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Aerosols are significant to the Earth’s climate, with nearly all atmospheric aerosols containing organic compounds that often contain amphiphilic molecules. However, the nature of how these compounds are arranged within an aerosol droplet remains unknown. It was recently demonstrated that fatty acids in proxies for atmospheric aerosols self-assemble into highly ordered three-dimensional nanostructures known as lyotropic liquid crystalline phases. This finding may have implications for environmentally important processes. In this research, acoustically trapped droplets of oleic acid/sodium oleate mixtures in a sodium chloride solution were analysed by simultaneous synchrotron small-angle X-ray scattering (SAXS) and Raman spectroscopy in a controlled gas-phase environment. It was demonstrated that the droplets contained crystal-like lyotropic phases including hexagonal and cubic close-packed arrangements of spherical and cylindrical micelles, and stacks of bilayers, whose structures responded to atmospherically relevant humidity changes and chemical reactions. Further experiments showed that self-assembly reduces the rate of reactions with atmospheric oxidants. These experiments at the High Throughput SAXS beamline (B21) demonstrated that lyotropic-phase formation also occurs in more complex mixtures more closely resembling compositions of atmospheric aerosols. This suggests that lyotropic-phase formation likely occurs in the atmosphere, with potential implications for cloud formation and properties, lifetimes of molecules in the atmosphere, and other important aerosol characteristics.
In these experiments single droplets were held in levitation and followed changes in these droplets at the same time using X-rays to track the arrangement of the molecules and Raman spectroscopy to follow chemical changes. These droplets were exposed to conditions encountered in the atmosphere: changes in humidity and presence of the atmospheric oxidant ozone. In all humidities, the droplets exhibited highly complex self-assembled three-dimensional structures. Exposure to ozone eventually broke down the molecules and destroyed the complex molecular arrangement; however, the molecules survived much longer in those arrangements than expected. This could explain longer lifetimes of such molecules found in the atmosphere compared to laboratory experiments where the complex arrangements that may be encountered in atmospheric conditions is not taken into account.
It was demonstrated that complex 3D self-assembly occurs in proxies for atmospheric aerosols. Many of these 3D structures are strongly anisotropic and are known to significantly affect optical properties, diffusion, viscosity, surface tension and water uptake; and therefore, in an atmospheric context, may have a dramatic impact on aerosol behaviour (Fig. 1).
During the experiments on more complex mixtures on B21, two further atmospheric aerosol components were introduced: first sugar (fructose) and then hydrocarbon (hexadecane). Two mixtures were prepared: fatty acid/sugar and fatty acid/sugar/hydrocarbon. The fatty acid/sugar/hydrocarbon ratios were chosen according to ratios found by Wang et al1. in field studies of real atmospheric aerosols in the Chinese city of Chongqing in winter, where the three main classes of organic components were fatty acids, sugars and alkanes (3244, 2799 and 948 ng m−3, respectively). For experimental ease, the mixtures were analysed not as levitated droplets but as dry coatings on the inside of X-ray capillary tubes, which were exposed to high and low relative humidities. As demonstrated in Figure 2, both the sodium oleate/oleic acid/fructose and the sodium oleate/oleic acid/fructose/hexadecane systems showed complex 3D self-assembly. SAXS patterns from the sodium oleate/oleic acid/sugar system on humidification clearly show three Bragg peaks from the inverse hexagonal (HII) phase, with further peaks indicating additional coexisting phases. On drying, the structure changes, but different Bragg peaks are nonetheless observed; rehumidifying showed the changes to be reversible, suggesting that they represent thermodynamic phases in equilibrium with water vapour in the surrounding environment. The sodium oleate/oleic acid/sugar/hydrocarbon mixture showed a different self-assembly; here, while it was not possible to assign the peaks to a particular symmetry phase—indeed, more than one phase may be present—the presence of multiple peaks shows the existence of periodic ordering on the nanometre-length scale, while the reversible responses to humidity changes again show lyotropic-phase formation.
While it is clear that further studies are urgently needed to test the impact of this complex self-assembly on the atmosphere, this work demonstrates the potential of these arrangements to explain substantially extended atmospheric lifetimes found for reactive organic molecules. Further investigations combining laboratory and field studies are required to establish the influence of complex three-dimensional self-assembly on a wider range of properties such as light scattering, hygroscopicity, viscosity and diffusion in aerosol particles. These properties potentially affect cloud nucleation and albedo, and atmospheric reaction rates, and so are key to understanding the impact of aerosols on the environment and climate change.
The research is a collaboration between Universities of Reading, Bath, Bristol and Lund with experiments carried out mainly at large-scale facilities in Sweden (MAXIV-lab) as well as Diamond Light Source.
Pfrang C, Rastogi K, Cabrera-Martinez ER, Seddon AM, Dicko C, Labrador A, Plivelic TS, Cowieson N, Squires AM. Complex three-dimensional self-assembly in proxies for atmospheric aerosols. Nature Communications 8, 1724, doi:10.1038/s41467-017-01918-1 (2017).
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