Raman microscopic characterization of phase separation in binary n-alkane mixtures

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Abstract

Binary solid mixtures of linear n-alkanes were studied using the micro-Raman spectroscopy method. The process of self-organization into mono-component domains was monitored by analysis of shape and shift of spectral bands of alkane samples. One of the mixture components was deuterated. Before measurements at room temperature, the solid samples were heated, melted, mixed and quenched. The influence of difference in length of molecular chain and molecular ratio of n-alkanes mixture on the effectiveness of separation process was examined. To show the dynamics of the phase separation process on the samples surface the spectral Raman imaging was applied.

Introduction

The process of phase separation in n-alkanes solid mixtures is an important model for understanding the behavior of petroleum waxes and other mixed solid phases of chain molecules in time. The phenomenon of phase separation of linear n-alkanes in binary solid solutions has been studied for many years. Certain methods have been applied in order to monitor this process: X-ray diffraction [1], differential scanning calorimetry [2], electron diffraction [3], small-angle neutron scattering [4], [5], [6], and infrared absorption spectroscopy [7]. In the past interesting Raman results for n-paraffins, investigated as a function of temperature, had been received by Boerio and Koenig [8]. Later on the Raman study had been carried out only for longitudinal acoustic mode of n-alkanes [9]. On the other hand, a number of theoretical [10] and computer simulation studies [11], [12] of simple mixtures of n-alkanes has been carried out, as a useful step in direction to characterize the more complex hydrocarbon systems of polymers and lipid biopolymers [13]. The micro-Raman spectroscopy has been applied in this study to enable the observation of changes of molecular structure on selected sample surface (Raman imaging), which was made possible by the coupling of Raman spectrometer with confocal microscope and computer controlled XY mapping stage. The object of our investigations were n-alkanes in solid state, of the general equation CnH2n+2, from which we created binary mixtures and where one of the components had replaced H atoms by deuterons in over 90%. Investigation had been done for samples with different lengths of n-alkanes chains and molar ratio. The spectra showed changes of intensity of the bands associated with mixtures organizing into domains in time. The intensity changes of these bands and bands splitting were the result of influences of two factors: Davydov splitting (factor group splitting) and Fermi resonance [14]. In the mixtures of linear n-alkanes it had been also possible to follow in time the shift of bands relative to their original position. This was connected with the dynamics of the separations processes. Periodical observation of the sample surface of the mixtures by Raman imaging can clearly show self-organization of hydrocarbon components over time. The aim of our study was the analysis of changes occurring in time on the selected wax mixtures spectra, for various length of molecular chain of components and their different molecular ratio.

Section snippets

Experimental

Samples were prepared in the following molar ratio: (3:1) (2:1) (1.2:1) (1:1) (1:2) and (1:3) of deuterated component and hydrogenated component. The whole of the ingredients was mixed before and after heating up to temperature above these hydrocarbons' melting point, and then quenched. Samples prepared in this way were measured by a Lab Ram System spectrometer, comprising an Olympus X-40 microscope with image magnification of 100. It was equipped with a grating monochromator and a

Results

We consider the changes of methylene bending mode intensity on the spectrum of the studied mixtures. Part of the Raman spectrum for mixture C24H50:C28D58, prepared in molar ratio 1:3, which represents bending oscillations of functional group –CH2–, is presented in Fig. 1. During the first few hours since the preparation of the sample it was noticed, that the most intensive band appeared at frequency 1435 cm−1. After 20 h we observed that the 1412 cm−1 band and 1435 cm−1 band had approximately the

Summary

The process of separation in hydrocarbon binary mixtures can be analyzed with the use of Raman method. This approach proves to be very useful. The changes in the demixing process of hydrocarbon chains in binary mixtures depend on a few factors: time, molar ratio and differences in n-paraffins length. For samples prepared in molar ratio 1:1, the self-organization process in domains occurs faster then for other ratios of mixtures. It has been clearly observed, that the bigger the difference of

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