Some physicochemical studies on organic eutectics and molecular complex:: Urea-p-nitrophenol system

The phase diagram of urea-p-nitrophenol system, in the form of a temperature-composition curve, shows the formation of a 1 :1 molecular complex surrounded by two eutectics containing 0.216 and 0.777 mole fraction of p-nitrophenol. Data on growth velocity (v), obtained by measuring the rate of movement of the interface at different undercoolings (Delta T), suggest that they obey the Hillig-Turnbull equation, i.e., v = u (Delta T)(n), where u and n are constants depending on the nature of materials involved. From the heat of fusion values, determined by the differential scanning calorimetry (DSC) method, heat of mixing, entropy of fusion, roughness parameter, interfacial energy, radius of the critical nucleus, and the excess thermodynamic functions were calculated. While the x-ray diffraction data show that the eutectics are not mechanical mixtures of the components under investigation, the microstructural investigations give their characteristic features.