Studies on Tilted and Untilted Rotator Phases of Long Chain Alkanes
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.
Rotator Phases, Alkanes, Phase Transitions
Ungar G, Structure of rotator phases in n-alkanes. J. Phys. Chem., 1983, 87: 689-695.
Sirota E B, King Jr. H E, Singer D M, Shao H H, Rotator phases of the normal alkanes: An x-ray scattering study. J. Chem. Phys., 1993, 98: 5809-5824.
Sirota E B, Singer D M, Phase transitions among the rotator phases of the normal alkanes. J. Chem. Phys., 1994, 101: 10873-10882.
Sirota E B, Remarks concerning the relation between rotator phases of bulk n-alkanes and those of Langmuir monolayers of alkyl-chain surfactant on water. Langmuir, 1997, 13: 3849-3859.
Dutta S and Prasad S K, Confinement-driven radical change in a sequence of rotator phases: a study on n-octacosane. Phys. Chem. Chem. Phys., 2018, 20: 24345-24352.
Dutta S, Srikanthamurthy S, Mukherjee P K and Prasad S K, Nanometer confinement-driven promotion and stabilization of a hexatic phase intervening between ordered rotator phases. J. Phys. Chem. B, 2018, 122: 10953–10963.
Kotula A P, Hight Walker A R and Migler K B, Raman analysis of bond conformations in the rotator state and premelting of normal alkanes. Soft Matter, 2016, 12: 5002-5010.
Schappert K, Gemmel L, Meisberger D and Pelster R, Elasticity and phase behaviour of n-heptane and n-nonane in nanopores. EuroPhys. Lett., 2015, 111: 56003-1-6.
Kumar M V, Prasad S K, Rao D S S S and Mukherjee P K, Competition between anisometric and aliphatic entities: an unusual phase sequence with the induction of a phase in an n-alkane–liquid crystal binary system. Langmuir, 2014, 30: 4465–4473.
Paoloni S, Mercuri F, Zammit U, Leys J, Glorieux C and Thoen J, Analysis of rotator phase transitions in the linear alkanes hexacosane to triacontane by adiabatic scanning calorimetry and by photopyroelectric calorimetry. J. Chem. Phys., 1018, 148: 094503-1-10.
Wurger A, Rotator phases and herringbone order in Langmuir monolayers and alkanes. J. Chem. Phys., 2000, 112: 3897-3908.
Mukherjee P K, Phase transitions among the rotator phases of the normal alkanes: A review. Phys. Rep., 2015, 588: 1-54.
Mukherjee P K, Effect of gauche molecular conformations and molecular flexibility on the rotator phase transitions of alkanes. J. Phys. Chem. B, 2012, 116: 1517-1523.
Mukherjee P K, Confinement-driven rotator-I to rotator-V phase transition of alkane. EuroPhys. Lett., 2017, 118: 26002-1-5.
Mukherjee P K, Effect of nanoparticles on the RII-RI-RV rotator phase transitions of alkane. Chem. Phys. Lett., 2017, 681: 75-79.
Flory P J, Statistical Mechanics of Chain Molecules. John Wiley and Sons, New York, 1976.
Birshtein T M, Ptitsyn O B, Conformations of Macromolecules. (translated from the 1964 Russian ed. S. N. Timashe_ and M. J. Timasheff), Interscience, New York, 1966.
Scott R A, Scheraga H A, Method for calculating internal rotation barriers. J. Chem. Phys., 1965, 42: 2209-2215.