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Physicochemical Properties of Pinic, Pinonic, Norpinic, and Norpinonic Acids as Relevant α-pinene Oxidation Products.

Here, the study is focused on the synthesis and determination of physicochemical properties of four α-pinene secondary organic aerosol (SOA) products: cis-pinic acid, cis-pinonic acid, cis-norpinic acid, cis-norpinonic acid. These encompasses their thermal properties, solid-liquid phase equilibria and dissociation constant (pKa). Thermal properties, including the melting temperature, enthalpy of fusion, temperature and enthalpy of the phase transitions were measured with differential scanning calorimetry technique (DSC). These SOA components exhibit relatively high melting temperatures from 364.32 K for cis-pinic acid to 440.68 K for cis-norpinic acid. The enthalpies of fusion vary from 14.75 kJ∙mol-1 for cis-norpinic acid to 30.35 kJ∙mol-1 for cis-pinonic acid. The solubility in water was determined with the dynamic method (solid-liquid phase equilibria method) and then experimental results were interpreted and correlated using three different mathematical models: Wilson, NRTL and UNIQUAC equations. The results of the correlation indicate that Wilson equation appears to work the best for all investigated compounds giving rise to the lowest value of a standard deviation. Cis-norpinic acid and cis-pinic acid (dicarboxylic acids) show better solubility in the aqueous solution, than cis-norpinonic acid and cis-pinonic acid (monocarboxylic acids), which affect the multiphase chemistry of α-pinene SOA processes. For cis-pinonic acid and cis-norpinonic acid also pH-profile solubility was determined. The intrinsic solubility (S0) for cis-norpinonic acid was measured to be 0.05 mmol · dm-3, while for cis-pinonic acid 0.043 mmol · dm-3. The acidity constants (pKa) at 298 K and 310 K using the Bates-Schwarzenbach spectrophotometric method were determined. The pKa values at 298.15 K for cis-norpinonic acid and cis-pinonic acid were found to be 4.56 and 5.19, respectively, whereas at 310.15 K - 4.76 and 5.25, respectively.