Application of additive estimation methods to vaporization properties of liquids. n-Alkanes

A modified group-additive method for estimation of standard Gibbs energies of vaporization, standard enthalpies of vaporization and standard entropies of vaporization is developed and tested using n-alkanes as a model series. To obtained estimates of these vaporization properties for individual n-alkanes, contributions from each of the methyl and methylene groups that compose the n-alkane are summed and then added to a term that accounts for contributions from relative liquid- and gas-phase molar volumes. To obtain vapour pressures from this method, first the estimated value for the standard Gibbs energy of vaporization is converted into the ideal-gas vapour pressure (i.e. fugacity) of the pure liquid. This hypothetical vapour pressure is then converted to the actual vapour pressure by explicitly accounting for any effects of vapour non-ideality. This additive method provides a convenient and highly accurate means of estimating vapour pressures of low-volatile n-alkanes for which contributions of vapour non-ideality to the magnitude of the vapour pressure may be ignored. It is also found that after explicitly correcting for vapour non-ideality using available non-additive estimation methods, vapour pressures up to ca. 0.5 atm may be estimated to a level of accuracy near that of experiment.