The thermodynamics of hydrocarbon solutions - II. The systems benzene+diphenyl, benzene+diphenylmethane and benzene+dibenzyl

Abstract

The vapour pressures of benzene solutions of diphenyl, diphenylmethane and dibenzyl have been measured over the temperature range 25 to 75° C. Small deviations from Raoult’s law are observed, which are positive for diphenyl and negative for diphenylmetharie and dibenzyl solutions. Activity coefficients ( y ) have been calculated after correcting the data for imperfection of the benzene vapour. The final results are compared with the theoretical equations deduced by Orr and by Guggenheim for mixtures of monomers and dimers.- In the range of mole fractions up to 0-6 of non-volatile component the agreement between theory and experiment is within + 0-0015 in log 10 y when the energy of interaction between molecules is taken as independent of temperature and the number of nearest neighbours (z) is six. The energy of interaction in the case of diphenyl leads to a heat of mixing in satisfactory agreement with that measured calorimetrically by Tompa. The energy of mixing Nw , where w refers to z nearest neighbour contacts, and N is the Avogadro number, is 640 J (153 cal) for diphenyl, 264 J (63.0 cal) for diphenylmethane, and 295 J (70.5 cal) for dibenzyl. For diphenyl solutions there is evidence of a systematic deviation between theory and experiment which exceeds the limits given above only in concentrated solutions. More detailed analysis suggests that the discrepancy is to be attributed to deviations of the entropy from the theoretical value, but experiments of a very much higher precision than has so far been attained in work of this kind would be needed to confirm this unequivocably. Baxendale & Eniisttin’s conclusions that both heat and entropy effects contribute to the deviations, and that the parameter w depends on the composition of the solution, are considered to be premature.