Dette Hans P, Koop Thomas
Faculty of Chemistry and Center for Molecular Materials, Bielefeld University, Universitätsstraße 25, D-33615 Bielefeld, Germany.
J Phys Chem A. 2015 May 14;119(19):4552-61. doi: 10.1021/jp5106967. Epub 2014 Dec 23.
Recent experiments suggest that organic aerosol particles may transform into a glassy state at room temperature under dry conditions. Information on glass forming processes in mixed inorganic/organic aerosol particles is sparse, however, because inorganic crystal nucleation is usually very likely in such mixtures. Here we investigate the glass transition temperatures Tg of various organics (trehalose, sucrose, citric acid, sorbitol, and glycerol as well as 3-MBTCA) in binary mixtures with either NaNO3 or NH4HSO4 at different mass fractions. The glassy samples were prepared with the MARBLES technique by atomizing dilute aqueous solutions into aerosol particles and subsequent diffusion drying. The resulting aerosol particles were collected and their phase behavior was investigated using differential scanning calorimetry. At small and intermediate inorganic mass fractions salt crystallization did not occur. Instead, the single-phase mixtures remained in an amorphous state upon drying such that determination of their Tg was possible. From these measurements the Tg value of pure NaNO3 and pure NH4HSO4 could be inferred through extrapolation, resulting in values of Tg(NaNO3) ≈ 290 K and Tg(NH4HSO4) ≈ 220 K. Upon drying of NH4HSO4/3-MBTCA mixtures, phase-separated samples formed in which the inorganic-rich and organic-rich phases each show an independent glass transition. Our measurements provide a route toward establishing Tg values of inorganic salts that usually crystallize readily, and they may explain the reported contradicting observations of NaNO3 aerosol particles to either crystallize or remain amorphous upon drying at room temperature.
近期实验表明，在干燥条件下，有机气溶胶颗粒在室温下可能转变为玻璃态。然而，关于无机/有机混合气溶胶颗粒中玻璃形成过程的信息却很稀少，因为在这类混合物中无机晶体成核通常很容易发生。在此，我们研究了不同质量分数下，各种有机物（海藻糖、蔗糖、柠檬酸、山梨醇、甘油以及3 - 甲基苯三甲酸）与硝酸钠（NaNO₃）或硫酸氢铵（NH₄HSO₄）二元混合物的玻璃化转变温度Tg。通过将稀水溶液雾化成气溶胶颗粒并随后进行扩散干燥，利用MARBLES技术制备玻璃态样品。收集所得的气溶胶颗粒，并使用差示扫描量热法研究其相行为。在较小和中等无机质量分数下，未发生盐结晶。相反，单相混合物在干燥后仍保持非晶态，从而能够测定其Tg。通过这些测量，可通过外推推断出纯NaNO₃和纯NH₄HSO₄的Tg值，得出Tg(NaNO₃)≈290 K和Tg(NH₄HSO₄)≈220 K。在干燥NH₄HSO₄/3 - 甲基苯三甲酸混合物时，形成了相分离的样品，其中富无机相和富有机相各自呈现独立的玻璃化转变。我们的测量为确定通常容易结晶的无机盐的Tg值提供了一条途径，并且可以解释关于NaNO₃气溶胶颗粒在室温下干燥时要么结晶要么保持非晶态的相互矛盾的报道观测结果。
