 4. SDS and DDA molecules optimized by Gaussian simulation.

Table 2. Bond Distances between Atoms in SDS and DDAHead Groups

bond distance (Å)

S−O1 1.461S−O2 1.582N−H1 1.006N−H2 1.085

Figure 5. Illustration of molecular packing of SDS molecules andDDA molecules at the air−water interfaces. The color code for atomsis as follows: white, H; cyan, C; red, O; yellow, S; purple, Na; blue, N;green, Cl.

Figure 6. MDS snapshots of interfacial SDS and DDA molecules after1 ns of simulation time; on top is the organization of SDS moleculesand on the bottom is the organization of DDA molecules. The colorcode for atoms is as follows: red, O; white, H; cyan, C; yellow, S; blue,Na; purple, N; green, Cl.

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groups. It was found that the spacing was 5.944 Å for SDS and4.775 Å for DDA molecules. These observations provideadditional support to confirm the hypothesis that DDAsurfactant molecules are more densely packed at the air−water interface.It is evident that SFG experimental measurements of anionic

SDS and cationic DDA surfactants at the air−water interfaceprovide a fundamental understanding of the interfacial waterstructure and the interaction between interfacial watermolecules and surfactant head groups at the interface.MDS Analysis of Interfacial Water Structures at Air−

Water Interfaces. To gain a better understanding of thebehavior of interfacial water structure at the air−waterinterface for anionic SDS and cationic DDA surfactantsolutions, molecular dynamics simulations (MDS) wereemployed. Results in the following sections indicate that theinterfacial water structure agrees well with the SFGexperimental results, as discussed in the previous section. Itis noted that in the analysis of the MDS results, which arepresented as plots of interfacial water characteristics versusdistance in the following sections, the position of the air−waterinterface, shown as 0 value on the x-axis, was defined by theinitial uniform location of the surfactant head groups. Figure 7shows the organization of surfactant molecules at the air−water interfaces at the beginning of simulation and thedefinition of the position of the air−water interface.Relative Number Density Profile. The number density

profiles of interfacial water molecules at the air−water interfaceprovide a quantitative distribution analysis obtained from thetrajectory of water coordinates. In this study, the position of asingle water molecule is defined as the position of the center ofmass for that water molecule. The number of water moleculesin each bin of 0.5 Å was counted and normalized by thenumber of water molecules in the bulk phase.Figure 8 presents the density distribution of water molecules

at the interfaces with high coverage of SDS and DDAsurfactants. The density profiles are very similar to the reportedstudies of surfactants at the interfaces.19,31,32 Simulation resultsindicate that the interfacial water molecules are more crowdedat the interface for the SDS solution in comparison with theDDA solution. As for the SDS solution, the interfacial waterdensity was about 84% of the bulk water density, whereas itwas only 46% in the case of DDA solution. The bulk waterdensity was retrieved at 2 Å from the initial interface of theSDS solution and 6 Å from the initial interface of the DDA

solution. The results imply that the atomic structure ofsurfactant molecules affects the distribution of interfacial watermolecules, probably by the establishment of a hydrogen bondnetwork between interfacial water−surfactant head groups at

Figure 7. Initial arrangement of surfactant molecules at the air−water interfaces and the definition of the position of the air−water interface; on theleft is the organization of SDS molecules and on the right is the organization of DDA molecules. The color code for atoms is as follows: red, O;white, H; cyan, C; for SDS molecules: yellow, S; blue, Na; for DDA molecules: blue, N; green, Cl.

Figure 8. Top figure: Relative water number density profiles ofanionic SDS and cationic DDA surfactant solutions; on the x axis, thepositive value is the distance toward the water phase and the negativevalue is the distance toward the air phase. Bottom figure: on the left isthe MDS snapshot of SDS molecules and on the right is the MDSsnapshot of DDA molecules at the air−water interfaces. The colorcode for atoms is as follows: red, O; white, H; cyan, C; for SDSmolecules: yellow, S; blue, Na; for DDA molecules: blue, N; green,Cl. The initial interface is the same in both cases. See Figure 7.

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the interface and between interfacial water−water in theaqueous phase.In addition, it is worth pointing out from Figure 8 that water

molecules penetrate the surfactant head groups. Interfacialwater molecules have a more prominent distribution aroundthe anionic sulfate group than around the cationic ammoniumgroup. Only a few water molecules coordinate with theammonium group, whereas more water molecules areaccommodated by the sulfate group. The results show thatmore water molecules are expected at the SDS surface than atthe DDA surface. Hence, the SFG experimental results shownin Figures 2 and 3 are supported by MDS.Hydrogen Bonding Network. Other significant informa-

tion provided from the simulation results is the hydrogenbonding, including the hydrogen bonding between two watermolecules and the hydrogen bonding between oxygen in thesulfate group and the water molecules. Two water moleculesfor the SPC/E water model are defined as being hydrogenbonded if the distance between two oxygen atoms is less than3.5 Å and the O···O−H angle is less than 30°.33 The numberof hydrogen bonds per water molecule is determined bydividing the total number of hydrogen bonds in each waterlayer by its corresponding water number.Figure 9 presents the average number of hydrogen bonds per

water molecule as a function of the distance from the air−water interface. The simulation results show that the averagenumber of hydrogen bonds per water molecule in the bulkphase is about 3.42, in good agreement with simulation resultsreported for the SPC/E model of bulk water.34,35 Generally,water molecules at the interfaces and in the air phase are lesshydrogen bonded than water molecules in the bulk phase.Figure 9 also shows that the water molecules penetrated withinthe sulfate group are more strongly hydrogen bonded than thewater molecules around the ammonium group. Also, at theair−water interface, there are 3.20 hydrogen bonds per watermolecule for the SDS solution, but just 2.83 hydrogen bondsper water molecule for the DDA solution. Results in Figure 9support the hypothesis mentioned in the previous section ofSFG spectra results, that the interfacial water network at thesurface of the SDS solution is more strongly connected thanfor the DDA solution. This finding suggests that the interfacialstructure is dependent not only on the surfactant propertiesbut also on the structural properties of the interfacial water atthe interface. Hence, as expected, water−water hydrogenbonds are in large part responsible for determining interfacialwater structure.Water Dipole Orientation at the Interface. In this

study, water dipole orientation is defined by the angle αbetween the water dipole moment (oriented from negative topositive) and the surface normal. To compute the densitydistribution for water dipoles, the simulation box size wasdivided into 0.5 Å bins parallel to the interface. In each bin, theangle α of water molecules was measured, and the numberdensity distribution for different degrees was determined. It isnoted that the range of angle α is from 0 to 180°. Results ofwater dipole orientation analysis of interfacial water moleculesare shown in Figure 10. The results indicate that at the air−water interface for the SDS solution, the average water dipoleis pointing approximately 16° away from the surface normal.Figure 11 illustrates the orientation of water molecules with a16° orientation, which demonstrates a highly ordered behaviorof water molecules occurring at the interface. This finding is inexcellent agreement with the literature regarding the orienta-

tional flip-flop of water molecules at charged interfaces.2,10

Tahara et al. applied the multiplex heterodyne-detected SFG tomeasure the second-order nonlinear susceptibility (χ(2))spectra for OH bands of interfacial water molecules. Theobtained χ(2) spectra suggested that interfacial water moleculesorient with their hydrogen up toward the negative charge ofthe sulfate head group, which agrees with our simulationresults. As for the DDA solution, it was noticed from ourresults that the water dipole distribution was randomlydistributed in the range of 0° to 180°. This observation,which is consistent with our results of relative density profilesand hydrogen bonding analysis, demonstrates the reliability ofthe SFG experimental results, which suggest the interfacial“ice-like” water structure at the interface for the SDS solutionwhen compared to the DDA solution.

■ CONCLUSIONSThe features of the interfacial water molecules for anionic SDSand cationic DDA surfactant solutions were investigated at theair−water interface. The SFG experimental results revealedthat interfacial water molecules strongly interacted with theanionic sulfate groups at the interface, whereas the interactionbetween the cationic ammonium groups with interfacial water

Figure 9. Top figure: Number of hydrogen bonds per water moleculeof SDS and DDA solutions; on the x-axis, the positive value is thedistance toward the water phase and the negative value is the distancetoward the air phase. Bottom figure: on the left is the MDS snapshotof SDS molecules and on the right is the MDS snapshot of DDAmolecules at the air−water interfaces. The color code for atoms is asfollows: red, O; white, H; cyan, C; for SDS molecules: yellow, S; blue,Na; for DDA molecules: blue, N; green, Cl. The initial interface is thesame in both cases. See Figure 7.

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molecules was not as strong as with the sulfate group. Morehighly ordered water molecules were accommodated at thesurface of the SDS solution than at the surface of the DDAsolution. This observation was confirmed by the analysis ofMDS results, including water number density profile, hydrogenbonding network, and water dipole orientation results. Ourstudy potentially demonstrates that the combination of SFGmeasurements with MDS simulations provides an excellentanalysis to gain a better understanding of interfacial waterfeatures at air−water interfaces. It is expected that the resultsfrom this study will provide a better understanding of the wide

range of surfactant applications in which the features ofinterfacial water are critically important, such as in the area offlotation technology.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jan.miller@utah.edu. Telephone: (+1)801-581-5160.ORCIDLiem X. Dang: 0000-0003-4878-2200Jan D. Miller: 0000-0003-4889-1108NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was funded by the Division of Chemical Sciences,Geosciences, and Biosciences, Office of Basic Energy Sciences(BES) of the US Department of Energy (DOE) through GrantNo. DE-FG03-93ER14315. Part of the computationalprocedure was carried out using computer resources providedby the Division of Chemical Sciences, Geosciences, andBiosciences, BES, of the DOE at the Pacific NorthwestNational Laboratory. The authors thank Dorrie Spurlock forher assistance in the preparation of the manuscript.

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Figure 10. Water dipole moment relative density distribution alongthe interface (toward the air phase) at the air−water interface of theSDS solution (top figure) and the DDA solution (bottom figure).

Figure 11. Illustration for the average water dipole orientation fromthe surface normal for the SDS solution surface.

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The Journal of Physical Chemistry B Article

DOI: 10.1021/acs.jpcb.9b01246J. Phys. Chem. B 2019, 123, 2397−2404

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