Alkyl chains acting as entropy reservoir in liquid crystalline materials

Abstract

The roles played by the conformational disordering of alkyl chains in determining the aggregation states of matter are reviewed for liquid crystalline materials from a thermodynamic perspective. Entropy, which is one of the most macroscopic concepts but which has a clear microscopic meaning, provides crucial microscopic information for complex systems for which a microscopic description is hard to establish. Starting from structural implication by absolute (third-law) entropy for crystalline solids, the existence of successive phase transitions caused by the successive conformational melting of alkyl chains in discotic mesogens is explained. An experimental basis is given for the "quasi-binary picture" of thermotropic liquid crystals, i.e., the highly disordered alkyl chains behave like a second component (solvent). A novel entropy transfer between the "components" of a molecule and the resulting "alkyl chains as entropy reservoir" mechanism are explained for cubic mesogens.

Universality of Molten State of Alkyl Chain in Liquid-Crystalline Mesophases: Smectic E Phase of 6-Alkyl-2-phenylazulene

“…10 J K ¹1 (mol of CH 2 ) ¹1 ). 19,20 On the other hand, a much smaller slope (ca. 5 J K ¹1 (mol of CH 2 )…”

“…2,3 Indeed, the present authors have shown that the large entropy capacity, in some cases, governs the phase sequence against temperature variation. 18,19,2123 Considering that equilibrium is characterized by the minimum Gibbs energy, the large contri- bution of the chain entropy strongly suggests that the molten state is not an accidental situation but the prerequisite for the appearance of most real liquid-crystalline mesophases as stable phases because most mesogens have flexible chain(s) at the end(s) of the core. Indeed, no simple molecular model has been reported for SmE phase to the authors' knowledge.…”

“…2429 Certainly the perception of the chain entropy as an important factor for LC formation contradicts the traditional in part, because thermotropic LC is now treated as quasi-binary consisting of rigid cores and flexible chains. 19,30,31 It is, however, emphasized that the contribution is not enthalpic but rather entropic. 32 Implication of this view is under study at this laboratory.…”

“…10 J K ¹1 (mol of CH 2 ) ¹1 ) 19,20 in comparison with both the change in entropy expected for orientational (20 30 J K ¹1 mol ¹1 ) and positional melting (14 J K ¹1 mol…”

Experimental assessment of quasi-binary picture of thermotropics: Induced smectic A phase in 7CB–n-heptane system

“…However, the experimental confirmation through the analyses of entropy itself were performed only recently. 16,17 Indeed, the molten state of chain in LC states were established through the analysis of absolute entropy for some mesogens showing cubic LC phases such as 4 -alkyloxy-3 -nitrobiphenyl carboxylic acid (ANBC(n), Fig. 1).…”