A Possible Critical Point for Nematic Order on the Basis of Landau Free Energy Having Dual Instabilities for Nano-Segregated Smectic Liquid Crystal

Landau expansion of free energy assuming dual instabilities for nano-segregated SmA phase is analyzed. In addition to known phase sequences (on cooling, disordered isotropic liquid => nematic phase!smectic phase, and disordered isotropic liquid => smectic phase), a new sequence (disordered isotropic liquid => density wave with subsidiary nematic order => smectic phase) and the existence of a critical point are demonstrated in case where the instability for density wave formation occurs at a higher temperature.
(Soft Matter, 11, 8493-8498 (2015))

Molecular Thermodynamics of Thermotropic Liquid Crystals while Paying a Special Attention on "Core and Tail" Structure

Through thermodynamic analyses of several liquid crystals (LCs) and related compounds, discussed are the roles played by alkyl chains attached to mesogenic cores. The roles identified include, i) an extension of molecular length, enhancing the shape anisotropy required for the formation of LCs by excluded volume effects, ii) an entropy reservoir, which, through the entropy term in Gibbs energy, stabilizes mesophases and governs the phase relation, and iii) intramolecular solvent (self-solvent), which induces micro-phase separated LCs including SmA phase. The last aspect opens the possibility that science and technology of thermotropic and lyotropic LCs be combined under the unified view of LCs. The implication of such a view on structures of layered LCs such as SmA phase is briefly discussed.
(Netsu Sokutei (Calorim. Therm. Anal.), 40(1), 2-9 (2013), account in Japanese in recognition of JSCTA Award 2012)

What Roles Do Alkyl Chains of Mesogens Play?

Through thermodynamic analyses of several liquid crystals (LCs) and related compounds, discussed are the roles played by alkyl chains attached to mesogenic cores. The roles identified include, i) an extension of molecular length, enhancing the shape anisotropy required for the formation of LCs by excluded volume effects, ii) an entropy reservoir, which, through the entropy term in Gibbs energy, stabilizes mesophases and governs the phase relation, and iii) intramolecular solvent (self-solvent), which induces micro-phase separated LCs including SmA phase. The last aspect opens the possibility that science and technology of thermotropic and lyotropic LCs be combined under the unified view of LCs.
(Liq. Cryst. XVI, Proc. SPIE, 8475, 84750C (2012), account)

Experimental Assessment of Quasi-Binary Picture of Thermotropics:
Induced Smectic A Phase in 7CB-n-Heptane System

Quasi-binary (QB) picture of thermotropics, which regards a neat thermotropic liquid crystal as a binary system consisting of (semi)rigid core and molten alkyl chain, was assessed experimentally for the most famous (and representative) thermotropic mesogenic series nCB. By adding n-heptane as solvent, the smectic A phase was induced in 7CB-n-heptane system. Small angle X-ray diffraction showed that the QB picture holds not only in the phase behavior but also in the structure. It is suggested that the melting of 8CB and 9CB to isotropic liquid via smectic and nematic liquid crystals can be understood as a thermotropic-lyotropic crossover.
(J. Chem. Phys., 135, 044705 (2011))

Microphase-Separated Multicontinuous Phase in Low-Molecular Mass Thermotropic Liquid Crystal

Extensive application of chemical thermodynamics to exotic aggregation formed in thermotropic liquid crystals is briefly described. Through thermodynamic analyses and considerations of experimental results on liquid crystals, the unexpected sharing of common properties by thermotropic and lyotropic liquid crystals is demonstrated.  In some thermotropic liquid crystals, the terminal alkyl chain attached to the molecular core is highly disordered as indicated by the magnitude of configurational entropy.  The molten chain serves as intramolecular solvent (self-solvent), as evidenced by the close similarity between phase diagrams against chain length and composition in binary system with n-alkane.  These facts lead to the quasi-binary picture of thermotropic liquid crystals.  Consideration on thermodynamic potential expanded in terms of density fluctuation gives a new insight on multicontinuous phases formed in simple systems consisting of anisotropic, rodlike particles.
(Pure & Appl. Chem., 81, 1783-1798 (2009), account)

Thermotropic and Lyotropic Liquid Crystals: Are They So Different?

Through thermodynamic analyses and considerations on the existing experimental results on cubic mesophases, the unexpected sharing of the common properties by thermotropic and lyotropic liquid crystals is demonstrated. In some thermotropic liquid crystals, the terminal alkyl chain attached to the molecular core is highly disordered as indicated by the magnitude of configurational entropy. The melt chain serves as intramolecular solvent (self-solvent), as evidenced by the close similarity between phase diagrams against chain-length and composition in binary system with n-alkane. These facts lead to the quasi-binary (QB) picture of thermotropic liquid crystals. The QB picture affords the basis to establish the structural models of cubic mesophases in classic cubic mesogens, and that to deduce the entropy difference between flat surfaces and complex surface(s) having complicated geometry such as Gyroid, a triply periodic minimal surface (TPMS).
(Netsu Sokutei, 32, 133-140 (2005), review in Japanese)

Quasi-Binary Picture of Thermotropic Liquid Crystals and Its Application to Cubic Mesophases

A quasi-binary (QB) picture of thermotropic liquid crystals is proposed on the basis of thermodynamic observations. The experimental conformational entropy of long alkyl chains attached to a (semi)rigid core of mesogenic molecules indicates that the chain is highly disordered in liquid crystalline states. These disordered chains serve as “intramolecular solvent” or “self-solvent” judging from a close resemblance between phase diagrams of neat (against chain length) and binary (against composition) systems. The application of the QB picture to the classic examples of thermotropic cubic mesophases (in ANBC series) shows that the essential structural motif is triply periodic minimal surface.
(Chem. Phys. Lett., 366, 56-61 (2002))

Thermodynamic Implication of the Dependence of Mesomorphic Transition Entropy on Chain-Length: Smectic A - Nematic Phase Transition

Meaning of the alkyl chain-length dependence of transition entropy in liquid crystal is discussed and clarified in a long-chain region. A method to deduce contributions of the alkyl chain and the molecular core is proposed. Application of the method suggests that, in most smectic A (SmA) - nematic (N) phase transitions, not only a molecular core (modeled by a hard-rod) but also a (terminal) alkyl chain is disordered upon the SmA-N transition.
(J. Therm. Anal. Calor., 70, 345-352 (2002))

See also Isotropic Liquid Crystals

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