Vapour-liquid equilibria in associating solutions

Isobaric vapour-liquid equilibria data were measured for the binary mixtures of acetic acid-propionic acid and formic acid-propionic acid at 760 mm Hg. The data were tested against a new thermodynamic model which extends the one proposed by Marek in that it is suitable for the case of binary mixtures containing two associating species and takes into account hetero-dimerization in the vapour phase. The proposed model converts into that of Christian for the particular case of an ideal liquid solution. The model fits well the system acetic acid-propionic acid with regard to the thermodynamic consistency of the overall activity coefficients, but does not fit the second mixture so well, probably because higher degrees of polymerization have to be included in it.