Emerging investigator series: exploring the surface properties of aqueous aerosols coated with mixed surfactants

Junyao Li , Siyang Li , Shumin Cheng , Narcisse T. Tsona and Lin Du *
Environment Research Institute, Shandong University, Binhai Road 72, Qingdao, 266237, China. E-mail: lindu@sdu.edu.cn; Tel: +86-532-58631980

Received 9th September 2018 , Accepted 15th October 2018

First published on 16th October 2018


Mixed Langmuir monolayers of cholesterol with both saturated and unsaturated fatty acids, stearic acid (SA), and oleic acid (OA) spread at the air–seawater surface were studied. The phase behavior, molecular interaction, and conformational order of the monolayers were investigated by surface pressure–area (π–A) isotherms and infrared reflection-absorption spectroscopy (IRRAS) measurements. The thermodynamic parameters of the mixed films, including excess molecular area and excess Gibbs free energy were calculated by using the isotherm data. The interaction between SA (or OA) and cholesterol varied with the molar fraction of the fatty acids and surface pressure. OA/chol monolayers showed the characteristics of miscibility, but they acted as nonideal systems. Cholesterol has been observed to have a stabilizing effect on OA monolayers. The negative values of the excess Gibbs free energy in the entire composition range demonstrated that mixed OA/chol monolayers were thermodynamically stable. IRRAS spectra showed that mixing with cholesterol changes the ordering of fatty acid monolayers at the air–seawater surface. The findings provide general information regarding the structural changes in the monolayer induced by lateral packing. These results help in the understanding of the mixing behavior of fatty acids and cholesterol and provide insights into the fate of the mixed-monolayer-coated sea salt aerosol in the ocean environment.



Environmental significance

This manuscript provides comprehensive information regarding the mixing behaviors of different surface-active species at the air–seawater interface, which has a significant impact on the surface characteristics of aqueous aerosols from ocean. The current work is important for understanding the fate and transport of aqueous aerosols coated by mixed surfactants from the ocean to the atmosphere, air–sea exchange of contaminants, and their link with the climate change.

1. Introduction

Oceans, which account for about 70% of the Earth's surface, provide reactive surfaces for the adsorption of organics, air–sea exchange, production of aerosols, as well as other chemical and photochemical reactions.1–4 Large amounts of naturally produced aerosols are derived from the exchange between the ocean and the atmosphere,5 which are interlinked via the exchange of chemical species, including evaporation, decomposition, and sea spray process.6 Sea salt is the most widely distributed natural source of aerosols over the oceans.7 Sea salt aerosols (SSAs) have been thoroughly and systematically investigated for their chemical complexity,8 size-dependent changes in their composition and properties,9 organic surfactants on them,10 and their implications on the climate.1,4,6

Recent studies have revealed that these particles may contain a large fraction of organic matter due to the high biological activity in seawater. The organic fraction dominates and contributes 63% to the submicrometer aerosol mass during bloom periods upon the measurements of the physical and chemical properties of aerosols collected in the northeast Atlantic marine air arriving at the Mace Head Atmospheric Research Station.11 Field observations combined with mass spectrometry studies also showed evidence that some organic coatings, particularly fatty acids, do exist on marine aerosols.12,13 As compared to other more soluble organic compounds, fatty acids are more likely to exist at the air–seawater interface.2 The surface-active species present at the air–sea interface could be more efficiently transferred into the aerosol phase relative to other surface inactive species.14 The sea–air exchange of trace metals, bacteria, gas-phase species, and transport of surface-active organic components from seawater to the atmosphere have aroused considerable interest in several research groups.3,15 The organic coatings on the aerosols produced by the sea that have significant climate-relevant effects, such as the cloud condensation nuclei (CCN) activity of sea salt particles, cloud reflectivity, rain formation, evaporation, and uptake of water and global climate change.16–20

In recent years, considerable attention has been paid to the potential roles of surfactants and organic coatings on atmospheric aerosols.4,6,21 A class of compounds referred to as amphipathic molecules could spread on the water surface and form stable Langmuir monolayers. These amphipathic compounds found on atmospheric aerosols are long-chain carboxylic acids, including both saturated and unsaturated fatty acids.22–25 Long-chain fatty acids are an important class of organic compounds found in submicron SSA particles.10,26 One of the most common unsaturated fatty acids is oleic acid (OA), which is a monounsaturated fatty acid. Stearic acid (SA) is a saturated long-chain fatty acid found on atmospheric aerosol particles.10 OA has been found to be the most abundant unsaturated fatty acid in the organic surfactants of SSAs.13 SA has been recognized as the most abundant fatty acid in seawater and is certainly one of the most frequently investigated fatty acids for monolayer studies. The structure of OA is nearly identical to that of SA, except that it contains a cis double bond in the middle of the chain. This induces the bent shape of OA relative to SA, a linear molecule. These organics are normally well mixed with other species on the surface of atmospheric species instead of existing in a pure state. Over the past decades, the behavior and surface properties of mixed monolayers of fatty acids with other organic species, including cholesterol, esters, alcohols, and amines, have attracted much attention and have been extensively studied at the molecular level.27–31 Using experimental techniques and molecular dynamics simulations, the interfacial behaviors and properties of mixed films at the air–water surface have been recently studied.32–37 This work provides important information regarding the effect of organic films on water transfer from aerosols, alteration of aerosol particles, and particle's impact on climate.

Cholesterol plays an essential role in the functioning of mammalians and is widely distributed or accumulated in animal tissues.38 Cholesterol could be abundant in the sea surface microlayer (SSML) via the biological behavior of marine biota such as excretion, exudates, or remains of dead bodies. Further, it could be brought to the SSML from bulk seawater, land, or atmosphere by different transport mechanisms.39 In addition, cholesterol was reported to have a surprisingly high CCN activity, and therefore, plays a significant role in the Earth's radiative balance.40 Mixed monolayers of cholesterol with other organic species have been widely investigated.25,38,41,42 The examination of the mixing behavior of different organic compounds is more realistic and significant for understanding the stability of mixed films and the impact on the surface properties of aqueous aerosols such as water evaporation and uptake. Langmuir troughs are an effective tool to investigate the intermolecular interactions between the membrane molecules and fatty acids. Langmuir monolayers at the air–water interface were used as a representative model for organic films on aqueous aerosols.

In the current work, considerable efforts have been devoted to elucidate the mixing behavior of SA/chol and OA/chol monolayers at the air–seawater interface by using a Langmuir trough. Simplified artificial seawater (NaCl solution) coated with mixed fatty acids and cholesterol films was used to simulate the real surface of SSAs coated by organic compounds. Surface pressure (π)–area (A) isotherms analysis was applied to examine the thermodynamic behaviors and physicochemical properties in mixed systems of cholesterol with SA and OA. A well-established experimental method—the infrared reflection absorption spectroscopy (IRRAS)—was used to investigate the molecular conformation and ordering of alkyl chains on sea salt droplets.37,43 The miscibility and stability of mixed films have been discussed to reveal the interaction between fatty acids and cholesterol and provide more insights into the air–sea exchange and the aqueous phase aerosols over the ocean.

2. Methods

2.1 Materials

Cholesterol (95%, Acros), SA (>98%, Aladdin), OA (99%, Alfa Aesar), spreading solvents (i.e., chloroform), and NaCl (>99%, Acros) were purchased and used without additional purification. Fatty acid molecules possessing more than 12 carbon atoms in the alkyl chain could be dissolved in proper volatile organic solvents such as chloroform. Stock solutions were prepared by dissolving cholesterol, SA, or OA in chloroform to a concentration of 1 mM immediately prior to the experiment. The same stock solutions were then used to prepare solutions of the desired mixture ratios to minimize the fluctuation of the relative concentration of the surfactants. Seven molar ratios of SA (or OA) to cholesterol were used, i.e., 1[thin space (1/6-em)]:[thin space (1/6-em)]0, 9[thin space (1/6-em)]:[thin space (1/6-em)]1, 8[thin space (1/6-em)]:[thin space (1/6-em)]2, 7[thin space (1/6-em)]:[thin space (1/6-em)]3, 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 3[thin space (1/6-em)]:[thin space (1/6-em)]7, and 0[thin space (1/6-em)]:[thin space (1/6-em)]1. Ultrapure water with a resistivity of 18.2 MΩ cm produced by a Milli-Q system was used in all the preparations. NaCl comprises the main composition of inorganic salts in seawater. Therefore, we simplified the system with NaCl as the sole sea salt constituent of seawater.44,45 The bulk NaCl solution with a concentration of 0.6 M was used as the subphase. All the solutions were sealed from the ambient environment. The temperature was maintained at approximately 20 °C with relative humidity of 30 ± 5% using a glass cover to avoid any possible contact with the environment.

2.2 π–A isotherm measurements

A Teflon trough with a working area of 65 mm × 280 mm × 3 mm equipped with 2 symmetrical barriers to allow film compression was placed on an antivibration table and sealed in an environmental chamber to perform the π–A isotherm measurements. The trough was cleaned several times with ethanol and ultrapure water and filled with the prepared NaCl solution as the aqueous subphase. A filter paper was changed to a plate hanging from a force transducer as the pressure sensor. The surface pressure is the difference of the surface tension between pure water and monolayer-covered water.46

A syringe was cleaned with the solution to be measured. With the syringe just above the water surface, the lipids mixture solutions were deposited in the form of drops onto the aqueous surface with the caution that the pressure value produced by adding the drop has returned to zero before introducing the next one. It lasted 10–15 min to allow the chloroform to evaporate and the lipid molecules to spread across the salt solution surface; then, the compression was initiated. The area and surface pressure of the monolayer were varied. The π–A isotherms were recorded during the movement of the barriers across the water surface with a speed of 3 mm min−1. Each experiment was repeated several times to ensure the reproducibility of the isotherms. Schematic experimental setup for the Langmuir trough and IRRAS is shown in Scheme 1.


image file: c8em00419f-s1.tif
Scheme 1 Schematic experimental setup for Langmuir trough and IRRAS.

2.3 IRRAS

IRRAS experiments were conducted on a Vertex 70 FTIR spectrometer (Bruker, Germany) with a modified external variable angle reflectance attachment. The IR beam was directed to the air–seawater interface and the angle of incidence could be adjusted. The reflected beam was collected and pointed to a liquid-nitrogen-cooled HgCdTe (MCT) detector. A Langmuir trough with two compartments was used for the IRRAS measurements. The reference counterpart was filled with NaCl solution as the subphase, while the sample counterpart contained the subphase solution covered by the investigated film. A computer-controlled stepper motor was employed to shuttle the trough to different positions and to collect the spectra from both film-covered and uncovered areas. A time delay of 60 s was allowed for equilibrium between the movement of the trough and the collection of the spectra. The IRRAS spectra for pure and mixed monolayers on artificial seawater were measured at the incidence angle of 40° with respect to the surface normal and was collected between 400 and 4000 cm−1. The resolution used was 8 cm−1 and the total number of scans was 2000. The spectra were recorded at a predefined surface pressure of 26 mN m−1, which was maintained constant during spectra accumulation.

3. Results and discussion

Pure components SA, OA, and cholesterol and their mixtures can form compressible monolayers at the air–seawater interface. Mixed monolayers of SA/chol and OA/chol were prepared by mixing SA (or OA) with cholesterol at various fixed molar ratios. The chemical structures of SA, OA, and cholesterol are shown in Scheme 2.
image file: c8em00419f-s2.tif
Scheme 2 Chemical configuration of SA, OA, and cholesterol.

3.1 π–A isotherms

The main source of the thermodynamic data about monolayers was the π–A isotherm measurements, which describe the increase in the surface pressure with decreasing molecular area upon film compression on the aqueous subphase. The basic properties of the monolayers were inferred based on the shape, course, and position of the π–A isotherms, including the stability, miscibility, and interaction mechanism of the mixed films. The π–A isotherms for the mixture of saturated (SA) and unsaturated (OA) fatty acids with cholesterol in different proportions of components are shown in Fig. 1, where the curves in black and yellow are the isotherms for the monolayers of pure SA (OA) and cholesterol, respectively. The π–A isotherms of the single-component SA and OA monolayers measured on the NaCl solution were compared with the respective mixed monolayers with cholesterol. As shown in Fig. 1, the lift-off molecular areas for SA and OA are 32 and 58 Å2 per molecule, respectively. This value for OA follows the previously reported values on pure water surface,28,29 indicating that NaCl in the subphase solution did not considerably influence the lift-off area of OA. The surface pressure for pure cholesterol monolayer in Fig. 1 shows a sharp increase, demonstrating a typical ordered, liquid-condensed monolayer resulting from the planar and rigid structure of the molecule.47 The cholesterol monolayer collapsed at approximately 32 mN m−1 with the molecular area of 45 Å2 per molecule (see in Table S1 in ESI). For pure SA monolayer on seawater, the surface area collapsed at ∼19 Å2 per molecule with the surface pressure of 43 mN m−1.
image file: c8em00419f-f1.tif
Fig. 1 π–A isotherms of mixtures of cholesterol with (a) SA and (b) OA at different mixing ratios at the air–seawater surface.

For the SA/chol mixed monolayers, with the increase in the molar fraction of the cholesterol in the mixture, the π–A isotherms become steeper, which is consistent with the previous studies.28,29 The sharp increase in the surface pressure for SA and its mixtures with cholesterol indicates the formation of rigid and condensed monolayers at the air–seawater interface. For the pure SA monolayer, the isotherms exhibit a phase transition from the liquid to the solid state at approximately 20 mN m−1, which is in reasonable consistency with the results of Seoane et al.28 Obviously, the shapes for all the mixed SA/chol isotherms changed regularly within the curves of the pure components. In the mixture with XSA = 0.9, a phase transition from liquid to the solid phase also occurred at ∼20 mN m−1. For the mixtures with XSA < 0.9, cholesterol-like condensed monolayers were observed. The lift-off areas were shifted to larger areas with increasing cholesterol ratios in the mixed monolayers. Evidently, from Fig. 1(a), the lift-off values for the SA/chol mixed system ranged from 31 Å2 per molecule in pure SA to 52 Å2 per molecule in cholesterol. For the equimolar SA/chol mixture, the value of the lift-off area, namely, 42 Å2 per molecule, is approximately 1.3 times higher than that for pure SA. The surface areas of the mixed monolayer at the surface pressure of 20 mN m−1 were shifted from 24 Å2 per molecule to 40 Å2 per molecule when the molar ratio of cholesterol increased from 0.1 to 0.7.

As compared to the SA monolayer on pure water surface, the isotherms on seawater were shifted to a higher molecular area. This indicates that the phase behavior is different between the two systems in a way. The isotherm of SA on pure water lifted off at ∼25 Å2 per molecule,29 while the lift-off area is 32 Å2 per molecule on the surface of seawater. Film collapse occurred at 44 mN m−1 for SA, which is smaller than that on pure water.28,29

The π–A isotherms for the mixed monolayers of OA and cholesterol are presented in Fig. 1(b). Different from single-component SA monolayers, the curve for pure OA exhibits no obvious transition point from the liquid phase to the solid phase. Instead, the transition from the gas phase to the liquid phase at 58 Å2 per molecule could be observed. No solid phase was found before collapsing. OA could not pack tightly as compared to SA due to the unsaturated double bond in the middle of the alkyl chain, and the surface pressure increased steadily upon compression. The mixed monolayers were more fluid and compressible, and the slope decreased with the increasing OA fraction in the mixture. OA/chol mixed monolayers varied with the proportion of OA and were gradual and much more expanded, contrary to the sharp decrease or solid-like collapse observed for SA/chol. The OA isotherm shows that the pure OA monolayer lifted off at 58 Å2 per molecule and collapsed at ∼24 Å2 per molecule with a surface pressure value of 35 mN m−1, which is consistent with the results of a previous study in which OA/stratum corneum lipid mixtures at the pure water surface were investigated.48 Upon mixing with cholesterol, as displayed in Fig. 1(b), the collapse pressures for the mixtures were slightly higher than that of pure OA film, except for XOA = 0.9. This indicates that the mixed OA/chol monolayers on seawater were more stable relative to the individual OA or cholesterol monolayer, and the presence of cholesterol exhibited a stabilizing effect on the monolayer.29,49 This effect could be attributed to the straightening of the alkyl chains of OA in the expanded state.28

The lift-off areas for OA/chol mixtures are also affected by the OA molar fraction. These areas decreased upon mixing with cholesterol relative to those for pure OA monolayers. The π–A isotherms for the mixtures in which XOA ≤ 0.5 were evidently shifted toward lower molecular areas with the increase in OA composition as compared to the isotherms for both individual component films. The areas for the initial rise in the surface pressure were 47 and 50 Å2 per molecule for XOA = 0.5 and 0.3, respectively, suggesting that the two components interacted with each other. The presence of cholesterol reduces the molecular area occupied by the mixed monolayers. This confirms the condensing effect of the investigated cholesterol.25 The isotherms of the OA/chol mixed monolayer did not shift regularly toward the curve of OA as the OA proportion increased. As shown in Fig. 1(b), the shape of the π–A isotherms for the mixture with OA molar proportion ≥ 70% changed approximately in the same way as that of the pure OA case, particularly in the low surface pressure regions. The curves for mixtures with 90% OA and pure OA are distinctly different from those with a dominant cholesterol composition. In the case of the mixed monolayers with 90% OA, the surface pressure started to increase very slowly at 58 Å2 per molecule and reached a value of only ∼5 mN m−1 at about 48 Å2 per molecule. The addition of cholesterol made the OA monolayer to shift from the liquid-expanded phase to a more condensed phase. The lift-off area and collapse pressure for single-component OA monolayer are in reasonable agreement with the recent study, exploring the impact of model surfactants on the CCN activity of SSA mimics.45 It is worth noting that the results reported by Forestieri et al. were determined by using the film model for microdroplets in which water uptake occurs quickly, while the Langmuir–Blodgett experiment is performed under equilibrium conditions. This marginal discrepancy could also be explained by the difference in the relative humidity and concentration of NaCl solution. The results are also consistent with the measured values of Voss et al. who studied the oxidation of OA and reported the lift-off area of 51 Å2 per molecule for OA at the air–seawater interface.44 The lift-off area of 58 Å2 per molecule for the OA monolayer is larger than that of the SA monolayer.

The shape of SA and OA isotherms at low and high surface pressures suggests two prominent mechanisms of collapse. Upon further compression, the interfacial pressure of the SA monolayer decreases precipitously at the collapse pressure, pointing to a fold-collapse mechanism. This distinct behavior has also been observed in the isotherms of myristic acid monolayer at the air–water surface.30 On the contrary, the interfacial pressure curve of the OA monolayer on pure water surface increases steadily toward a plateau-like region.44 The OA monolayer on seawater surface also proceeded through a steady increase, but it did not reach the plateau-like region at the surface pressure of 29 mN m−1, as observed in the slope decrease in Fig. 1(b). The interval of the mean molecular area between lift-off and collapse for OA is 34 Å2 per molecule, while this value is only 12 Å2 per molecule for SA.

3.2 Miscibility of the mixed monolayers

The interaction between two film-forming components can be evaluated based on the miscibility of the mixed monolayer, which can be best described quantitatively by the evolution of the excess area (ΔAexc) of the mixed monolayer with varying molar fractions at a particular surface pressure. The excess area of mixing can be obtained by comparing the mean area per molecule of the mixed monolayer with an ideal unmixed, single-component monolayer:50,51
 
ΔAexc = A12Aid(1)

A 12 denotes the mean molecular area of the mixed system, and Aid is defined as the mean molecular area of the ideal mixing situation. The ideal value for both mixed monolayers Aid can be calculated from the molar fractions of the two components:

 
Aid = A1X1 + A2X2(2)
where X1 and X2 are the molar fractions of components 1 and 2 in the mixed film, respectively; A1 and A2 are the area per molecule of the pure monolayers at the same surface pressure. If the mixture is ideal or the two components are completely immiscible, the excess area will be zero and ΔAexc will be linear in Xi. The deviation from zero value indicates that various types of interactions occur in the film.52 The positive and negative deviations represent the intermolecular repulsion and attraction, respectively. In addition, the positive and negative deviations from the additivity rule indicate miscibility or nonideality. Fig. 2(a) shows the excess molecular area as a function of the molar fraction of SA at different surface pressures and provides information on the miscibility between SA and cholesterol at the air–seawater surface. The biggest deviation from zero was observed at the SA proportion of 0.7, and the value of ΔAexc at this composition was −2.2 Å2 per molecule. The plots can be divided into two regions at the boundary of XSA = 0.5. The ΔAexc values in XSA = 0.3 decreased with increasing surface pressure and were slightly deviated from zero at the surface pressure of 10–20 mN m−1, suggesting the weak attractive interaction between SA and cholesterol. ΔAexc was positive at 5 mN m−1, with a value of ∼0.3 Å2 per molecule, which indicated the weak intermolecular repulsive interaction between SA and cholesterol and that the binary system is metastable. This might be ascribed to the disordered phase at low surface pressures.


image file: c8em00419f-f2.tif
Fig. 2 Excess area per molecule of mixing (ΔAexc) as a function of the composition of fatty acid for mixed monolayers of (a) SA/chol and (b) OA/chol at different surface pressures.

On the other hand, at 0.5 < XSA< 1, similar values of ΔAexc against the surface pressure were observed at XSA = 0.7. Meanwhile, the minimum values of ΔAexc were also produced at XSA = 0.7 in the entire range of the molar fraction, suggesting that the molecules were packed relatively closely and hardly changed upon compression and the film was at the most stable state at this SA composition. At XSA = 0.5, ΔAexc was enhanced with the increase in the surface pressure. Furthermore, the maximum value of ΔAexc was obtained at XSA = 0.5, and all the ΔAexc values were almost positive at this composition over the studied surface pressure range. In addition, the intermolecular interactions between SA and cholesterol strengthened and the repulsive forces enlarged at this SA proportion, except for the low surface pressure of 5 mN m−1. The ΔAexc values showed a negative deviation from zero at XSA = 0.7 and 0.9, indicating that SA and cholesterol exhibited an attractive force at this composition range at the air–seawater interface. It is worth noting that ΔAexc approached zero in the molar fraction range of XSA = 0.3–0.5 at 10 mN m−1 surface pressure, which is consistent with that observed by Seoane et al.29 In some previous studies, the miscibility of the components of SA/chol systems were studied at the pure water surface. The linear plots of the mean molecular areas of SA/chol mixed monolayers vs. the molar fraction of cholesterol show that cholesterol and SA are immiscible regardless of the composition.28,29 The biggest deviation from ΔAexc = 0 appeared at XSA = 0.5, namely, ∼2 Å2 per molecule, which is much smaller than the deviation in the OA case (discussed later).

With regard to the excess molecular area of OA and OA/chol mixtures, ΔAexc is negative for all the compositions and surface pressures studied. These negative deviations confirm the attractive interaction and miscibility between OA and cholesterol and also indicate film compression. From Fig. 2(b), it is evident that the mixed OA/chol monolayers were miscible but not ideally mixed within the entire composition and surface pressure range. Further, the relation of the excess mean area of the OA/chol mixtures with the OA proportion exhibits marked a deviation from the linearity value, indicating strong attractive interactions between OA and cholesterol. For the excess area of OA/chol monolayer, the maximum negative value of ΔAexc was obtained at the equimolar proportion of OA and cholesterol. The molecules were packed closely at the air–seawater interface with stronger attractive interaction. The ΔAexc values decreased with the surface pressure at XOA = 0.5 and 0.7. In addition, the ΔAexc values of mixed OA/chol monolayers were lower than those of mixed SA/chol at the SA (or OA) molar ratios smaller than 0.7. The intermolecular interactions between OA and cholesterol were more attractive than those between SA and cholesterol.

3.3 Thermodynamic stability of the mixed monolayers

The stability of the monolayers can be quantitatively analyzed in thermodynamics treatment as an excess Gibbs free energy function, ΔGexc. The ΔGexc value can be evaluated by the integration of π–A isotherms from zero to a particular surface pressure:53,54
 
image file: c8em00419f-t1.tif(3)
where π is the surface pressure of the monolayer. If the film is an ideal mixture, A12 equals A1X1 + A2X2, and therefore, ΔGexc should be zero, i.e., the interactions are identical between the two components or the two components are completely immiscible. A positive value of ΔGexc indicates that molecules in the mixed monolayer have a lesser tendency to mix together due to the repulsive interaction and prefer to stay alone, while a negative value of ΔGexc suggests that the interactions between the molecules are more attractive or less repulsive as compared to their respective single-component films and indicates the miscibility between the two components.55 Moreover, a more negative ΔGexc value suggests more stable mixed monolayers. The ΔGexc values were calculated from the π–A isotherms in Fig. 1. In Fig. 3, the excess Gibbs free energy of the mixed monolayer vs. composition for SA/chol (XSA) and OA/chol (XOA) plots are shown at several selected surface pressures. It is obvious that the plot of ΔGexc values vs. molar fractions of OA exhibit a negative deviation from the linearity in the entire range of compositions and surface pressures, which indicates that the sum of the forces corresponds to the attractive interaction between the molecules and that the orders of the mixed monolayers are increased. These results also demonstrated that the mixing of OA with cholesterol contributes toward the stability of the monolayers. As shown in Fig. 3(a), ΔGexc exhibits similar variation tendency with ΔAexc as the molar fraction changes and positive ΔGexc values were observed. The ΔGexc values for SA/chol monolayers depend on the mixing ratios and surface pressure. It is evident that similar ΔGexc values were obtained at SA molar ratios of 0.3 and 0.5 for lower surface pressures (≤10 mN m−1). These ΔGexc values were close to zero, which is in agreement with the results of ΔAexc, indicating that the SA and cholesterol system was close to being ideally mixed at these composition ranges. The ΔGexc values were observed to be positive and increased with the surface pressure for 1[thin space (1/6-em)]:[thin space (1/6-em)]1 SA/chol mixed monolayer, which is consistent with the maximum ΔAexc values in Fig. 2(a). SA and cholesterol tended to separate from each other due to their repulsive interaction. The stability of SA/chol mixed monolayer was the weakest at the equimolar composition. Lower values of ΔGexc indicate a higher tendency to pack molecules of monolayers and more stable mixed films.24,25 The most negative value of ΔGexc was obtained at XSA = 0.7. The ΔGexc values were negative and decreased with the increasing surface pressure. This implies that the affinity between SA and cholesterol monolayers becomes the largest at this composition. From Fig. 3(a), it is evident that the ΔGexc values were indistinguishable at XSA = 0.3, indicating that the variation in the surface pressure had a marginal impact on the 3[thin space (1/6-em)]:[thin space (1/6-em)]7 SA/chol mixed monolayer on the air–seawater surface.

image file: c8em00419f-f3.tif
Fig. 3 Excess Gibbs free energy of mixing (ΔGexc) as a function of composition of fatty acid for mixed monolayers of (a) SA/chol and (b) OA/chol at different surface pressures.

For the OA/chol monolayer, the distinct negative deviation from the ideal line is favorable for the occurrence of molecular condensation and enhancement of thermodynamic stability of the mixed system.56 From Fig. 3(b), it is evident that the ΔGexc values become more negative as the pressure increases, indicating that the interactions become stronger when the films are compressed and the molecules occupy smaller areas at the air–seawater surface. Besides, the ΔGexc values of OA/chol mixed monolayers decreased with an increase in the OA proportions till the equimolar composition of OA and cholesterol is reached; this is when ΔGexc values reached their minima and then increased with the molar ratios of OA. In contrast to SA/chol mixtures, the maximum negative values of ΔGexc were observed at OA molar proportion of 0.5 for all the surface pressures (Fig. 3(b)). The strongest interaction existed in the mixed OA/chol system at XOA = 0.5, and the most stable monolayer was formed in the equimolar OA and cholesterol composition. The smallest deviation of ΔGexc from zero was observed at XOA = 0.9, as shown in Fig. 3(b), demonstrating that the OA/chol mixed monolayer is less stable at this ratio. In addition, the ΔGexc values are more negative for OA/chol than those for SA/chol monolayers. As shown in Fig. 3, ΔGexc is −156 J mol−1 for OA/chol monolayers, which is approximately three times lower than that of the SA/chol monolayer (ΔGexc = −51 J mol−1). This indicates stronger intermolecular interactions in the OA/chol mixed systems.

3.4 IRRAS spectra of the monolayers at the air–seawater interface

IRRAS is an in situ method that is well suited and widely used for investigating the conformation order and organization of monolayers at the air–seawater interface.57,58 Typical IRRAS spectra of methylene stretching vibrations at 2800–3000 cm−1 for pure and mixed SA/chol and OA/chol monolayers at the air–seawater surface are shown in Fig. 4. The assignments of the corresponding main vibrational bands are listed in Table S8. The strongest bands at 2916–2919 cm−1 and 2850 cm−1 can be attributed to the antisymmetric (νa(CH2)) and symmetric (νs(CH2)) stretching vibrations of methylene, respectively. The CH2 vibration bands are known to be conformational sensitive and they can be used to monitor the conformational order of alkyl chains.57 In the case of the IRRAS spectra for SA/chol mixtures, the antisymmetric and symmetric stretching vibrations of pure SA monolayer were found to be at 2916 and 2850 cm−1, respectively, which are in good agreement with the results obtained at the pure water surface,59 indicating that the presence of NaCl in the subphase has a marginal impact on the position of CH2 vibrational bands of SA. In the mixed monolayers of SA with cholesterol at different SA proportions (Fig. 4(a)), the νa(CH2) and νs(CH2) bands maintained consistency with the wavenumber of the one-component SA monolayer.
image file: c8em00419f-f4.tif
Fig. 4 IRRAS spectra of (a) SA/chol and (b) OA/chol monolayers in the range of 2830–3029 cm−1 on the 0.6 M NaCl solution surface.

As shown in Fig. 4(a), the intensities of the antisymmetric and symmetric bands of methylene groups have a sensitive response to the molar fraction of SA. The peak intensities of νa(CH2) and νs(CH2) bands decreased with the increase in the cholesterol composition. The νa(CH2) and νs(CH2) band intensities of pure SA are 2.4 and 2.5 times stronger than those of the mixed monolayers at XSA = 0.7. It is worth mentioning that the intensities of νa(CH2) and νs(CH2) vibrations for the SA monolayer on seawater increased relative to those on the pure water surface.59 Sea salt (NaCl) in the subphase increased the conformational order of the surface molecules of fatty acids at the aqueous interface. The ratios of the peak-height intensity between CH2 antisymmetric and symmetric vibrational bands (Ias/Is) were also calculated to analyze the relative ordering of the alkyl chain in the monolayers. High values of Ias/Is correspond to a more closely packed layer and a larger number of trans bonds.60 The ratios of the two bands (Ias/Is) for the mixed films with XSA = 0.9, 0.8, and 0.7 were 1.19, 1.25, and 1.24, respectively (Table S9). They were slightly higher than that of pure SA film (1.18), indicating that the mixing with cholesterol increases the order of the monolayers.

With regard to the OA/chol mixed monolayer on seawater, the bands at 2924 and 2854 cm−1 due to the antisymmetric and symmetric stretches of the methylene groups were observed on seawater, which are consistent with the literature values.44,61 Similar to the SA/chol mixtures, in the presence of cholesterol, the intensities of νa(CH2) and νs(CH2) bands for the OA/chol mixed monolayers also show negative dependence to the cholesterol molar fraction (Fig. 4(b)). The presence of C[double bond, length as m-dash]C in the alkyl chain of OA affected the CH stretching vibration, as shown in Fig. 4. The νa(CH2) and νs(CH2) values at around 2920 and 2850 cm−1 for the SA/chol mixed monolayer are about 4.5 times more intense than those for the mixed OA/chol film. As opposed to SA and its mixtures with cholesterol, the spectra of OA/chol mixed monolayers exhibit the signature of the HC[double bond, length as m-dash]CH stretching in 3001–3016 cm−1, which is characteristic of unsaturated fatty acids. This is consistent with the assignment in the broad-bandwidth sum frequency generation spectroscopy experiment conducted by Voss et al. who reported the position of the C[double bond, length as m-dash]C stretching vibration at 3014 cm−1.44 The intensities of C[double bond, length as m-dash]C bonds also decreased with the increase in the cholesterol molar fractions. For the OA/chol mixtures, the Ias/Is ratios for the mixed monolayers were notably higher than that of the pure OA film. They were observed to be 1.36, 1.65, and 1.88 for XOA = 0.9, 0.8, and 0.7, respectively. The alkyl chains are formed in a more well-ordered state in the mixed OA/chol monolayers.

The spectra in Fig. 5 provide information regarding the antisymmetric and symmetric stretching vibrations of the carboxylate group (νa(COO) and νs(COO)), the stretching vibration of carbonyl group (ν(C[double bond, length as m-dash]O)), and the scissoring mode of CH2 (δ(CH2)), which appeared between 1380 and 1710 cm−1. This indicated the sensitivity of these bands toward the molar fraction of the fatty acids. The bands at 1704 cm−1 were assigned to the C[double bond, length as m-dash]O stretching vibration (ν(C[double bond, length as m-dash]O)) in the COOH group for SA and OA monolayers and their mixtures with cholesterol. Upon interaction with cholesterol, the intensities of the C[double bond, length as m-dash]O vibration bands for OA/chol mixtures were significantly increased at all the compositions under investigation, while ν(C[double bond, length as m-dash]O) for the SA/chol mixed monolayers appeared to be insensitive to cholesterol incorporation and exhibited marginal changes.


image file: c8em00419f-f5.tif
Fig. 5 IRRAS spectra of (a) SA/chol and (b) OA/chol monolayers in the range of 1300–1800 cm−1 on 0.6 M NaCl solution surface.

In the presence of cholesterol with different ratios, as evident from Fig. 5(a), the intensities and positions of ν(C[double bond, length as m-dash]O), νa(COO), νs(COO), and δ(CH2) bands for the SA/chol monolayers remain nearly unchanged. The spectral features of SA at the air–seawater interface were very similar to those at the pure water surface.59 However, Fig. 5(b) shows that all these bands for the OA/chol mixtures at various compositions are enhanced when compared with those of the pure OA monolayer. The increase in the peak intensity is characteristic of the closely packed structure of the mixed monolayer. Upon mixing with cholesterol, the formation of highly ordered conformation indicates that the OA/chol mixed films are more stable than single components in the pure state. This is consistent with the results of the excess Gibbs free energy. The intensities of the sharp νa(COO) bands for both SA and OA mixtures with cholesterol found at 1520 cm−1 are comparable with that of the νa(CH2) bands, while the symmetric COO vibration bands at 1400 cm−1 are relatively weak, which is in agreement with a previous study.59 The δ(CH2) bands of the SA chain on seawater were located at 1465 cm−1 in the present spectra, which is similar to their location on the pure water surface.59 The δ(CH2) mode indicates that the alkyl chains in both single-component and mixed monolayers are packed in a hexagonal subcell structure.62,63

4. Atmospheric implications

Both fatty acids and sterol are important organic compounds found in submicron SSA particles.10,26,39 Surface-active compounds on the SSAs could alter the amount of water absorbed by the aerosol particles and the hygroscopic properties of aerosols.45,64 Organic compounds could also affect the refraction and size of the sea salt droplets and then influence the aerosol extinction at a particular wavelength. Organic coatings present in aerosols may increase their CCN activities.45 The CCN activities of pure cholesterol have been investigated, and it was found that cholesterol is a good source of CCN in the atmosphere.40 Therefore, mixing with other organic species such as long-chain fatty acids might affect its CCN activity by altering the chemical composition and physical properties. Clearly, the negative values of the excess Gibbs free energy for SA (or OA)/chol mixed films coated on the SSA surface in our result showed that the mixing of cholesterol with SA and OA at the air–seawater interface can improve the stability of the monolayers coated onto the aqueous SSAs. High Ias/Is ratios proved that the mixed monolayers were packed more densely and formed in a highly ordered state, thereby prolonging their lifetime in the atmosphere and inhibiting the air–sea exchange of surface-active organics and other atmospheric gas-phase species due to the more stable conformation.65,66 The surface monolayers are well ordered, and therefore, may inhibit the water evaporation from the subphase, which eventually leads to larger aerosols with closely packed surfactants.67,68

Fatty acids coated on aqueous aerosols might react with oxidants in the atmosphere such as O3, NO3, and OH.44 In the future, considering the high reactivity of the C[double bond, length as m-dash]C bond toward ozone, we will explore the mechanism and kinetics of the oxidation and photooxidation of mixed monolayers by ozone at the air–seawater surface. Furthermore, the present work can be combined with chamber studies to explore the surfactant-coated sea salt reactions with other gas-phase pollutants to determine the transformation of aerosols, reaction products, reaction rates, and productivity. This study could be expanded from involving molecular-level interactions to macroscopic thermodynamics and aqueous aerosol chemistry and regional- and global-scale climate change.

5. Conclusions

The miscibility, stability, and structural characteristics of the SA/chol and OA/chol mixed monolayers on the air–seawater interface have been investigated by means of the Langmuir technique. Coupling the π–A isotherms with IRRAS on the air–seawater subphase yields important information regarding the phase behavior and chain packing of the SA/chol and OA/chol mixed monolayers. We have shown that the isotherms on seawater were shifted to higher molecular areas as compared to SA/chol mixed monolayers on pure water surface. The curves for the OA/chol mixtures increased steadily upon compression. The lift-off areas for OA monolayers decreased upon mixing with cholesterol, indicating the condensing effect of cholesterol.

The interactions between SA (or OA) and cholesterol were affected by their composition and surface pressure. The OA/chol mixed system shows miscibility between the two components, but OA and cholesterol do not form ideally mixed monolayers. The negative excess Gibbs free energies for OA/chol mixed monolayers suggest that the mixed OA/chol films were more stable than single-component monolayers. Lowe ΔGexc values of OA/chol relative to SA/chol mixed system indicate strong intermolecular interactions between OA and cholesterol. High Ias/Is ratios demonstrate that mixing with cholesterol changes the ordering of fatty acid films and alkyl chains are formed in a more well-ordered state in the mixed monolayers at the air–seawater interface. The composition of surface-active organics on atmospheric aerosols impact the water evaporation and uptake, and it might affect the lifetime and transfer of aerosols on the ocean surface. The mixing behavior and surface properties of mixed monolayers explored in this study provide information regarding the air–sea exchange and the characteristic changes in organic-species-coated SSAs.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors acknowledge support from National Natural Science Foundation of China (91644214, 21876098), and Shandong Natural Science Fund for Distinguished Young Scholars (JQ201705).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8em00419f

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