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.

No abstract

We report high-precision measurements of phase transitions in the starch-water system by using for the first time adiabatic scanning calorimetry (ASC). Potato starch and nixtamalized corn flours were studied by this technique as a function of the moisture content. We calculated the percentage of gelatinized starch granules, as a function of the temperature, for both flours. For bi-phasic behavior in potato starch, at intermediate water contents, we propose an explanation based on previous hypotheses and on the experimental behavior as deduced from ASC measurements. ASC results suggest, in the case of nixtamalized corn flour, that uniform moisture is present in corn starch granules for all the considered moistures. A shift to higher temperatures of the peak temperature is observed when comparing the c(p)(T) curves with the classical DSC endotherms as measured for this flour. For both flours we estimated from the ASC results the melting temperatures of the starch granules, at zero moisture content, by using Flory's equation.

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.

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.