Studies on Tilted and Untilted Rotator Phases of Long Chain Alkanes
[1]
Prabir Kumar Mukherjee, Department of Physics, Government College of Engineering and Textile Technology, Serampore, India.
Long chain hydrocarbon molecules show significantly similar thermal properties and phase sequences. These chain alkanes (CnH2n+2 from n =20 to 33 and above) exhibit different rotator phases depending on their chain lengths. The phase behaviour of these rotator phases has been studied both experimentally and theoretically. All these rotator phases correspond to different orderings and dynamic behaviours caused by the intermolecular interaction field coupled with thermal agitation. A unified description of all probable phase sequence irrespective of chain lengths and the reason behind the generation of tilt in relatively longer chain alkanes is presented. We calculate the Fourier transform of the approximate atom to atom potential and thereby construct the free energy density. We obtain all theoretical phase sequences in terms of structural order parameters. We compare these phase sequences with those obtained from Landau mean-field theory and find explicit chain length dependence with molecular tilt within Landau theory framework as well. The theoretical predictions conform with existing experimental studies.
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