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: -pinic acid, -pinonic acid, -norpinic acid, and -norpinonic acid. These encompass their thermal properties, solid-liquid phase equilibria, and dissociation constant (p). Thermal properties, including the melting temperature, enthalpy of fusion, temperature, and enthalpy of the phase transitions, were measured with the differential scanning calorimetry technique. These SOA components exhibit relatively high melting temperatures from 364.32 K for -pinic acid to 440.68 K for -norpinic acid. The enthalpies of fusion vary from 14.75 kJ·mol for -norpinic acid to 30.35 kJ·mol for -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, non-random two-liquid model, and universal quasichemical equations. The results of the correlation indicate that the Wilson equation appears to work the best for all investigated compounds, giving rise to the lowest value of a standard deviation. -Norpinic acid and -pinic acid (dicarboxylic acids) show better solubility in the aqueous solution than -norpinonic acid and -pinonic acid (monocarboxylic acids), which affect the multiphase chemistry of α-pinene SOA processes. For -pinonic acid and -norpinonic acid, also pH-profile solubility was determined. The intrinsic solubility () for -norpinonic acid was measured to be 0.05 mmol·dm, while for -pinonic acid, it was found to be 0.043 mmol·dm. The acidity constants (p) at 298 and 310 K using the Bates-Schwarzenbach spectrophotometric method were determined. The p values at 298.15 K for -norpinonic acid and -pinonic acid were found to be 4.56 and 5.19, respectively, whereas at 310.15 K, p values were found to be -4.76 and 5.25, respectively.