The Effect of Quasispherical Solutes on the Smectic-A−Nematic and Nematic−Isotropic Phase Equilibria in p-n-Alkyl-p‘-Cyanobiphenyl Liquid Crystals

Abstract

Density as a function of temperature was measured in the nematic and isotropic phases in the immediate vicinity of the nematic-to-isotropic transition temperature (T NI) of p-n-hexyl-p‘-cyanobiphenyl (6CB) and p-n-octyl-p‘-cyanobiphenyl (8CB) and in the smectic-A and nematic phases in the immediate vicinity of the smectic-A-to-nematic transition temperature (T AN ‘) of 8CB. Similar measurements were made on dilute solutions of the quasispherical solutes Et4C (tetraethylmethane) and R4Sn (R = C2H5, n-C3H7, and n-C4H9) in 6CB and 8CB. Addition of these solutes led, as is expected for first-order phase transitions, to a nematic−isotropic two-phase region bounded by temperatures T N and T I, which are lower than T NI, and in the case of the 8CB systems to a smectic-A−nematic two-phase region bounded by temperatures T A and T N ‘, which are lower than T AN ‘. Volume changes at the transitions of the liquid crystals confirm that they are weakly first-order. βN and βI, the moduli of the slopes of the nematic and isotropic boundary lines, respectively, of the nematic−isotropic two-phase regions in the 6CB and 8CB systems were obtained assuming that the respective reduced temperatures T N* (= T N/T NI) and T I* (= T I/T NI) versus solute mole fraction (x 2) diagrams are linear. βA and βN ‘, similarly defined for the smectic-A−nematic two-phase regions in the 8CB systems, were obtained from the respective linear T A* (= T A/T AN ‘) and T N ‘* (= T N ‘ / T AN ‘) versus x 2 diagrams. The dependencies of βN and βA on solute and solvent sizes and the nature of the liquid crystalline phase are discussed.

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

“…The values for 6CB are in good agreement with those previously reported [8,12]. The density presents a jump at the nematic to isotropic transition, as expected taking into account that the transition is first order.…”

“…Therefore accurate determinations of the density of a liquid crystal about its transition temperatures are of great interest [2][3][4][5]. In literature [6][7][8][9][10][11][12] there are a number of studies concerning the density measurements for members of alkyl cyanobiphenyl homologous series. In this paper the results for the density vs. temperature in the nCB series are completed with new data for the compounds 3CB in the isotropic phase and for 4CB and 6CB in the nematic and isotropic phases.…”

Expected and Unexpected Behavior of the Orientational Order and Dynamics Induced by Azobenzene Solutes in a Nematic

“…The presence of nonmesomorphic solutes in a nematic LC can lead, in a temperature range close to the T NI , to the appearance of a biphasic region, consisting of both N and I phases which coexist in equilibrium. − This actually seemed to be the case for some of the samples doped with the cis azo-derivatives, whose ESR spectra exhibited a line shape that could be attributed to the superposition of two different spectra, one characteristic of a nematic phase and the other of an isotropic environment. To verify for all the samples the existence and the limits of the biphasic region, if any, we also performed a series of polarizing microscope observations under crossed polarizers.…”