Abstract
The solid to liquid phase transition of n-alkanes with more than ten carbon atoms is an interesting phenomenon relevant to many fields, from cosmetics to automotive. Here we report Raman spectroscopy of tetradecane, pentadecane and hexadecane as a function of temperature. In order to gain information on the structural changes that the hydrocarbons undergo during melting, and to determine the temperature and the speed at which the phase change occurs, their temperature-dependent Raman spectra are acquired. The spectra are analysed not only with respect to frequency shifts, band widths, and intensity ratio of certain bands, but also using a principal component analysis. The spectroscopic data suggest that the solid to liquid phase transition in hexadecane, differently from tetradecane and pentadecane, is almost instantaneous. Tetradecane shows a slightly faster transition than pentadecane. In addition, a rotator phase as an intermediate state between the liquid and crystalline solid phases is identified in pentadecane. Different characteristic features in the solid spectra of the hydrocarbons relate tetradecane and hexadecane to a tryclinic crystalline structure, and pentadecane to an orthorhombic structure.
Highlights
The crystallisation of n-alkanes is an interesting subject area to explore because of its relevance to important industrial applications, such as the processing of fats, surfactants, and oils.[1] n-Alkanes represent the main building blocks of complex organic compounds, such as lipids, surfactants, liquid crystals and polymers, but are the main constituents of traditional hydrocarbon fuels
The Raman spectra in the CH stretching and fingerprint region are studied with respect to bands position, frequency shifts, bands widths and intensity ratios of specific bands in order to identify the phase transition temperature and to investigate characteristic changes in the spectra
In this paper we investigated the solid to liquid phase transition of tetradecane, pentadecane, and hexadecane analysing their temperature dependent Raman spectra
Summary
The crystallisation of n-alkanes is an interesting subject area to explore because of its relevance to important industrial applications, such as the processing of fats, surfactants, and oils.[1] n-Alkanes represent the main building blocks of complex organic compounds, such as lipids, surfactants, liquid crystals and polymers, but are the main constituents of traditional hydrocarbon fuels. The liquid to solid phase transitions in n-alkanes become important for those compounds with more than ten carbon atoms as their melting point is above À30 1C. A greater stability is associated with even n-alkanes, which crystallize into the triclinic crystal structure This is consistent with a heterogeneous freezing mechanism, in which the nucleation starts at the free surface of the sample, leading to surface freezing.[1] X-ray and surface tension measurements have been used by Wu et al.[11] to observe the formation of a crystalline monolayer on the surface of liquid n-alkanes a few degrees above the bulk solidification temperature. Ocko et al.[12] studied the structure of the monolayer in detail determining its existence in alkanes with carbon number (n) of 16 o n o 50
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