Investigation of Mixed Surfactant Films at Water Surface Using

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Investigation of mixed surfactant films at water surface using molecular dynamics simulations Alena Habartova, Martina Roeselova, and Lukasz Cwiklik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02854 • Publication Date (Web): 06 Oct 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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Investigation of mixed surfactant films at water surface using molecular dynamics simulations

Alena Habartová,1 M. Roeselová,1 and Lukasz Cwiklik2*

1

Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,

Flemingovo nám. 2, 16610, Prague 6, Czech Republic 2

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,

v.v.i., Dolejškova 3, 18223 Prague 8, Czech Republic *corresponding author, e-mail: [email protected]

Abstract Multi-component Langmuir monolayers are important models of organic coatings of naturally occurring water-vapor interfaces such as the surfaces of oceans or aerosol particles. We investigated mixed monolayers comprised of palmitic acid, C15H31COOH (PA) and 1-bromoalkanes of different chain length (C5, C10, and C16) at the air-water interface employing classical molecular dynamics simulations. Different composition ratios and lateral compression of the monolayers were considered. The structural parameters, such as density profiles, and deuterium order parameter, evaluated as functions of composition and the lateral film packing, provide microscopic information about 1

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organization and dynamics of the mixed monolayers. Simulations demonstrate that stable and well mixed monolayers are formed by the mixtures of PA and BrC16, whereas the two considered shorter bromoalkanes, BrC5 and BrC10, do not form stable films. This is in accord with earlier experimental studies. Under high lateral pressures, in PA/BrC10 mixed systems molecules of the bromoalkane readily flip in the monolayer and subsequently leave the film, while the molecules of the longer BrC16 are expelled from the PA film but no flipping occurs. These results suggest that the film collapse under pressure is preceded by squeezing-out of bromoalkanes from the PA monolayer.

Introduction Organic films coat oceans, lakes as well as aqueous atmospheric aerosols surfaces1, 2 and affect a wide range of physical and chemical processes, from water evaporation and condensation to light scattering and heterogeneous chemistry.3, 4 Langmuir monolayers (LM) of organic surfactants at the air-water interface are useful model systems for studying these important processes under controlled conditions. Because a huge number of organic compounds are emitted and present in the environment, the naturally occurring surfaces have a complex composition. Therefore, the focus in both experimental and computational studies has shift from mono- to multi-component LMs recently. Palmitic acid, C15H31COOH (PA) is the most prevalent saturated fatty acid found in marine aerosols.1,

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Together with other fatty acids, it is released primarily during the decomposition of

phospholipid cellular membranes of marine organisms, mostly phytoplankton.4 It accumulates at the ocean surface and is then transferred to the sea-spray aerosols. Palmitic acid is also ubiquitous in inland atmospheric aerosols originating from decomposition of terrestrial higher plant waxes as well as from anthropogenic sources such as combustion of fossil fuels or cooking.5 Haloalkanes (halons) are another 2


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class of surfactants present in atmospheric aerosols and recognized to be serious atmospheric and environmental pollutants.6,

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In particular, bromine-containing haloalkanes present in atmospheric

aerosols are a source of stratospheric bromine participating in ozone destruction.8 In multi-component organic coatings of atmospheric aerosol particles, interactions between individual film components can potentially alter film properties such as its stability and chemical reactivity. Recently, incorporation of haloalkanes into fatty acid monolayers has been studied to assess how the haloalkanes incorporation is affected by the length of the alkyl chain, presence of bromine, composition, and lateral compression.9 These experiments have shown a dependence of bromoalkane incorporation on the molecule chain length. Namely, the chain length of at least fifteen carbons is necessary for the bromoalkane incorporation into a PA monolayer. Moreover, when lateral pressure is applied in such a mixed monolayer, haloalkane molecules are squeezed out from the film and form aggregates on the top of the PA monolayer. These observations are particularly important from the point of view of mixed films formation and their subsequent restructuring/degradation upon varying surface pressure which occurs during aerosol particle ageing in the atmosphere. As the experimental methods employed in the previous study do not provide detailed insight into molecular-level properties of the films, a full atomistic explanation regarding molecular-level basis of the observed bromoalkanes incorporation into PA monolayers is missing. Molecular dynamics (MD) simulations are a convenient method for studying molecular-level properties of condensed systems, including surface films.10, 11 In particular, MD was shown to be useful for investigation of lipid and fatty acid monolayers in various contexts, ranging from biology to atmospheric physical chemistry.12, 13, 14, 15, 16 Such properties as incorporation of molecules into a film, monolayer compression-decompression, as well as squeezing-out of monolayer components can be studied at atomistic level. Still, systematic MD studies of multi-component lipid monolayers are very 3


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scarce. In the present study, we employed MD simulations to investigate the molecular-level basis of bromoalkane incorporation into PA monolayers. In particular, we focused on the issue of the role of the bromoalkane molecule structure, namely the chain length and the presence of the polar group, in its stability into the monolayers of various compositions.

Methodology The simulated system consisted of 6770 water molecules in a central slab geometry and 232 surfactant molecules forming a Langmuir monolayer of 116 molecules at each of the two water-vapor interfaces (Fig. 1A). The Langmuir monolayer was a two-component mixture of palmitic acid C15H31COOH (PA) and either 1-bromopentane BrC5H11 (BrC5), 1-bromodecane BrC10H21 (BrC10), or 1-bromohexadecane BrC16H31 (BrC16) (see Fig. 1B). These mixed surfactant films were created from equilibrated pure PA monolayer by a random replacement of one quarter, one half, or three quarters of PA molecules by the 1-bromoalkanes yielding a ratio PA to desired bromoalkane component of 3:1, 1:1, or 1:3. Each of the investigated systems was placed in a rectangular simulation box with x-, y-, and zdimension of 5.4 nm, 5.4 nm, and 25 nm, respectively, the z-axis being parallel with the normal to the water-vapor interface. This conditions result in LM area per lipid (APL), counted per sum of PA and bromoalkane molecules, of 0.25 nm2. To mimic various compression stages, a semi-isotropic lateral pressure was applied to either reduce the two lateral dimensions to 4.8 nm or increase them to 6.0 nm, yielding APL of 0.20 or 0.31 nm2, respectively. The thickness of the liquid water slab varied, depending on compression stage between 6 nm (the relaxed stage) and 9 nm (the compressed stage). Periodic boundary conditions were applied in all three dimensions.

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Fig. 1. (A) Simulation box containing water slab in the center and mixed monolayer of palmitic acid (in cyan) and long chain bromoalkanes (in ochre) on the air/water interface . (B) Molecules of palmitic acid (PA) were randomly mixed with bromohexadecane (BrC16, bromodecane (BrC10), or bromopentane (BrC5) at several ratios.

The system was propagated for 100 ns in an isochoric-isothermal (NVT) ensemble at T = 293 K using the leap-frog integrator with the time step of 2 fs. Note that the NVT ensemble in the case of monolayer simulations is known for slow convergence.12 We have verified that a good the convergence was achieved in the considered systems by assuring that the lateral properties of the two monolayers present in the simulation box do not differ. System configurations were saved at 2 ps intervals.17 The temperature was controlled by the Nose-Hoover thermostat with the coupling constant of 1 ps.18 The cut-off distance of 1.0 nm was employed for both Lennard-Jones interactions and the short-range part of the Coulomb interactions. The neighbor list was updated every 10 integration steps. The long-range part of Coulomb interactions was evaluated using the Particle-Mesh Ewald method19 with a relative

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tolerance of 10-5, fourth order cubic interpolation, and a Fourier spacing parameter of 0.12. All bonds were constrained using the LINCS algorithm.20 The OPLS force field21 was employed to model PA and bromoalkane molecules using RESP charges22, 23 (force field parameters are provided in the Supporting Information). For derivation of the RESP charges, the Hartree-Fock method with cc-pVTZ basis set was employed. These electronic structure calculations were performed using the Gaussian software package.24 The force field was tested against the original OPLS parameterization as well as experimental data (see the Supporting Information). Overall, RESP charges give less polarized C-H bonds; also the conformational flexibility of the headgroup region of palmitic acid is increased. The SPCE model was employed for water.25 All molecular dynamics calculations were performed using the GROMACS (version 4.5.5) software package26. All trajectories were propagated for 100 ns. The first 90 ns of each trajectory were excluded from the analysis to allow for equilibration and only the last 10 ns were used for analysis. As equilibration criterion, convergence of density profiles and lateral components of the pressure tensor were employed. Analysis of the trajectories was performed using standard GROMACS tools26 and the VMD program27 was employed for visualization. In simulations of high lateral compression, the unit cells with APL=0.20 nm2 were used as starting configurations and then lateral pressure of up to 350 bar was applied using the Berendsen barostat.28

Results and Discussion Final snapshots of 100 ns MD trajectories of mixed palmitic acids-bromoalkanes monolayers at APL = 0.25 nm2 are presented in Fig. 2. This value of APL corresponds to the onset of the non-zero surface pressure at the experimental surface pressure-area isotherm of pure palmitic acid. Note that the surface 6


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pressure obtained in MD simulations in this isotherm region is negative (see Supporting Information); lowering of the surface pressure is a well-known artifact often occurring in simulations (see Ref. 12 for a comprehensive discussion of this issue). Nevertheless, the trends in surface pressure changes observed here with changing monolayer composition follow those obtained in experiments.9 As evidenced from the presented snapshots, in all cases the mixtures exist in the form of a stable film covering the water-vapor interface. Significant differences between the systems occur depending of the identity of the haloalkane admixed with the palmitic acid. In the case of BrC5, i.e., the haloalkane with the shortest chain among those considered ones, a non-ideal mixing leading to separation of PA and BrC5 is observed (Fig. 2, left column). Namely, at all considered PA to BrC5 number ratios, regions of the interface covered exclusively by either PA or BrC5 are present. When PA is the dominant component (PA:BrC5 = 3:1), the air-water interface is relatively well covered by the surfactants, while in the cases of more prevalent presence of BrC5 in the surface film (1:1 and 1:3), water pores occur in the monolayer. This behavior demonstrates that BrC5 on its own is not able to cover well the regions of the water-air interface which it occupies. The snapshots show that at all compositions BrC5 molecules are orientationally disordered, as demonstrated by the presence of different orientations of the bromine atom with respect to the interface.

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Fig. 2. Visualization of the final snapshots of trajectories (top and side view) for different film compositions at APL=0.25 nm2. Color coding: PA in cyan, bromoalkanes in orange, bromines as red beads.

In the case of BrC10 (Fig. 2, middle column), a relatively good mixing of PA and the bromoalkane occurs whilst PA is the major component (PA:BrC10 = 3:1). Molecules of the bromoalkane are 8


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oriented, in general, in parallel to the molecules of palmitic acid and together they form a surface film. Still, some of BrC10 molecules are flipped with their bromine atoms directed toward the gas phase. At the PA:BrC10 ratio equal to 1:1, this flipping is more pronounced (with approximately 10% of BrC10 molecules flipped) and a separation between PA and BrC10 molecules is visible. When the bromoalkane is the major component (PA:BrC10 = 1:3), its molecules are highly orientationally disordered with some of them even relocated atop of the film. Lateral separation between PA and bromoalkanes becomes more pronounced and water pores are formed in the film. A qualitatively different behavior is observed in the system containing BrC16. Namely, regardless of the bromoalkane to PA ratio, molecules of BrC16 readily incorporate in-between the PA and together form a relatively well ordered monolayers at the air-water interface. All BrC16 molecules are oriented with their Br atom toward the water phase. No lateral separation between BrC16 and PA is observed in the course of the simulation.

Fig. 3. Density profile along the normal to the air-water interface (zero is set in the middle of the box, only one interface is shown for clarity) for APL=0.25 nm2. All profiles are normalized such that their maximum values are equal to one.

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The structural features of the considered lipid films which were qualitatively described above based on the simulation snapshots, can be quantified by means of density profiles. In Fig. 3, density profiles of selected atoms along the normal to the water-air interface are depicted for the 1:1 bromoalkane to PA ratio. Only the data for 1:1 mixtures are presented because both the observed effects are the strongest in the equimolar mixture and this was also the primary system reported in the previous experimental study.9 Regardless of the type of bromoalkane present, molecules of PA form a well-structured monolayer, as evidenced by completely separated density peaks of PA heads (the C1 atoms) and tails (the C16 atoms). In all cases, hydrophilic PA headgroups are directed toward the water phase whereas hydrophobic tails are oriented toward the gas phase. In this respect, the behavior of PA in the mixed system with bromoalkanes reminds that observed in pure PA monolayers.9 Regarding the behavior of bromoalkanes, it differs significantly with the varying length of the carbon chain. In the case of BrC5, molecules are accumulated at the water-vapor interface, as evidenced by the presence of the density profiles with maximum at 4 nm in Fig. 3A. The overlap of the peaks of Br and the terminal carbon of BrC5 demonstrates that the molecules are orientationally disordered. The density profiles of the BrC10/PA mixture (Fig. 3B) reveal that molecules of the bromoalkane are incorporated relatively well in the PA monolayer, as the peaks of Br and the terminal carbon are well separated. The distance between these peaks is lower than that between the end atoms of PA; this is because of the shorter chain length of BrC10 compared to that of PA. While most of the bromoalkane chains are oriented with their Br atoms toward the water phase, there is a non-negligible population of BrC10 flipped, i.e., with Br atoms oriented toward the gas phase. This is in full agreement with the conclusions based on visual analysis of the corresponding snapshots (see Fig. 2 middle column where both types of BrC10 orientations are visible in the 1:1 mixture). In the case of BrC16, the haloalkane molecules behave virtually the same as PA. Namely, two well separated peaks mostly overlapping with those of PA are 10


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present; hence, in terms of orientation, a good incorporation between the two types of molecules occurs. The distances between the peaks in the bromoalkane and PA are comparable; this is due to a similar length of both molecules. All BrC16 molecules have their Br atoms directed toward water and the terminal carbon toward the gas phase. Note that the peak of the terminal carbon of the hydrophobic tail of BrC16 is somewhat more distant from the water phase than that of PA; this suggests a more extended conformation of the BrC16 than that of PA.

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Fig. 4. Three-dimensional radial distribution functions calculated between in the PA/BrC16 system at APL=0.25 nm2for selected atoms of film components and oxygen atoms of water (a) and between Br atoms of BrC16 and both hydrogen and oxygen atoms of water (b).

Further structural information regarding incorporation of BrC16 in the PA monolayer can be obtained from radial distribution functions (RDFs). It is, in particular, interesting what is the local three-dimensional arrangement of water molecules hydrating individual atoms of surfactant headgroups. In Fig. 4, 3D RDFs calculated between selected atoms of both PA and BrC16 and oxygen atoms of water in the 1:1 PA/BrC16 monolayer presented. Significant binding between hydrogen atom (H1) of the hydroxyl moiety of the carboxyl group with water oxygen (i.e., hydrogen bond donated to water oxygen) is clear based on the presence of the peak at r = 1.8 nm. The peaks between the oxygen atoms of the carboxyl group (O1, O2) arise from the hydrogen bond acceptance from water. The different shape of the RDF curves of O1 and O2 originates from their different chemical identity (see Fig. 1). 12


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Namely, orientation of O2 that belongs to the hydroxyl group is more rigid because of the hydrogen bonds formed by hydroxyl hydrogen atom whereas orientation of the carbonyl O1 is somewhat more flexible as demonstrated by the presence of two maxima at r = 0.28 and 0.31 nm. Regarding orientation of the bromine atom of the BrC16 at the interface, the RDFs presented in Fig. 4B demonstrate that molecules of water are in close proximity of Br atoms (starting from ~0.2 nm) with their hydrogen atoms preferentially oriented toward bromine.

Fig. 5. Two-dimensional radial distribution functions calculated between center of masses of monolayer components in the PA/BrC16 systems at APL=0.25 nm2 with varying PA to BrC10 ratio (see Figs. S4 and S5 in the Supporting Information for the PA/BrC5 and PA/BrC16 data). The full and dashed lines represent the data calculated for the two monolayers present in the simulation box.

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Lateral structure of monolayers can be analyzed based on two-dimensional RDFs calculated between individual monolayer components. In Fig. 5, 2D RDFs calculated between center of masses of PA/BrC16 monolayer components with varying PA to BrC10 ratio at APL=0.25 nm2 are presented (the data for PA/BrC5 and PA/BrC16 are shown in Figs. S4 and S5 in the Supporting Information). In the case of PA/BrC5 (Fig. S5), at each considered PA to BrC5 ratio, there is no lateral order observed between BrC5 molecules as well as between BrC5 and PA. In contrast, there is well visible structure of the 2D RDFs calculated between PA molecules. This result demonstrates that BrC5 is disordered in the monolayer whereas molecules of PA form relatively well ordered pools. This is in accord with the observation based on the simulation snapshots that was discussed beforehand. In the case of Br/C10 systems (Fig. 5), the two-dimensional ordering depends on monolayer composition. At the PA to BrC10 ratio equal to both 3:1 and 1:1, well-structured mixed monolayers are formed, as can be deduced from the well-structured RDFs calculated between all types of components. This behavior of RDFs proves that in these systems BrC10 can incorporate between PA molecules. In contrast, in the monolayer with PA:BrC10=1:3, i.e., with prevailing BrC10, the relatively high lateral order between PA molecules is preserved whereas BrC10 is laterally disordered. In the case of PA/BrC16 mixed monolayers (Fig. S5), at all Br to C16 ratios, well laterally-oriented systems are formed at the water-air interface. It supports the above-mentioned observation, based on simulation snapshots, that BrC16 be very efficiently mixed into PA monolayers. The influence of the presence of admixed bromoalkanes on the structural properties of PA molecules can be evaluated by means of the deuterium order parameter. In Fig. 6, PA order parameter calculated for the pure PA monolayer and 1:1 mixed systems at APL = 0.25 nm2 is presented. Regarding carbon C2 which is located in the proximity of the hydrophilic PA headgroup, its order parameter is nearly intact by the presence of both BrC5 and BrC16. In the first case, this is because 14


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BrC5 resides predominantly in the middle part of the surface film (see Fig. 3) and hence does not influence the PA headgroup region. Concerning BrC16, it does not affect the C2 order parameter because this bromoalkane incorporates well in the PA monolayer and does not distort the neighboring PA chains. Similarly, in the data concerning the remaining PA carbons, namely C3-C15, order parameter in the presence of BrC16 is almost unchanged in comparison with that of pure PA monolayer. By contrast, both BrC5 and BrC10 affect order parameter in this region of PA. Namely, PA order parameters of C5-C13 are significantly increased in the presence of these two bromoalkanes. This effect is due to lateral separation of both BrC5 and BrC10 from PA molecules (see Fig. 2) leading to a reduction of the available surface area for PA and hence its increased lateral compression. A reduction of the order parameter by BrC10 at the carbons C2 and C15 originates from the presence of Br atoms in the vicinity of those carbons. These Br atoms originate either from regularly oriented or flipped BrC15 molecules, the former affecting carbon C2 and the latter interacting with the carbon C15 of PA.

Fig. 6. Lipid order parameters along carbon chain of palmitic acid calculated in pure PA monolayer (dashed line) and PA mixed with bromoalkanes (solid lines) at APL=0.25 nm2.

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In order to explore the conditions experienced by the considered films at various surface pressures we simulated the considered surfactant films under different lateral packing conditions, corresponding to APL equal to 0.20 and 0.31 nm2. Based on the experimental PA isotherm this region refers to the tilted condensed state of the PA monolayer.9 In Fig. 7, final simulation snapshots of such mixed PA/BrCx (1:1) systems are presented. The APL = 0.20 nm2 at the PA isotherm corresponds to pressure close to the monolayer collapse. At this high surface packing (Fig. 7A), BrC5 molecules are laterally separated from their PA counterparts. Also, the molecules of BrC5 are orientationally disordered and some of them are squeezed out of the monolayer and reside as admolecules at the surface of the lipid film. In the case of BrC10, no lateral separation between the film components is observed. Despite of the increased surfactant density (and hence decreased freedom for molecular reorientations), there is still a significant population of BrC10 molecules with their Br atoms oriented toward the gas phase. Some of molecules of BrC10 are squeezed out of the monolayer and reside at the top of the film but this is to a significantly lower extent than in the case of the shorter BrC5 chains. In the case of BrC16, all bromoalkane molecules are well incorporated into the surface mixture and no squeezing out was observed. As surfactant molecules are more stretched at this surface packing, the monolayer thickness measured as the distance between the peaks of the density profiles of terminal PA carbon atoms increases to 2 nm. While discussing the highly laterally packed conditions, it should be noted that MD simulations described here are unable to reproduce quantitatively monolayer collapse as this phenomenon requires much longer lateral sizes (such as those accessible in coarse grained MD simulations) than the system sizes considered in the present atomistic-level MD studies.13, 29 Rather, the simulation results obtained for high lateral compression presented above should thus be treated as a qualitative description of monolayer behavior close to the film collapse.

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Regarding the less packed conditions, APL = 0.31 nm2 corresponds to lipids in the two dimensional gas phase at the PA isotherm. At such a low surface packing (Fig. 7B), in the case of the two shorter bromoalkanes, BrC5 and BrC10 (Fig. 7B, middle columns) formation of laterally separated patches of PA and the bromoalkane was observed in the course of simulation. Moreover, under these conditions the surface coverage by the organic molecules was not complete as evidenced by formation of patches of uncovered water. On the other hand, the mixed PA/BrC16 system (Fig. 7B, right column) stays stable with the film thickness of 1.1 nm. Similarly to the case of high lateral pressures because of relatively small system sizes, the process of monolayer breaking and formation of water pores observed in MD simulations should be treated as a qualitative description of the surface film behavior at low pressures. Note that formation of water pores under low surfactant densities may be promoted by simulation artifacts.16

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Fig. 7. Final snapshots (top and side view) of laterally compressed at APL=0.20 nm2 (A) and relaxed at APL=0.31 nm2 (B) mixtures of 1:1 PA/BrCn.

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Fig. 8. Simulation snapshots (top and side view) of the systems with BrC10 (A) and BrC16 (B) squeezed out of the PA film (lateral pressures of 100 and 350 bar were used for BrC10 and BrC16 system, accordingly).

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Fig. 9. Density profiles of the systems with BrC10 (A) and BrC16 (B) squeezed out of the PA film. All profiles are normalized such that their maximum values are equal to one (lateral pressures of 100 and 350 bar were used for BrC10 and BrC16 system, accordingly).

To gain additional insight into the experimentally reported process of squeezing out of selected surface film component close to monolayer collapse, we performed additional MD simulations employing severely increased lateral pressure. Namely, simulations with lateral pressure up to 350 bar were performed for the systems containing BrC10 and BrC16. Note that such high values of pressure at the length- and timescale of simulations resulted here in similar linear compression of the monolayer as that used in experiments.9 Nevertheless, one should be aware that high pressures may also lead to some artifacts in the obtained compressed structures. The system including BrC5 was not considered here as BrC5 molecules were observed to be efficiently squeezed out of the monolayer already at the 20


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simulation with APL = 0.20 nm2, as described above. In the case of the PA/BrC10 mixture, the lateral pressure of 100 bar caused squeezing out of bromoalkane molecules. This is evidenced by both simulation snapshot and density profiles in Figs. 8 and 9. The second peak and the Br atoms density profile correspond to the population of bromodecane molecules expelled from the monolayer. The snapshot in Fig. 8 shows a typical conformation of the system upon squeezing out the bromodecane where molecules of BrC10 form a relatively disordered cluster residing at the interface between the chains of PA and air. In the case of BrC16, the lateral pressure required for expulsion of the bromoalkane from the monolayer was equal to 350 bar. The process of expulsion was qualitatively the same as in the case of the shorted BrC10, as evidenced by simulation snapshots and density profiles in Figs. 7 and 8. In the case of both BrC5 and BrC10, the higher stability of bromoalkane in the surface film can be rationalized by a higher PA-PA stabilization due to van der Waals interaction between the PA chain atoms. In the BrC16/PA mixed films, the higher PA stability in the film arises due to higher stabilization of the strongly hydrophilic carboxylic moiety of PA than that of the Br atom at the water/lipid interface. These results of the high-lateral pressure simulations indicate that at high lateral compression of the mixed PA/bromoalkanes an initial squeezing out of the bromoalkane occurs prior to monolayer collapse. As already noted, the simulated lipid film under high lateral pressures cannot properly reproduce the experimentally observed macroscopic monolayer collapse and hence should be treated only as a qualitative model of the monolayer close to the collapse conditions. Note, however, that the observed process of squeezing-out the less polar component of the mixed surface film is in accord with previous simulation studies of mixed lipid films under high lateral pressures30, 31 as well as in accord with experimental results.9

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Conclusions We have characterized main structural features of mixed palmitic acids-bromoalkanes films at the airwater interface employing atomistic molecular dynamics simulations. Additionally, we have addressed the issue of the mixed film stability under high surface pressures. At ambient pressure, a stable and well mixed monolayer is formed by the mixture of PA and BrC16 at several compositions studied. In contrast , the two considered shorter bromoalkanes, BrC5 and BrC10, do not form stable films with PA, as lateral demixing of the film components occur. The shorter BrC5 completely laterally separates from PA forming disordered domains in the middle of the film; it is also able to easily diffuse out of the monolayer and in part cover the PA- air interface. The longer BrC10 partially laterally demixes from PA and its molecules flip attaining two conformations with Br atoms preferentially oriented in the direction of either water or the gas phase. These results are in accord with earlier experiments which demonstrated a stable character of PA/BrC16 mixed monolayers, in contrast to the mixtures containing the two shorter bromoalkanes. Regarding stability under high lateral pressure, in PA/BrC10 mixed systems molecules of the bromoalkane readily flip in the monolayer and leave the film, while the molecules of the longer BrC16 are also expelled from the PA film, however no flipping occurs. Overall, we have thus gained detailed molecular-level knowledge about the onset of processes characteristic for the experimental behavior of the mixed PA/bromoalkanes monolayers under high lateral compression. Namely the simulations suggest that the process of film collapse is preceded by squeezing-out of bromoalkanes from the PA monolayer.

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Supporting Information Force field parameters of palmitic acid, BrC5, BrC10, and BrC16. Force field verification. Surface tension and surface pressure values. Two-dimensional RDFs for PA+Br5 and PA+Br16 monolayers. The Supporting Information is available free of charge on the ACS Publications website.

The authors declare no competing financial interest.

Acknowledgments This paper is dedicated to the memory of late Dr. Martina Roeselova. A.H. and L.C. acknowledge Prof. Pavel Jungwirth for discussions and useful comments. This work was supported by grant 13-06181S from the Czech Science Foundation.

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TOC Graphic

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Investigation of Mixed Surfactant Films at Water Surface Using

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