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Glass-forming properties of 3-methylbutane-1,2,3-tricarboxylic acid and its mixtures with water and pinonic acid.

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.