Estimating the vapor pressures of multi-functional oxygen-containing organic compounds using group contribution methods

A UNIFAC-based method for estimating the vapor pressure (pLo) values of oxygen-containing compounds of intermediate-to-low volatility has been developed as an aid in modeling the formation and behavior of organic aerosols. This UNIFAC-pLo method was constructed using a set of 76 compounds with experimentally determined pLo values. The compounds chosen are of intermediate-to-low volatility and contain multiple oxygen-containing functionalities. For test and development purposes, the 76 compounds were divided into a basis set of 43 compounds used to generate the coefficients required in the UNIFAC-pLo method and a second set of 33 compounds that was used to test the coefficients generated using the basis set. Both the basis and test sets contained compounds that possessed similar structures and functionalities. For the 33 compounds in the test set, on average UNIFAC-pLo predicted the pLo values to within a factor of 2 over the temperature range 290–320 K. Furthermore, the UNIFAC-pLo method did not show any correlation in prediction error with pLo so that it was equally likely to underpredict as overpredict pLo regardless of volatility. For comparison, three other vapor pressure estimation methods were applied to the test set of compounds. On average, these other methods all predicted the test set pLo values to within a factor of 3 over the temperature range 290–320 K. In contrast to the UNIFAC-pLo method, the prediction errors from the methods were found to be correlated with pLo so that the other methods overpredicted pLo as volatility decreased.