Study of the temperature and enthalpy of thermally induced phase-transitions in n-alkanes, their mixtures and Fischer-Tropsch waxes

A precise determination of the temperatures and enthalpies of thermally ind uced various phases of n-alkanes, especially those having the molecular wei ghts in the petroleum wax range, have been done by using a differential sca nning calorimeter (DSC). These phase transitional parameters are shown to b e dependent on the rate of heating/cooling, particularly on the faster rate s. Also, the parameters measured during heating cycle (melting) differ from those obtained during the cooling cycle (crystallization). The typical sha pe of the solid-solid transition curve in the DSC thermograms(cooling cycle ) of even n-alkanes readily distinguishes these from the odd n-alkanes, whi ch is greately influenced by the increase in the carbon numbers in the chai n (n-C-34 and beyond) and the purity of the sample. High molecular weight n -alkanes also showed instability in the structures at the phase transitions , The formation of solid solutions of n-alkanes have been studied by measurin g phase transitional parameters and studying nature of the DSC thermograms of various binary and multicomponent mixtures of n-tetracosane (n-C-24) wit h other n-alkanes. It is observed that the immiscibility between two n-alka nes of the binary mixture begins when chain-length difference between them is more than four carbon atoms. In between the two extremes of solid soluti on and eutectic formations, there also existed the partial miscibility of t wo n-alkanes, particularly at high temperatures. The DSC thermograms of multicomponent n-alkane mixtures and Fischer-Tropsch waxes revealed that perfect solid solution is formed when the involved n-a lkanes have carbon number very close to each other or the carbon number dis tribution is in a narrow range with smaller chain n-alkanes dominating.