Weakly first-order character of the nematic-isotropic phase transition in liquid crystals.

The classification of phase transitions in first-order and second-order (or continuous) ones is widely used. The nematic-to-isotropic (NI) transition in liquid crystals is a weakly first-order transition, with only small discontinuities in enthalpy and specific volume at the transition which are not always easy to measure. On the other hand, fluctuation effects near the transition, typical for a continuous transition, are present because of the only weakly first-order character. In a recent paper [Phys. Rev. E 69, 022701 (2004)], it was concluded from the static dielectric permittivity in the isotropic phase near the NI transition that less polar mesogens (with little or no pretransitional effects) are characteristic for a first-order NI phase transition, whereas in the case of strongly polar ones (with large pretransitional effects) the NI transition is close to second order. In this paper, we address the question whether it is, indeed, possible to use these fluctuation effects in the isotropic phase to quantify the "strength" of a weakly first-order transition, i.e., how far it is from second order. Therefore, we measured the temperature dependence of the enthalpy near the NI transition of seven liquid crystals with adiabatic scanning calorimetry and compared the measured values of the latent heat with pretransitional effects in the dielectric constant and the specific heat capacity. The compounds used in the comparison are MBBA, 5CB, 8CB, 5NCS, 5CN, 8CHBT, and D7AB. From our analysis we find, contrary to the assertion in the above reference, no correlation between the strength of the NI transition of a given compound and the pretransitional effects observed, neither dielectrically, nor thermally.

Characterization of the smectic-A-hexatic-B transition in 650BC by adiabatic scanning calorimetry.

We investigated the smectic-A-hexatic-B (SmA-HexB) transition in the liquid-crystal n-hexyl-4'-n-pentyloxybiphenyl-4-carboxilate (650BC) with adiabatic scanning calorimetry. We were able to prove in a direct way that this transition is indeed very weakly first order, as was already suggested in the literature. The latent heat at the transition was determined to be deltaHL = 0.04 +/- 0.02 J/g. Our experiments confirm the high value for the heat capacity critical exponent earlier reported, yielding alpha = 0.64 +/- 0.05.

First-order character of the smectic-A to chiral nematic transition in chiral liquid-crystal mixtures.

An investigation into the smectic-A to chiral nematic (N'A) transition in liquid crystals is presented by using adiabatic scanning calorimetry (ASC). It is predicted theoretically that chirality drives this transition to first order. This transition is studied in mixtures of the nonchiral liquid crystal octyloxycyanobiphenyl (8OCB) and the chiral 4-(2-methylbutyl)-4(')-cyanobiphenyl (CB15), a system with a large (chiral) nematic region that widens upon increasing the chiral (CB15) fraction. An ASC measurement on pure 8OCB showed no evidence for a latent heat, in agreement with previous ac calorimetric studies, with an upper boundary for the latent heat (if any) of 1.8 J/kg. Since pure 8OCB has no measurable latent heat, and taking into account the widening of the chiral nematic region, the possibility of a continuous to first-order crossover due to the coupling of the nematic and the smectic order parameters, as occurring in several cases of smectic-A to nematic (NA) transitions, can be excluded. However, for all examined mixtures a latent heat could be determined at the smectic-A to chiral nematic transition. This confirms theoretical predictions of the first order character of this transition. Quantitatively, theoretical predictions of the evolution of the entropy discontinuities and latent heats of this transition were not consistent with the experimental results. It was further observed that the transition temperature decreases linearly in agreement with theoretical predictions and a previous ac calorimetric study. Finally, it was observed that the pretransitional specific heat capacity shows an interesting evolution upon increasing chiral fraction, and it may be concluded that any theoretical model based on Landau theory is not sufficient to describe this transition.

Systematic calorimetric investigation of the effect of silica aerosils on the nematic to isotropic transition in heptylcyanobiphenyl.

The effect of confinement on liquid crystal phase transitions was investigated in mixtures of the liquid crystal heptylcyanobiphenyl with hydrophilic aerosils. The influence of the aerosil density on the nematic to isotropic transition was studied by adiabatic scanning calorimetry. Mixtures have been investigated with an aerosil content between rho(S)=0.1 and rho(S)=0.7, the latter being substantially higher than that investigated in previous studies with other liquid crystal-aerosil mixtures. The transitions in the examined mixtures exhibited an unusually large broadening, while the transition temperatures showed peculiar behavior. Notwithstanding, the transition (latent) heat behaved as could be expected on the basis of previous studies. The observed behavior can be explained, in the context of the elastic-strain approach usually employed to describe transition temperature shifts, by variations in the distribution of the radius of curvature R of the aerosil voids originating from sample preparation. It is important in this regard to separate the behavior of the transition temperatures and the transition (latent) heats, the former being influenced by the radius of curvature of the voids and the latter by the total void surface per unit volume. Three quantitative models were compared with experimental results. Both the pinned boundary layer and the random field model yield an evolution of transition temperatures not in agreement with experimental observations. More importantly, they predict a decrease of the pretransitional specific heat capacity, not supported by experiment. The difficulties with these models are avoided in a third, mean-field surface-induced order model.

Phase behavior and blue-phase-III-isotropic critical point in (R)-(S) mixtures of a chiral liquid crystal with a direct twist-grain-boundary to blue-phase transition

We have investigated mixtures of the (R) and (S) enantiomers of a chiral liquid crystal, (R)- or (S)-1-methylheptyl 3'-fluoro-4'-(3-fluoro-4-octadecyloxybenzoyloxy)tolane-4-carboxylate using high-resolution adiabatic scanning calorimetry. The pure (R) compound has a direct transition from the twist-grain-boundary to the blue phase without an intermediary chiral nematic phase. For the blue phases a different kind of phase behavior as a function of enantiomeric excess is observed, most probably related to the presence of a twist-grain-boundary-A instead of a chiral nematic phase below the blue phases. The general form of this phase diagram is compared with traditional blue-phase behavior. Furthermore a blue-phase-III-isotropic phase critical point, analoguous to that of a liquid-gas system, is observed, consistent with experimental and theoretical work recently published in this field. Finally, the effect of changing enantiomeric excess on the latent heats of the different first order phase transitions is measured and discussed.

Glass transition temperature of honey as a function of water content as determined by differential scanning calorimetry.

The glass transition of pure and diluted honey and the glass transition of the maximally freeze-concentrated solution of honey were investigated by differential scanning calorimetry (DSC). The glass transition temperature, of the pure honey samples accepted as unadulterated varied between -42 and -51 degrees C. Dilution of honey to 90 wt % honey content resulted in a shift of the glass transition temperature by -13 to -20 degrees C. The concentration of the maximally freeze-concentrated honey solutions, as expressed in terms of honey content is approximately 102-103%, i.e., slightly more concentrated in sugars than honey itself. The application of DSC measurements of and in characterization of honey may be considered, but requires systematic study on a number of honeys.