The Entropy Effects in Binary Mixtures of Polar Mesogenic Solvent/Nonpolar Solute

Abstract

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= 1.52 J/(mol K)] was evaluated from the analysis of the phase diagram of the mixture 6CB + 2CyPh9 with use of the Landau-Lifshitz theory; the resulting value of the transition enthalpy (Delta H (0) NI = T NIDelta S (0) NI = 0.50 kJ/mol) agrees well to that obtained with the calorimetric methods. The field-induced entropy increment (Delta S) was calculated, at the given temperature, from the static dielectric permittivity derivative value (depsilon s/d T), with use of the Fröhlich theory. The singularities in dependence of the entropy increment on the temperature and on the mixtures composition are discussed in terms of the prenematic molecular self-organization extent in mesogenic liquids of different density of dipoles.

Temperature Behavior of the Electric Field-Induced Entropy Increment within a Homologous Series of Nematogenic Compounds

“…Figure 2 presents the experimental results on the temperature dependence of the static permittivity, its derivative, and the corresponding ∆F and ∆S increments, obtained for nCPAB homologous series. Similarly as observed for other nematogenic liquids, [14][15][16][17] in the vicinity of the phase transition from the isotropic liquid to the nematic liquid crystalline phase of nCPABs, the entropy increment shows a critical-like temperature dependence, and, a few degrees before the phase transition, the increment changes its sign from a negative value to a positive one. The negative value of the electric field-induced entropy increment corresponds to the normal field-ordering effects in polar liquids, and in the case of nonmesogenic polar liquids or in mesogenic liquids far from the I-N phase transition, ∆S shows only a slow increase when the temperature decreases, reflecting a change of the number of molecules per unit volume.…”

Temperature Behavior of Electric Relaxational Effects due to Ionic Conductivity in Liquid Lactones

“…As was shown recently, the S (T ) dependence reflects molecular self-association such as, for example, dipolar coupling in isotropic mesogenic liquids in the vicinity of the transition to the nematic phase [37][38][39][40] or hydrogen bond formation in liquid amides [30]. The negative values of S (Fig.…”

Comprehensive review on Froehlich entropy estimation: a technique to investigate the state of order in condensed matter

“…A further set of compounds where FE evaluation is fruitfully employed is liquid crystals. In particular, this approach was used in 2008 to describe the dynamics of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene molecules in prenematic conditions [90]. Notably, the authors claimed that this work, and other of the same group [41,71,91], were the first to experimentally exploit Fröhlich equations and interpretation, although the previously discussed works on Ga nanoparticles had been published four years before ( [38][39][40], section 4.2).…”

“…) in mesogenic solvent of n-hexylcyanobiphenyl [90],4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene and 4-cyanophenyl-4'-n-heptylbenzoate [43,44], while in 4-cyanophenyl 4'-alkylbenzoates and 4n-octyl(4'-cyanophenyl)benzoate the dipolar orientational and isotropy features are correlated to the molecular structures and dielectric properties [42,45].…”

“…In Fig. 12 the trend of FE as a function of temperature is displayed for different concentrations of a nonpolar admixture of 4-n-ethylcyclohexyl-4'-nnonylphenyl in a mesogenic solvent [90]. This is calculated from the dielectric permittivity values reported in Fig.…”