Application of Langmuir Monolayers in the Investigation of Surface Properties of Sea Spray Aerosols

Shumin Cheng; Lin Du; Xiuhui Zhang; Maofa Ge

doi:10.7536/PC200841

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1721-1730 DOI: 10.7536/PC200841

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Application of Langmuir Monolayers in the Investigation of Surface Properties of Sea Spray Aerosols

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Abstract

Aqueous aerosols coated by surface-active organics, such as sea spray aerosol(SSA), commonly possess inverse micelle structure with an aqueous core coated by a hydrophobic organic monolayer. The surface organization and chemical composition of organic films of SSAs have important influence on their physical, chemical and optical properties. Langmuir monolayers are monomolecular layers formed by the diffusion of long-chain surface-active organics with low volatility, such as fatty acids, fatty alcohols and phospholipids at the air-water interface. With Langmuir trough, the changes in surface pressure(π) along with decreasing molecular area(A) of single and mixed component monolayers at the air-water interface can be obtained. In addition, π-A isotherms provide information about the interfacial behavior of corresponding monolayers above an aqueous surface. Understanding the interfacial properties of monolayers will enable the prediction of the fate and behaviors of surfactants in the real-world SSA. This paper reviews the commonly used interfacial characterization techniques of atmospheric aerosols, established models for SSA based on Langmuir monolayer, and the implications of organic films on atmospheric behavior of SSA. Although the physical properties and morphological changes of SSA-related monolayers have been studied in depth, little attention has been paid to the interfacial changes induced by environmental factors such as reactive gases, irradiation and bioactive substances. These knowledge gaps shed light on future laboratory simulation and model research of SSA.

Contents

1 Introduction

2 Commonly used interfacial characterization techniques of atmospheric aerosols

3 Established models for SSA based on Langmuir monolayer

3.1 Single component monolayers

3.2 Mixed component monolayers

4 Implications of organic films on atmospheric behaviors of SSA

5 Conclusion and outlook

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Keywords

monolayer / sea spray aerosol(SSA) / surface property / air-water interface / atmospheric behavior

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Shumin Cheng, Lin Du, Xiuhui Zhang, Maofa Ge. Application of Langmuir Monolayers in the Investigation of Surface Properties of Sea Spray Aerosols. Progress in Chemistry, 2021, 33(10): 1721-1730 DOI:10.7536/PC200841

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1 Introduction

Sea surface microlayer (SSML) is a layer of water located at the air-Sea interface, which is rich in organic matter, inorganic ions, bacteria and other microorganisms.The thickness of SSML is usually 1 ~ 1000 μm^[1⇓~3]. Organic matter in SSML is mainly produced by the decomposition of marine bacteria and phytoplankton^[4,5]. These organic substances mainly include small molecular fatty acids and their derivatives, phospholipids, amino acids, sugars and sterols, as well as macromolecular proteins and humic substances^[6⇓~8]. In addition, abiotic sources of organic matter, including transformation of photochemical precursors and riverine transport, also increase the compositional diversity of SSML^[2,9]. In addition, the content of metal ions in SSML is generally high through the complexation of organic matter^[1,10].

Sea spray aerosol (SSA) is one of the largest sources of primary aerosol particles in the world (about Sea spray aerosol), and it is also the main liquid-phase aerosol in the atmosphere^[1,11,12]. SSA is produced during the bursting of bubbles formed by the wind-driven waves of the SSML^[13,14]. Therefore, the components in SSA are strongly influenced by SSML. What is most interesting in this process is the high enrichment of organics with higher surface activity in SSA relative to SSML (Fig. 1)^{[7,15⇓⇓~18]}. In the 1970s, MacIntyre proposed that water droplets covered by organic films were ejected into the atmosphere when breaking waves broke at the surface of the ocean^[19,20]. Ellison et al. Proposed a reverse micelle model of marine spray aerosols in 1999, which consists of a hydrophobic surface formed by organic molecules and an aqueous core^[4]. In 2002, Tervahattu et al. Confirmed the presence of fatty acid-dominated organic films at the SSA interface by analyzing collected SSA samples^[17,18]. In recent years, a series of laboratory simulations of SSA using monolayers have shown that the selective enrichment of metal ions in SSA by interaction with organic compounds has a certain impact on the arrangement and stability of SSA interface molecules^{[10,21⇓~23]}.

Changes in the composition and morphology of the organic thin film present at the SSA interface can affect its physical, chemical, and optical properties, thereby altering its atmospheric processes^[25,26]^[27,28]^[29]^[4,30]. For example, the compactness of organic molecules will affect their permeability to water molecules and trace gases, and then change the moisture absorption process of aerosols, the activity of cloud condensation nuclei, radiation absorption and scattering, and the reactivity with atmospheric gases^[4,31,32]^[8]^[33]^[29,34]^[35,36]. In addition, due to the high specific surface area of SSA, its water-air interface is particularly important for the atmospheric heterogeneous reaction and nucleation process. Therefore, it is very important to study and characterize the composition, molecular arrangement and reactivity of SSA interface chemicals at the molecular level in order to improve the accuracy of aerosol climate impact prediction^[17].

Langmuir monolayer technique was developed in the 1930s^[37,38]. Since the 1980s, with the development of surface detection technology, monolayers have been widely used in the fields of physics, chemistry and biology^[37,39]^[21,40]^[41,42]. In recent years, the study of monolayers has been extended to the field of environmental chemistry, and has been used as an ideal simplified model for liquid aerosols wrapped by organic membranes^[22,23,43]. Monolayers of low volatility surfactants such as long-chain fatty acids and phospholipids are spread over pure water or simple aqueous salt solutions in a Langmuir cell. On this aerosol modeling system, environmental conditions such as surfactant and liquid phase ion species can be easily changed. The monolayers with a thickness of only one molecule can be obtained at room temperature by Langmuir cell, and the monolayers can be ordered by applying transverse pressure to them, and then the surface pressure-molecular area (π-a) curve of the monolayer can be obtained. As the monolayer is compressed, the surface tension (γ) at the water-gas interface decreases, while the surface pressure increases (π=γ₀-γ)^[44⇓~46]. Langmuir monolayers have the advantages of controllable molecular orientation, simple film formation conditions and good uniformity.

At the air-water interface, the change in the shape of the π-a curve is related to the orientation of the surfactant molecules^[47]. Stearic acid is the main representative of saturated fatty acids in SSA organic membranes, and the π-a curve of its monolayer at the gas-liquid interface is shown in Figure 2^[48]. With the compression of the monolayer and the decrease of the molecular area, the packing density and arrangement order of the surface active molecules show a two-dimensional phase behavior similar to that of gas, liquid and solid phases^[48]. Three stages can be clearly seen: in the Gaseous film, the surfactant molecules spread out at the gas-liquid interface in a dispersed state, each molecule occupies a large area and hardly interacts with each other, and the surface pressure is almost zero; After continuous compression into the Liquid film, the molecular area decreases and interactions occur, and the surface pressure begins to rise slowly. When the monolayer is further compressed, the molecular area decreases rapidly and the surface pressure increases rapidly, forming a Solid film. The surfactant molecules are now tightly packed at the interface and the alkyl chains are almost perpendicular to the surface. At this point, the compression continues, the molecular distance reaches a minimum, and the monolayer reaches its Collapse pressure.

2 Commonly used atmospheric aerosol interface characterization technique

Because of the complexity and importance of surface chemistry, it has been a hot area of aerosol research for many years. A variety of experimental techniques have been used to characterize the surface properties of aerosols at the microscopic level, including chemical structure, ordered arrangement, interaction and morphology^[37,49]. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) can be used to monitor the heterogeneous reaction process of atmospheric organic and inorganic pollutants on the surface of mineral dust aerosol, which is one of the important components of atmospheric aerosol, and to characterize the structural information of its surface chemical composition^[50]^[50⇓~52]. In addition, Attenuated total reflection Fourier transform infrared spectroscopy (Attenuated total internal reflectance-Fourier transform infrared spectroscopy, ATR-FTIR) can be used to analyze the surface chemical composition of aerosol samples collected in the laboratory and field^[53,54]^[55⇓~57]. The technology has the characteristics of simple sample preparation, high detection sensitivity, simple operation and the like. In addition, some researchers have used transmission and scanning electron microscopy techniques to reveal the physical and chemical characteristics of the surface and interior of the particles^[58,59]. The interface of SSA carries a wide range of physical and chemical processes, and self-assembled monolayers have a very prominent advantage in the study of its organic film model^[21,49].

Although Langmuir monolayers have been studied since the beginning of the 20th century, the π-a curve can only show the macroscopic phase change of monolayers under constant compression^[60]. In the mid-1980s, with the application of fluorescence microscopy and surface sensitive techniques such as X-ray reflection and diffraction to the study of molecular structure and orientation, people began to obtain microstructure information from Langmuir films^[37,40,61]. The development of these surface detection techniques has played an important role in promoting the study of Langmuir monolayers. At present, the commonly used methods for the detection of SSA-associated Langmuir monolayers are vibrational spectroscopy and microscopic imaging (Table 1).

Vibrational spectroscopy, such as Infrared reflection absorption spectroscopy (IRRAS), can be used to analyze the monolayer in situ (Figure 3) to obtain information such as the configuration of functional groups and alkyl chains and the order of molecular arrangement, and then analyze the stability of the monolayer at the interface^[62⇓~64]. The IRRAS technique was first used in 1985 by Dluhy et al., who demonstrated the feasibility of obtaining Langmuir monolayer infrared spectra of fatty acids and phospholipids in situ at the water-gas interface, and has now become a powerful interface detection method^[65]. The results show that the wave number of methylene (—CH₂—) stretching vibration is sensitive to the conformation of the alkyl chain, and the lower wave number represents a more ordered conformation, that is, the molecular arrangement at the interface is more compact, otherwise it is more disordered^[63,66]. The interaction between liquid phase ions and interfacial molecules can be analyzed according to the changes of the characteristic peaks of the head groups of surfactants, such as carboxylic acid and phosphoric acid groups. The effects of sea salt ions such as Na⁺, Ca²⁺ and Mg²⁺ on the ordering and orientation of alkyl chains, the orientation of head groups, the ionization state and the compressibility of monolayers of long-chain fatty acids and phospholipids have been studied in depth before^{[10,22,67,68]}. Understanding the ionization state of lipid molecules and their interactions with metal ions can further explain their effects on the interfacial properties of SSA.

Atomic force microscopy (AFM) and Brewster angle microscopy (BAM) can be used to observe the images of monolayers under different surface pressures, and then to observe the formation process and morphology changes of monolayers^[37,70]. For example, in the BAM image (Fig. 4), the darkest areas represent negligible surfactant coverage (gaseous film), the lighter areas represent tightly packed monolayers (liquid/solid film), and the lighter areas show the presence of three-dimensional structures after monolayer collapse^[21]. Combined with π-a curves and microscopic observations, the morphologies of monolayers of single and complex components such as long-chain fatty acids, phospholipids and sterols in pure water and sea salt solutions have been studied in detail^{[10,23,49,68]}.

Vibrational spectroscopy provides molecular insights into the interfacial properties of SSA, which are difficult to obtain by other methods. Although vibrational spectroscopy is a reliable technique for detecting the properties of aerosol interfaces, the correlation of the spectral characteristics of organic films with the microstructure and morphology of the interfaces requires the assistance of microscopic imaging techniques^[42]. Therefore, it is often necessary to combine the two techniques in the model study of SSA organic films^[21,23,49].

3 SSA Model Based on Langmuir Monolayer

3.1 Ingle-component monomolecular film

The study of Langmuir monolayers promotes the development of aerosol interfacial chemistry. Allen and Vaida et al. First established a series of SSA models by choosing long-chain low-volatility organic molecules as the model molecules of aerosol surface according to the composition of the organic film on the real SSA surface^[71⇓~73]. As for that single component monolayer, typical organic compound mainly used include aliphatic compound such as fatty acid, fatty alcohol and fatty amine, and phosphatidic compounds such as Dipalmitoylphosphatidic acid (DPPA) and Dipalmitoylphosphatidylcholine (DPPC). The effects of different organics on the surface properties of SSA and the interaction mechanism between organics and sea salt ions can be studied by single component monolayer.

3.1.1 Fatty acid monolayer

Long-chain fatty acids (C₁₄~C₁₉) are widely present in the organic membranes of SSA^[17,30]. Among them, stearic acid, palmitic acid, arachidic acid and oleic acid are commonly used as model molecules because of their large abundance in organic membranes^{[23,28,67,74]}. Compared with the bulk phase, the pK_a value (~ 8.34) of long-chain fatty acids at the water-air interface is significantly increased, which leads to the partial dissociation of carboxylic acid groups at the seawater (pH ≈ 8) interface and the interaction with sea salt ions in the liquid phase^[67,75]. Li et al. Studied the response of stearic acid and arachidic acid molecules at the interface to atmospheric inorganic ions^[74]. It was found that after adding inorganic ions such as Fe²⁺, Ca²⁺ and Al³⁺ into pure water subphase, the change of π-a curve of fatty acid monolayers indicated that the monolayers contracted or expanded in different degrees, and it was speculated that there was interaction between inorganic ions and fatty acid molecules. The difference between the asymmetric and symmetric stretching vibration peaks of the carboxyl group in the IRRAS spectrum further confirms that the inorganic ions change the surface characteristics of the fatty acid monolayer through coordination. In combination with the IRRAS spectrum, Wellen Rudd et al. studied the infrared spectrum of palmitic acid monolayer in different concentrations of Ca²⁺ solution, and found that the signal of COO^- group gradually increased with the increase of Ca²⁺ concentration (Figure 5), indicating that Ca²⁺ can dissociate palmitic acid molecules and effectively combine with COO^-^[23]. Similarly, Tang et al. Found through VSFG spectroscopy that the effect on the interfacial hydrogen bonding structure between palmitic acid monolayer and water molecules is greater than that of Mg²⁺,Ca²⁺, which is consistent with the fact that Ca²⁺ has greater binding force to carboxylic acid groups, thus explaining the high enrichment of Ca²⁺ in SSA relative to other metal ions^[76]^[34]. In addition, some researchers have considered the spectral response of carboxylates at the water-air interface induced by Na⁺^[67]. It was found that with the increase of pH and the concentration of the Na⁺, the infrared signal of the dissociated palmitic acid molecules at the interface gradually increased, and the stability of the monolayer also increased, which was due to the formation of —COO^-∶Na⁺ ion pairs. This conclusion is consistent with the results of Brzozowska et al. In the study of stearic acid monolayer in artificial seawater. When the mixed solution of inorganic salts is used as the subphase, the stability of the monolayer increases with the increase of pH value and/or salt concentration according to the change of π-a curve of the monolayer^[77]. The above results show that the presence of large amounts of sea salt ions in SSA can affect many chemical properties of the surface organic film, such as surface activity, solubility and interfacial molecular structure. These properties have important effects on the atmospheric processes of aerosols.

3.1.2 Phospholipid monolayer

Saturated phospholipids containing 14 to 18 alkyl carbons, such as DPPA and DPPC, are widespread in SSML and abundant in the organic membranes of SSA^[7,78]. Compared with fatty acids, phospholipids have double alkyl chains and larger head groups, and their chemical structures are more complex.

The exploration of phospholipid monolayers is mainly focused on the enrichment of cations in liquid phase. Adams et al. Studied the effects of four constant sea salt cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) on the interfacial properties of DPPC monolayers, and the change of π-a curve showed that:Except for the contraction effect of Ca²⁺ on DPPC monolayer, the other three ions all made the DPPC monolayer more expansive, because the 2∶1 DPPC∶Ca²⁺ complex was more stable than the other 1:1 complexes (Ca²⁺>Mg²⁺>Na⁺>K⁺)^[10]. This was further confirmed by the subsequent BAM morphology analysis and the changes in IRRAS spectra. When the interface molecules were replaced by DPPA, it was found that the binding forces of the four cations with DPPA monolayers showed the same rule^[68]. In addition to these four common ions, some trace and transition metal ions can also be selectively enriched in SSA by binding to organic substances such as phospholipids^[39,49,78]. BAM, IRRAS and VSFG techniques were used to measure the DPPC monolayers spread on the surface of SrCl₂ and ZnCl₂, respectively. It was found that the interaction between Zn²⁺ and DPPC molecules (2∶1 DPPC∶Zn²⁺ complex) was stronger and promoted the contraction of the monolayer, which was consistent with the greater abundance of Zn²⁺ in the collected SSA samples^[39]. Zhang et al. Further compared the interactions between various trace metal ions and DPPC monolayers, and found that the binding force between these ions and DPPC has the following rule :Al³⁺>Fe³⁺>Zn²⁺>Mg²⁺>Ni²⁺>Mn²⁺~Ca²⁺^[49]. Similar to Zn²⁺, the formation of complexes of Fe³⁺ and Al³⁺ with DPPC makes the monolayer more stable, which is also the reason why they are selectively enriched in SSA. At the same time, it can also be explained that the binding force between phospholipid molecules and trace metal elements is stronger than that of major elements. The above series of studies show that the interaction between metal ions and lipid molecules is specific, and the effect of metal ions with the same charge on organic membranes may be different. Phospholipid monolayers contribute to the enrichment of cations, especially multivalent cations, in SSA. In addition, it was found that there were interactions between inorganic anions and phospholipid monolayers, and these anions made the molecular area and methylene stretching vibration wavenumber of DPPC smaller^[79]. In addition, compared with anions such as Br^-, Cl^- and

NO 3 -

, phospholipid molecules are more likely to bind to

SO 4 2 -

, thus promoting the enrichment of organic sulfates in SSA. These studies provide a more comprehensive assessment of the response of phospholipid monolayers to sea salt ions.

In addition to inorganic ions, the interaction between phospholipid monolayers and soluble organic substances in SSA, such as sugars and brown carbon, has also been explored^[43,80,81]. Soluble sugars such as glucosamine and glucuronic acid can not be adsorbed on the water-air interface alone, but with the increase of the concentration of sugars in the liquid phase, the VSFG vibration signal of H₂O molecules gradually weakens.It is speculated that the negatively charged sugar molecules are attracted to the positively charged DPPC film and replace the H₂O molecules interacting with it, resulting in the co-adsorption of soluble sugars at the interface (Fig. 6), which may partly explain the presence of a large number of sugar components in SSA^[43]. There are not only physical interactions but also chemical interactions between the dissolved organic matter adsorbed on the SSA interface and the organic membrane. Photosensitive water-soluble brown carbons, such as imidazolidinedialdehyde and humic acid, are widely present in SSA, which can reduce the stability of the organic film at the SSA interface by promoting the photochemical reaction of unsaturated phospholipids in the organic film to form water-soluble products^[81].

3.2 Composite monomolecular film

Multicomponent monolayers are monolayers that contain more than one film-forming chemical component^[73]. They are good models for exploring intermolecular interactions at the interface. The diversity of organic sources in real SSA makes it have high chemical composition complexity, and the intermolecular interactions between different membrane components may change its surface properties, so the multi-component monolayer is more representative of the organic coating of SSA^[4,46].

3.2.1 Aliphatic composite monomolecular film

NaCl solutions with pH values of 8.2, 5.6 and 2.0 were used to simulate the liquid phase cores of nascent and aged SSAs, and the π-a curves and BAM images of the monolayers of stearic acid, palmitic acid and myristic acid as well as their mixtures were measured, respectively. It was found that the phase transition, breakdown pressure and morphology of the monolayers were different between single and composite components, and changed with the pH of the liquid phase^[82]. When different kinds of surfactants such as fatty acids, fatty alcohols and fatty amines are mixed in pairs, the interfacial arrangement of the composite membrane is affected by the length and structure of the tail chain, the types of head groups and the intermolecular forces, and the difference becomes more significant^[83,84]. In addition, surface seawater is also an important sink for PFAs, which can enter the atmosphere through enrichment in SSA^[85]. These organic compounds with high surface activity can form stable monolayers and intermolecular interactions with fatty acids at the interface^[47,69].

When fatty acids are mixed with non-surface-active alkanes, it is found that the residence time of the organic matter at the interface is positively correlated with its surface activity and chain length^[86]. The evolution of the composition of the SSA organic film in the atmosphere can be inferred by exploring the change of the composition of the aliphatic composite film at the water-air interface over time. The Langmuir cell was used to obtain the composite membranes with different components and determine the composition of the organic membranes at different times. It was found that fatty acids were dominant in the aged organic membranes, which was consistent with the field observation results^[72,86]^[18]. These findings show the importance of using more realistic and complex model systems to elucidate atmosphere-related changes in the SSA organic membrane interface.

3.2.2 Multicomponent monolayers of aliphatic and other surface-active organic compound

In addition to composite membranes composed of long-chain hydrocarbon molecules with similar hydrophobic structures, some studies have explained the interfacial stability and morphological changes caused by surface-active organic compounds whose structures are significantly different from those of aliphatic molecules. Cerebroside is a kind of glycolipid and a typical component of marine biomembrane. Adams et al. Found that the formation of intermolecular hydrogen bonds after the formation of a composite membrane with palmitic acid enhanced the stability of cerebroside molecules at the interface^[21]. Griffith et al. Found that water-soluble phenylalanine had a destructive effect on stearic acid monolayer at the interface through BAM images^[87]. In addition, the decrease of infrared signal of stearic acid and the appearance of amino acid characteristic peak at the 1600 cm^-1 were detected by IRRAS, which indicated the formation of phenylalanine/stearic acid composite film. Similarly, other biogenic and anthropogenic sources of sterols, phthalates, and petroleum spill-derived oxygenated aromatic hydrocarbons (benzaldehyde, benzoic acid, etc.) are widespread in SSML and SSA and can form complex membranes with fatty acid molecules at the gas-liquid interface and reduce the order of their molecular arrangement^[88]^[89,90]^[91]. Therefore, the surface morphology of the aerosol is affected by both the chemical composition of the liquid phase core and the organic film itself, and the composition of the entire aerosol must be considered in the study of SSA interface processes.

4 Effect of organic film on the atmospheric behavior of SSA

SSA has a wide and profound impact on global total aerosol and climate change. In order to accurately assess these effects, it is necessary to understand their particle size, composition, and morphology. Surface-active organic compounds are important components of the organic film on SSA surface. The diverse sources of these organics form a diverse and complex organic surface composition. Combining Langmuir groove with spectroscopy and microscopy techniques such as IRRAS, VSFG and BAM, the molecular arrangement and conformational information of surface films can be monitored from macro and micro levels. This information is key to understanding the SSA interface process^[26,92].

The composition of SSA, especially the composition of organic film, can affect the deposition and evaporation of water molecules, and the transport of reactive gases and radicals from the gas phase to the particle phase by changing its interface morphology, thus affecting its cloud condensation nucleus activity and a series of heterogeneous chemical reactions^[14,31]. The formation of highly ordered organic monolayers such as octadecanol and stearic acid can inhibit these processes, while the interactions between fatty acids, fatty alcohols, phospholipids and sterols at the real SSA interface can accelerate the atmospheric evolution of SSA by reducing the molecular density and stability of the interface^[93]. Since the optical properties of aerosols depend on their size, the ability of the surface of particles to absorb or evaporate water is important for the simulation of global radiation balance^[94,95].

Ionic and dissolved organic matter within aerosols may have the ability to significantly affect their surface composition and morphology^[91,96]. Both field observations and laboratory analyses of SSA particles show a significant enrichment of Ca²⁺ relative to other metal ions^[97]. This is consistent with the stronger binding of lipid monolayers such as fatty acids and phospholipids to Ca²⁺^[21]. In addition, the enrichment of some trace heavy metal ions by SSA also provides an important way for the transfer of pollutants to the atmosphere^[49]. In turn, the presence of these metal ions can also affect the molecular arrangement and ionization state of surfactants at the SSA interface, thus affecting the stability of the organic film and the reactivity of the interface molecules^[76,98].

SSMLs are also important sinks for persistent organic pollutants such as perfluorinated fatty acids, phthalates, and oxygenated aromatic hydrocarbons. SSA can not form a stable monolayer by itself, but the formation of a complex monolayer with lipid molecules can make SSA an important route for the sea-air transport of these pollutants^[69,85,89]. Enrichment in SSA particles may increase the long-range transport and global distribution of these compounds, such as pollution of polar regions and atmospheric transport into coastal and inland areas, thus endangering human health^[85,89].

5 Conclusion and prospect

The effective application of Langmuir monolayer and surface detection technology in the study of SSA surface characteristics reflects the close relationship between various disciplines. The organic monolayer at the water-air interface is an ideal experimental model to study the surface properties of SSA. Based on laboratory simulation, the models of single component and multi-component surface organic films were established, and the key chemical processes such as the interactions between surfactant molecules, between surfactant molecules and liquid inorganic ions, and between water soluble organic compounds and interfacial molecules at the gas-liquid interface were studied.It is helpful to further explore the physical, chemical and optical properties of SSA on the basis of the study of the gas-liquid interface characteristics of surface active organic films.

In current atmospheric simulations, the differences in aerosol size and hygroscopicity caused by organic films are usually ignored. Laboratory simulations involving SSA organic films are important to better predict aerosol reactivity and nucleation capabilities in climate models. Understanding the interactions between organic thin films and environment-related ions at the water-air interface and the effects of interactions between different organics on the surface properties of SSA will help to improve atmospheric aerosol climate models.

The interface can reduce the energy required for the chemical reaction and thus promote the reaction^[99]. Shrestha's first exploration of the interfacial reaction of fatty acid monolayers shows that the interfacial photochemical reaction in the presence of photosensitizers reduces the stability of palmitic acid monolayers, thus enhancing their permeability to water molecules and reactive gases^[28]. In this study, the constructed SSA surface organic film was successfully applied to simulate the interfacial heterogeneous reaction of aerosol. At present, some SSA surface film models have been established through the study of single-component and multi-component organic films, and then it is necessary to introduce other atmospheric components and environmental factors into the study system of monolayer films to study the interfacial reaction process and potential reaction mechanism of aerosols at the molecular level.

The formation of SSA involves not only the participation of sea salt and organic matter, but also the sea-air transport of a large number of active substances such as biological enzymes, bacteria and viruses^{[100⇓⇓~103]}. In the laboratory and field observation studies of Malfatti et al., it was first revealed that active marine microbial enzymes were directly transferred from seawater to nascent SSA, and the activity in SSA particles was 1 to 2 orders of magnitude higher than that in seawater^[100]. Studies on lipase/lipid composite membranes by Schiffer et al. And Luo et al. Have demonstrated that the transfer of these microbial enzymes from the ocean to the atmosphere profoundly affects the chemical composition and properties of the SSA interface^[104]^[105]. Future studies can explore the effects of these active substances on the composition of the marine atmosphere in combination with biomacromolecule-involved complex component monolayers, photochemistry, and other external factors.

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