Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland.
Faraday Discuss. 2013;165:289-316. doi: 10.1039/c3fd00049d.
Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.
尽管在了解对流层气溶胶粒子的性质方面取得了重大进展，但要了解其物理状态和形态仍然具有挑战性。为了更详细地了解内部混合的有机/无机气溶胶粒子的相、相转变和形态，我们评估了 33 种有机/硫酸铵（AS）/H2O 体系的液-液相分离（LLPS）、吸湿相对湿度（DRH）和反吸湿相对湿度（ERH），这些体系的数据来自我们自己的和文献中的数据。有机部分由单化合物或混合物组成，含有最多 10 个脂肪族和/或芳香族成分，具有羧酸、羟基、羰基、醚和酯官能团，O:C 比在 0.29 到 1.33 之间。在这些 33 个体系中，有 13 个体系在任何研究的有机-无机混合比下都没有显示出 LLPS，16 个体系经历了 LLPS，显示出核壳形态，4 个体系根据有机-无机比和 RH 显示出核壳和部分包裹两种配置。在所有情况下，具有部分包裹配置的体系中的有机部分都由二羧酸组成。AS 在混合有机/AS/H2O 粒子中在 70%到 84%RH 时吸湿。AS 在低于 58%RH 时反吸湿或保持在单相状态。在具有 LLPS 的液滴中，AS 总是在 30%到 50%RH 之间发生反吸湿，这清楚地表明 LLPS 的存在促进了 AS 的反吸湿。对于含有聚乙二醇 400（PEG-400）和混合二羧酸的体系，富有机相在富 AS 相上的扩展系数与沉积在疏水性基底上的液滴的光学观察到的形态一致。对由 PEG-400/AS/H2O 组成的单个悬浮液滴的高分辨率弹性米氏共振光谱的分析允许检测到 LLPS，而在（2-甲基戊二酸+3-甲基戊二酸+2,2-二甲基琥珀酸）/AS/H2O 中很难检测到 LLPS。在 93.5%和 80.9%RH 下测量的 PEG-400/AS/H2O 的米氏光谱分别与均匀和核壳配置的计算米氏光谱一致，证实了从沉积在疏水性基底上的液滴获得的结果。基于呈现的证据，因此我们认为核壳形态是对流层有机/AS/H2O 粒子发生液-液相分离的主要形态。
