Magnetic effect on the phase transitions of n-docosane by means of a high resolution and super-sensitive DSC
Introduction
The development of refrigerator-cooled super-conducting magnet has made it possible to obtain a strong magnet field easily, for example, 10T. It has become possible to study the magnetic effect of paramagnetic and diamagnetic substances using such a magnet [1], [2], [3], [4]. These studies can make it clear and definite of the magnetic effect of these substances quantitatively, being helpful on the safer and more reliable application of these materials.
Since the magnetic energy at 10T is still weaker than the thermal energy kT at room temperature, it is necessary to construct a highly sensitive apparatus under the strong magnetic field in order to detect the magnetic effect of diamagnetic substances. If we want to detect the magnetic effect on the phase transition resulting from the orientation of diamagnetic organic materials, the following three conditions should be fulfilled [5].
- (1)
The anisotropy of diamagnetic susceptibility is sufficiently large.
- (2)
The molecules are sufficiently mobile at the measuring temperature.
- (3)
The sensitivity and the resolution of temperature in the measuring system are sufficient.
In the previous studies [5], [6], we have developed a high resolution and super-sensitive differential scanning calorimeter capable of measuring heat as small as the order of 20 nW with a temperature resolution of 1 mK working in a magnetic bore and measured the magnetic effect on the transitions of n-C32H66 using the calorimeter. It has been shown that the solidification temperature of n-C32H66 was shifted to a higher temperature by 22–60 mK depending on the cooling rate by applying magnetic field of 5T. This temperature shift due to the magnetic orientation was about four orders of magnitude larger than the calculated value based on the Clapeyron equation. The magnetic effect on the transitions of EBBA (N-p-ethoxybenzylidene-p′-butylaniline) [7] was also measured and the temperature due to liquid crystal to isotropic liquid transition was shifted to a higher temperature by 34 mK by applying magnetic field of 5T. The experimental temperature shift due to the transition was about two orders of magnitude larger than the calculated value in this case.
The n-alkanes, CnH2n+2, have many structurally different phases depending on the carbon number n and temperature [8], [9], [10], [11], [12], [13], [14]. The n-alkanes with 9 ≦ n (odd) ≦ 39 and with 20 ≦ n (even) ≦ 38 [15], have characteristic crystalline phases, which are commonly called as rotator phases (R phases). In these phases, molecules revolve about their long axes and behave as if they were in a liquid state. The crystals undergo solid–solid phase transition at a few degrees below the melting point from a low temperature ordered phase to rotator phases. Sirota et al. [13] found five R phases, which are characterized in terms of the molecular tilt, side packing, azimuthal ordering and layer stacking by X-ray scattering study. They [14] also made a calorimetric study of the n-alkanes: CnH2n+2 (20 ≦ n ≦ 30) using an adiabatic scanning calorimeter and found rotator to rotator transitions corresponding to the rotator phases found by the X-ray scattering study [13].
Wu et al. [16], [17] discovered the surface freezing phenomenon, in which a thin layer at an air/liquid interface crystallizes at about 3 K higher than the bulk melting point in molten normal alkanes CnH2n+2 (16 ≦ n ≦ 50) using X-ray and surface tension measurements. Yamamoto et al. [18], [19], [20] investigated this phenomenon using the molecular simulation method.
In our previous study [21], the phase transitions of n-C22H46 have been measured using a high resolution and super-sensitive DSC. The crystal–rotator transition and the rotator–melt transition were observed both in the heating and in the cooling run; although a rotator–rotator transition was observed only in the cooling run. A new peak was detected at about 3 K above the melting temperature of bulk sample. It is considered to be due to the surface freezing phenomenon. When the cooling rate was below 0.1 mKs−1, the peak due to solidification was separated into two peaks showing the existence of another phase transition.
Since n-alkanes have linear chains and diamagnetic anisotropy and have multiple phase transitions near the melting point, the three conditions to detect the magnetic effect described above may be fulfilled at the temperatures around the rotator transition and above. The comparison of the magnetic effect on the phase transitions among n-C22H46, n-C32H66 and EBBA and the study of the magnetic effect on the surface freezing phenomenon are very interesting. It will be helpful on the study of the magnetic effect from microscopic perspective, such as the chain length, the molecular structure and molecular arrangement.
In the present study, the magnetic effect on the phase transitions and the surface freezing phenomenon of docosane have been measured using a high resolution and super-sensitive DSC [5] working in a magnetic bore and the magnetic effect have been discussed.
Section snippets
DSC
The magnet is the refrigerator-cooled superconducting magnet with a maximum field 6T (JMTD-6T100HH). In the present study, the magnet field was set to be 5T. We have used the high resolution and super-sensitive heat-flux type differential scanning calorimeter [6] for the measurement of the magnetic effect of 5T on the phase transitions of docosane. The schematic drawing of the calorimeter is shown in Fig. 1. The DSC was vertically set in the cylindrical magnetic bore with 100 mm in diameter. The
Results
The thermal measurements of C22H46 transitions were made between 306 and 323 K at very slow heating and cooling rates from 0.05 to 0.5 mKs−1. We mainly show and discuss the results of the measurements with heating and cooling rate of 0.5 mKs−1 in this study, which were repeated and have a wide measurement temperature range to cover all of the transitions.
The DSC curve of the second heating run at the rate of 0.5 mKs−1 is shown in Fig. 2, where we can see two large endothermic peaks at 317.2 K (peak
Discussion
The DSC curves with and without magnetic field for solid–solid transition and melting or solidification of C22H46 show that the low temperature phases become relatively stable by the application of magnetic field. This effect is considered to be due to the orientation of linear molecules of n-C22H46 by magnetic field, resulting from the difference in diamagnetic susceptibility between parallel and perpendicular axis of molecule. When we consider linear molecules of docosane at temperatures near
Conclusions
- 1.
The transition temperatures due to solid–solid transition and due to melting or solidification were shifted to higher temperatures by about 10 mK by applying magnetic field of 5T. This effect is considered to be due to the orientation of linear molecules of n-C22H46 perpendicularly to the magnetic field.
- 2.
The transition temperatures due to melting and forming of the ordered monolayer on the surface of the liquid alkane were shifted to lower temperatures by about 10 mK by applying magnetic field of 5
Acknowledgements
The author would like to thank Associate Professor Tsunehisa Kimura of Faculty of Technology, Tokyo Metropolitan University for his helpful discussions. This work has been partially supported by the Japan Society for the Promotion of Science through the Research for the Future Program.
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