Fugacity coefficients in vapor-phase mixtures of water and carboxylic acids

Abstract

Strong nonideality in carboxylic acid vapors is attributed to the formation of dimers and trimers. To a good approximation, the enthalpy and entropy of dimerisation are the same for all carboxylic acids; similarly, the enthalpy and entropy of trimerisation are the same for all carboxylic acids. The weak nonideality in the water vapor can also be attributed to dimerisation. The cross-dimerisation between water and carboxylic acid monomers is probably very weak. Based on the ‘chemical theory’ of gas imperfection, equations are given for the fugacity coefficients of water and carboxylic acid in their binary vapor mixtures. The fugacity coefficient of the carboxylic acid deviates significantly from unity even at one atm. This large deviation must be taken into account in the calculation of activity coefficients in aqueous solution.