J. Phys. Chem. A 2010, 114, 5787–5795
5787
Equilibrium Morphology of Mixed Organic/Inorganic/Aqueous Aerosol Droplets: Investigating the Effect of Relative Humidity and Surfactants N.-O. A. Kwamena, J. Buajarern, and J. P. Reid* School of Chemistry, UniVersity of Bristol, Bristol, BS8 1TS, United Kingdom ReceiVed: January 14, 2010; ReVised Manuscript ReceiVed: April 11, 2010
There is considerable uncertainty regarding the phase, morphology, and composition of atmospheric aerosol. In particular, it is important to understand the microphysical structure of mixed inorganic/organic aerosol given that the structure can influence the surface composition, the role of heterogeneous chemistry, gasparticle partitioning of semivolatile organics, and water uptake in the sub- and supersaturated regimes. We present here a thermodynamic model that predicts the equilibrium morphology of mixed inorganic/organic aerosol. The model uses an iterative process to calculate the total surface free energy of all possible morphologies when two immiscible droplets are brought into contact, with the configuration with the lowest total surface free energy representing the final equilibrium structure. Sensitivity tests and validation experiments were performed by investigating the decane/NaCl/aqueous system. The addition of a water-soluble surfactant was found to promote spreading of decane on the aqueous droplet. This was confirmed by laboratory experiments, although the importance of considering the relative volumes of the aqueous and organic phases was found to play a significant role in determining the equilibrium structure. Decreasing the relative humidity (RH) of the surrounding gas phase was found to decrease the spreading of decane on the aqueous droplet, leading to thicker organic lenses on smaller aqueous droplets. We conclude that a core-shell structure is not always predicted to be the thermodynamically favored state of aerosol containing distinct hydrophobic and hydrophilic phases. Gaining a more reliable picture of the microphysical structure of aerosol is crucial to be able to model aerosol behavior and properties in the atmosphere, particularly when aerosol is dominated by internal mixtures of inorganic and organic components and when the organic is present in a (subcooled) liquid state. 1. Introduction The size, composition, and morphology of atmospheric aerosol must be known to assess accurately the impact of aerosol on global and regional climate, atmospheric chemistry and human health. The composition of atmospheric aerosol includes inorganic salts, mineral dust, carbonaceous organic compounds as well as water-soluble and insoluble organic compounds.1,2 However, the exact chemical composition is highly uncertain3,4 and varies depending on location.1 Organic compounds, covering a wide range of physical and chemical properties,1,3 are a major component of atmospheric aerosols5 and are the second most abundant class of compounds after sulfate and nitrate in particles 99%, analytical grade, Fisher Scientific), decane (>99% anhydrous, Sigma Aldrich), and SDS (> 99.0% ACS reagent, Sigma Aldrich) were all used as received without further purification. It should be noted that pure commercial samples of SDS typically contain 0.1 to 1% of dodecanol. Even at this low concentration, dodecanol has a dramatic effect on the interfacial properties of SDS solutions below the critical micelle concentration (CMC). In this study, all SDS solutions were made at concentrations that exceeded the CMC, and aqueous solutions were made using deionized water.
Figure 4. Comparison of the equilibrium morphology of two immiscible droplets when brought in contact. The structures in the third vertical column are results from Buajarern et al.16 based on the method of Torza and Mason.25 The results obtained from this work are shown in the fourth column.
4. Results and Discussion This section describes how the microphysical model can be used to predict the equilibrium morphology of mixed organic/ inorganic aerosol. The model is first validated using a system that has been previously studied, the decane/NaCl/water system.16 We then extend the application of the model by investigating the effects of surfactants and RH on the equilibrium morphology of decane/NaCl/aqueous droplets. 4.1. Model Validation. An iterative approach to predict the equilibrium structure of mixed-phase particles was used in our microphysical model. In contrast, previous models16,25 determined the equilibrium morphology of biphase droplets by calculating contact angles and radii of the final structure based on the surface and interfacial tensions of each phase as well as the initial volumes of the individual droplets. Despite the differences in the model approach, the qualitative similarity of our results compared with previous work using a different methodology (Figure 4) provides a good indication of the performance of our model. In effect, increasing the volume fraction of the organic component leads to greater engulfment of the aqueous phase by the organic droplet. The current model is advantageous because it provides a more general framework that can be easily tailored and modified to predict the equilibrium structures of more complex systems containing many phasesegregated domains as well as including the effects of gravity and the influence of external changes in environmental conditions. 4.2. Modeling More Complex Systems. Simulations were performed to investigate the effect of increasing and decreasing the surface tension of water and the interfacial tension between decane and water. The introduction of a surfactant to the decane/ NaCl/aqueous system decreases the surface energy of water and the interfacial tension. Conversely, decreasing the RH increases the surface energy of water and interfacial tension due to the increasing concentration of the inorganic solute. 4.2.1. Effect of a Surfactant on the Equilibrium Morphology. Surfactants are surface-active compounds that tend to partition to the surface of liquids and lower the surface tension of solutions. The lowering of the surface tension of a droplet may lead to an increase in the number of activated aerosols that become cloud droplets, as a result leading to an increase in cloud albedo.37-38 There are limited studies on the surface tensions of mixed organic/inorganic systems, especially on how
5792
J. Phys. Chem. A, Vol. 114, No. 18, 2010
they vary over wide concentration and temperature ranges and at the inorganic solute concentrations typically found in atmospheric aerosol.37 Furthermore, very little work has looked at the effect of surface tension depression, due to the presence of surface active compounds like surfactants, on the equilibrium morphology of mixed phase particles. In this study, the effect of adding a surfactant to a mixed organic/inorganic droplet was investigated. SDS, which is an anionic, nontoxic surfactant, was used as the model surfactant. Although SDS is not found in the atmosphere, it is often used as a surrogate for soluble atmospheric surfactants.39-42 To probe the effect of surfactants on the equilibrium morphology of the decane/NaCl/aqueous system, simulations were performed using surface and interfacial tensions with and without the surface tension depression due to the presence of SDS (Table 1). The radius of the aqueous droplet was kept constant at 3 µm for these simulations. In addition, an SDS concentration of 5 mM was assumed, the concentration at which laboratory experiments were performed. The decrease in the surface and interfacial tensions was expected to allow decane to spread more readily on the SDS-doped aqueous NaCl droplets. In fact, as shown in Table 1, the calculated spreading coefficients suggest that decane should completely engulf the surfactant-doped aqueous phase, contrasting with the partially engulfed behavior predicted for the surfactant free system. However, more detailed model simulations accounting for the volumes of the two phases suggest that only partial engulfment should be expected for all volume ratios investigated. This difference in predicted structure is a result of the breakdown of the assumption of equivalent surface area changes and the neglect of a distinct volume ratio, the assumption governing eq 2. This further highlights the importance of quantifying both the interfacial tensions and volumes of the phases to be able to predict the final equilibrium structure accurately. In general, the equilibrium structures predicted for the SDS/ decane/NaCl/aqueous and decane/NaCl/aqueous systems were similar for a given volume ratio. The difference in the two systems arises in the degree to which decane spreads on the aqueous droplets. Figure 5 illustrates how the surface area of the aqueous and organic phases vary as well as the interfacial area as the volume fraction of the organic component increases. It can be seen that the aqueous phase surface area decreases whereas the surface area of the organic phase and the interfacial area increase as the radius of the organic droplet increases. Furthermore, there is a greater extent of spreading on the aqueous phase in the presence of a surfactant when the organic phase volume arises from a droplet with radius
