Molecular Arrangement in Mesophases of Some p-n-Alkoxybenzoic Acids

Publisher Website
Google Scholar
Abstract
The properties of an even homologous series of alkoxy benzoic acids have been investigated in the crystalline and mesomorphic states by means of thermal microscopy, x-ray scattering and dilatometry. Structural parameters such as inter-lamellar distance d, and the molar volume Vm were studied as a function of the number n of methylene units of the alcoxy radical. In the crystalline state and the smectic C state the variation of d and of Vm with n is perfectly linear. The angle of tilt of the paraffinic chains α′ and of the aromatic stems φ was determined in the crystalline state. It was found that both moieties have the same magnitude of the tilt. In the smectic C state the value of α′ was found to be 25° as compared to 60.7° in the crystalline state indicating a “pulling up” of hydrocarbon tails when passing from the crystalline state to the smectic state. The passage from crystalline into the smectic state is also accompanied by a strong increase in Vm. Analysis of data indicates that this increase is entirely due to the aromatic part of the molecule while the aliphatic chains have the same Vm as in the crystalline state. It is concluded that while in the crystalline state the cohesion of the phase is insured to a large extent by dimerized, hydrogen bonded molecules, in the smectic C state the cohesion is insured by attraction between extended aliphatic chains. Evidence is also given for the formation of cybotactic groups in the nematic phase of octyloxy benzoic acid.