Thermochimica Acta 496 (2009) 13–17 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Thermal, solid–liquid equilibrium, crystallization, and microstructural studies of organic monotectic alloy: 4,4 ′ -Dibromobiphenyl–succinonitrile R.N. Rai ∗ , R.S.B. Reddi Department of Chemistry, Banaras Hindu University, Varanasi 221005, India article info Article history: Received 8 May 2009 Received in revised form 5 June 2009 Accepted 10 June 2009 Available online 18 June 2009 Keywords: Phase diagram Miscibility gap Thermochemistry Eutectic Organic monotectic abstract The phase equilibrium data on an organic analogue of a metal–nonmetal system involving succinonitrile–4,4 ′ -dibromobiphenyl shows two immiscible liquid phases are in equilibrium with a single liquid phase. The phase diagram study infers the formation of a eutectic and a monotectic with a large miscibility gap containing 0.9997 and 0.15mole fractions of succinonitrile, respectively. The consolute temperature being 67.0 ◦ C above the monotectic horizontal. The thermal study such as heat of mixing, entropy of fusion, roughness parameter, interfacial energy and excess thermodynamic functions were calculated from the enthalpy of fusion values, determined using differential scanning calorimeter (DSC) method. The effects of solid–liquid interfacial energy on morphological change of monotectic have also been discussed. The microstructures of monotectic, eutectic and pure components show their peculiar characteristic features. © 2009 Published by Elsevier B.V. 1. Introduction The investigations on the temperature dependent solidification behaviour of monotectic alloy are of potential importance both from fundamental understanding of the development of self-lubricating alloys and for industrial applications [1,2]. Although, metallic sys- tems constitute an interesting area of investigations [3–5], they are not suitable for detail study due to high transformation temperature and wide density difference of the components involved. How- ever, low transformation temperature, transparency, wider choice of materials and minimised convection effects are the special fea- tures that have prompted a number of research groups [6,7] to work on organic eutectics, monotectics and olecular complexes. As such organic systems are used as model systems for detailed investigation of the parameters which control the mechanism of solidification which decides the properties of materials. In addition, these materials are being used for various physicochemical inves- tigations for their use for non-linear optical effects and different electronic applications [8–10]. The monotectic alloys have been less studied due to several dif- ficulties associated with the miscibility gap systems while some of the articles [2,11,12] explain various interesting phenomena of monotectic alloys. The main problem arises due to a wide freez- ing range and large density difference between two liquid phases. The role of wetting behaviour, interfacial energy, thermal conduc- ∗ Corresponding author. Tel.: +91 0542 6701597; fax: +91 0542 2368127. E-mail address: rn rai@yahoo.co.in (R.N. Rai). tivity and buoyancy in a phase separation process has been the subject of great discussion. 4,4 ′ -Dibromobiphenyl (DBBP) is a mate- rial of high enthalpy of fusion (28.38 kJ/mole) and simulates the nonmetallic solidification (faceted morphology) where as succi- nonitrile (SCN) is a material of low enthalpy of fusion (3.70 kJ/mole) and corresponds the metallic solidification (nonfaceted morphol- ogy). Therefore, the present DBBP–SCN system is very good organic analog of metal–nonmetal systems like Al–Si and Zn–Bi. In the present paper, the details concerning phase diagram, thermochem- istry, linear velocity of crystallization at different undercoolings, heat of fusion, Jackson’s roughness parameter, interfacial energy and microstructure of DBBP–SCN system are reported. 2. Experimental 2.1. Materials and purification Succinonitrile, obtained from Aldrich, Germany, was purified by repeated distillation under reduced pressure while 4,4 ′ - dibromobiphenyl (Aldrich, Germany) was used as received. The melting temperatures of DBBP and SCN were found to be 167.5 ◦ C and 56.5 ◦ C, respectively which are quite close to their respective values reported [13]. 2.2. Phase diagram The phase diagram of DBBP–SCN system was determined by the thaw–melt method in the form of temperature–composition curve. In this method [14,15], mixtures of two components covering the 0040-6031/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.tca.2009.06.012