Deviations of binary organic eutectic melt systems

The deviations resulting in owing to molecular interactions in the condensed phases, from idea behaviour of binary organic eutectic melt systems, namely, benzil–azobenzene, acenaphthene–benzil, naphthalene–phenanthrene, naphthalene–acenaphthene, benzoic acid–benzil, benzoic acid–phenylacetic acid and benzoic acid–salicylic acid have been presented quantitatively in terms of excess thermodynamic functions. Since direct experimental techniques for measuring these functions at the liquidus temperatures of the mixtures are firstly, not developed and secondly, by no means simple, the solidus–liquidus equilibrium data of the systems investigated by thaw-point melting technique, have been subjected to thermodynamic analysis to computing the activity coefficients of the eutectic phases for various compositions of their mixtures covered by the state diagrams. The excess functions: GE and SE for the pre-, post-, and eutectic compositions, have been calculated by utilising the computed activity coefficient data of the eutectic phases together with their excess chemical potentials μiE(i=1,2). GE rapidly falls to lower values on either side of the solidus–liquidus equilibrium curves of the systems and acquires minima at the eutectic compositions, which is consistent with the criteria of spontaneity. Contrariwise, SE shows maxima at the eutectic compositions, which is in accord with the Planck formulation S=k ln w (k and w, respectively being the Boltzmann constant and the weight of configuration of eutectic phase molecules), since three phases coexist at this composition. The reliability of the excess functions has been authenticated by the application of Guggenheim lattice theory which offers supporting compliment to the essence of thermodynamical model for the eutectic phenomenon.