Determination of evaporation rates and vapor pressures of very low volatility compounds:: A study of the C4-C10 and C12 dicarboxylic acids

A method for the measurement of evaporation rates and vapor pressures of low volatility compounds was developed and applied to the homologous series of C-4-C-10 and C-12 dicarboxylic acids. Proton-transfer chemical ionization mass spectrometry was used to follow directly the temperature- dependent evaporation rates of aerosol samples collected on a cold plate that could be heated at a known rate. The vapor pressures of the deposited compounds were derived from observed evaporation rates through application of the Hertz-Knudsen equation. Temperature programmed desorption allowed for quantification of the enthalpy (Delta H-sub) and entropy (Delta S-sub) of sublimation of the diacids and is described. A strong odd-even dependence with respect to the total carbon number was observed in the derived diacid vapor pressures, consistent with previous measurements. However, the vapor pressures from this method were systematically lower than previous measurements. Though seen in the vapor pressure, no odd-even carbon chain length dependence was readily discernible in the measured values of Delta H-sub and Delta S-sub. Perhaps most importantly, these experimental results also suggest that residual solvent molecules (from the aerosol generation process) trapped in the diacid samples can have a considerable influence on the measured thermodynamic parameters and, if not properly accounted for, may give erroneous results.