RESUMO
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.
RESUMO
3-Methylbutane-1,2,3-tricarboxylic acid (3-MBTCA) is an atmospheric oxidation product of α-pinene and has been identified as the most relevant tracer compound for atmospheric terpene secondary organic aerosol (SOA) particles. Little is known, however, of its physicochemical properties such as water solubility and phase state (e.g., liquid, crystalline, glassy). To gain knowledge, we synthesized 3-MBCTA from methyl 2-methylpropanoate and dimethyl maleate via a Michael addition and subsequent hydrolysis with 78% overall yield. It was found that 3-MBTCA transforms into anhydrides upon melting at Tm = 426 ± 1 K, thus preventing a determination of the glass transition temperature Tg by differential scanning calorimetry (DSC) through melting and subsequent cooling. Therefore, we designed the novel technique MARBLES (metastable aerosol by low temperature evaporation of solvent) for transferring a substance into a glassy state without heating. In MARBLES an aqueous solution is atomized into wet aerosol particles that are subsequently dried in several diffusion dryers resulting in glass formation of the residual particles for several solutes. The glassy aerosol particles are collected in an impactor until enough mass has accumulated that the sample's Tg can be determined by DSC. Using this method, the glass transition temperature of 3-MBTCA was found to be Tg ≈ 305 ± 2 K. Moreover, we have determined the glass transition Tg' of the maximal freeze-concentrated aqueous solution of 3-MBTCA, and Tg of mixtures of 3-MBTCA with water and pinonic acid. The latter data indicate a dependence of Tg upon the atomic oxygen-to-carbon ratio of the mixture, with implications for parametrizing the glass-forming behavior of α-pinene SOA particles in the atmosphere.
