The entropy effects in binary mixtures of polar mesogenic solvent/nonpolar solute.

The paper concerns two aspects of the entropy in mesogenic systems: (i) the entropy jump (Delta S (0) NI) at the phase transition from the isotropic liquid (I) to the nematic liquid crystalline state (N), and (ii) the entropy increment (Delta S) caused by the ordering action of the probing electric field applied to the dipolar system. The system studied are the mixtures of strongly polar mesogenic solvent n-hexylcyanobiphenyl (C 6H 13PhPhCN, 6CB) and the nonpolar nonmesogenic admixture 4-ethylcyclohexyl-4'- n-nonylphenyl (C 2H 5CyHxPhC 9H 19, 2CyPh9). The entropy jump at the I-N phase transition in pure 6CB [Delta S (0) NI= 1.

Dynamics of the self-assembling of mesogenic molecules in the prenematic region of isotropic liquid.

The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematic region. A perturbation of the equilibrium by a static electric field of high strength (E0 approximately 10(7) V/m) is followed by a nonlinear dielectric relaxation process that, measured with the probing electric field E(omega) of small amplitude and variable high frequency (up to 100 MHz), reflects the rate of the domains formation. The fraction of the mesogenic molecules involved in the prenematic domains at different temperatures is estimated.

Contribution to understanding of the molecular dynamics in liquids.

The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C(7)H(15)PhPhCN, 7CB (molecular dipole moment mu approximately 5D) and 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene, C(6)H(13)CyHxPhNCS, 6CHBT (mu approximately 2.5D); advantageously, the temperatures of the I-N phase transition for the two compounds are very close to each other (T(NI) = 316.6 +/- 0.

Prenematic behavior of the electric-field-induced increment of the basic thermodynamic quantities of isotropic mesogenic liquids of different polarity.

Temperature dependences of the static dielectric permittivity and its derivative, obtained for isotropic mesogenic liquids composed of the molecules of different polarity in relation to the basic thermodynamic quantities (internal energy, entropy, and Helmholtz free energy), are analyzed. A role of the molecular polarity in the dielectric behavior of the liquids in the vicinity of the isotropic to nematic phase transition is discussed.

Dielectric and viscous properties of mesogenic n-nonylcyanobiphenyl (9CB).

Static and dynamic linear dielectric properties and shear viscosity of freely flowing 9CB in the isotropic (I), nematic (N) and smectic A (S(A)) phases were investigated. Pretransitional effects observed in the vicinity of the I to N phase transition manifest themselves as anomalies in the temperature dependence of the static permittivity, the rotational diffusion exponent and the activation energy of molecules rotating around their short axis. As a presmectic effect, a jump in the temperature dependence of the shear viscosity of freely flowing nematic 9CB is observed at about 0.

Pretransitional critical-like behavior of dielectric permittivity in mixtures of mesomorphic and nonmesomorphic compounds.

Static dielectric measurements are performed in the temperature range of the isotropic-nematic phase transition of binary mixtures of mesomorphic n-heptylcyanobiphenyl (7CB) and nonmesomorphic: (a) n-heptylcyanophenyl (7CP) and (b) carbon tetrachloride (CCl4) , the solutes of different molecular shape and polarity. In the whole studied range of the solutes mole fraction (x) (x(max) approximately equal to 0.17) , the critical-like temperature behavior of the permittivity in the vicinity of the transition from the isotropic phase to the two-phase (nematic+isotropic) region can be well described with a critical exponent close to 0.

Dipolar interactions in liquids and linear dielectric relaxation spectroscopy.

Linear dielectric relaxation studies performed on two isotropic liquids composed of the molecules of the same moment of inertia and a quite different polarity: C10H21-O-Ph-COO-Ph-CN (the dipole moment of about 5 D) and C10H21-O-Ph-OOC-Ph-CN (2.5 D) showed that, at given temperature, the relaxation times corresponding to the rotation around the short axis of the two kinds of molecules coincide to each other, regardless the polarity of the molecules and their abilities to accomplish dipolar aggregation. The studies allow one to estimate the lifetime of the intermolecular aggregates due to the dipolar interactions in liquids as no longer than 0.

Static and dynamic dielectric behavior of mesogenic compounds of different polarity in the vicinity of the isotropic to nematic phase transition.

Mesogenic compounds belonging to the two well-known -cyanophenyl, and -isothiocyanatophenyl homologous series, which distinctly differ in the molecular polarity (-C identical with N, 5D; -N=C=S, 2.5D), show an essential difference in the pretransitional dielectric behavior in the vicinity of the isotropic to nematic (I-N) phase transition. Taking into account the results presented in Phys.

Static and dynamic dielectric properties of strongly polar liquids in the vicinity of first order and weakly first order phase transitions.

The paper presents the results of measurements of the linear dielectric properties of the compounds from the homologous series of alkylcyanobiphenyls (C(n)H(2n+1)PhPhCN, nCB) in the vicinity of the first order transition (from the isotropic liquid to the crystalline phase) of nonmesogenic nCB's (n=2-4) and the weakly first order transition (from the isotropic liquid to the nematic phase) of 5CB. The experimental method for the separation of the critical part of the static permittivity derivative and the activation energy for rotation of the mesogenic molecules, in the vicinity of weakly first order phase transition, is proposed. It is shown that the critical temperature dependence of the permittivity and the activation energy can be described with a function of (T-T*)(-alpha) type, with the same values of the temperature of virtual transition of the second order (T*) and the critical exponent (alpha).