Analysis of the Order Character of the RII−RI and the RI−RV Rotator Phase Transitions in Alkanes by Photopyroelectric Calorimetry

Rotator phases in alkane systems: In bulk, surface layers and micro/nano-confinements

Medium- and long-chain alkanes and their mixtures possess a remarkable physical property - they form intermediate structured phases between their isotropic liquid phase and their fully ordered crystal phase. These intermediate phases are called "rotator phases" or "plastic phases" (soft solids) because the incorporated alkane molecules possess a long-range positional order while preserving certain mobility to rotate, which results in complex visco-plastic rheological behaviour. The current article presents a brief overview of our current understanding of the main phenomena involved in the formation of rotator phases from single alkanes and their mixtures. In bulk, five rotator phases with different structures were identified and studied in detail. Along with the thermodynamically stable rotator phases, metastable and transient (short living) rotator phases were observed. Bulk rotator phases provided important information about several interfacial phenomena of high scientific interest, such as the energy of crystal nucleation, entropy and enthalpy of alkane freezing, interfacial energy between a crystal and its melt, etc. In alkane mixtures, the region of existence of rotator phases increases significantly, reflecting the disturbed packing of different molecules. All these phenomena are very important in the context of alkane applications as lubricants, in cosmetics, as phase-change materials for energy storage, etc. Significant expansion of the domain of rotator phases was observed also in confinements - in the pores of solid materials impregnated with alkanes, in polymeric microcapsules containing alkanes, and in micrometer sized emulsion droplets. The rotator phases were invoked to explain the mechanisms of two recently discovered phenomena in cooled alkane-in-water emulsions - the spontaneous "self-shaping" and the spontaneous "self-bursting" (fragmentation) of emulsion drops. The so-called "α-phases" formed by fatty acids and alcohols, and the "gel phase" formed in phospholipid and soap systems exhibit structural characteristics similar to those in the alkane rotator phases. The subtle connections between all these diverse systems are outlined, providing a unified outlook of the main phenomena related to the formation of such soft solid materials. The occurrence of alkane rotator phases in natural materials and in several technological applications is also reviewed to illustrate the general importance of these unique materials and the related phenomena.

Simultaneous calorimetric and polarization microscopy investigations of light induced changes over phase transitions in a liquid crystal-napthopyran mixture

We have studied the specific heat and the thermal conductivity in a 4-(n-octyl)-4'-cyanobiphenyl liquid crystal (LC)-photochromic molecules mixture, before, during, and after the photo-activation of the dispersed photochromic molecules, over both the smectic A-nematic and the nematic-isotropic phase transitions. The evaluation of the specific heat has enabled the determination of the changes of the phase transition characteristics induced by the photochromic molecules photoisomerization, while that of the thermal conductivity could be used to monitor the modifications induced in the average LC molecular orientation. The polarization microscopy imaging of the sample texture constituted a valuable support for the interpretation of the obtained thermal conductivity results.

Effect of cholesterol on the phase behavior of solid-supported lipid vesicle layers

The interest in solid-supported biomimetic membranes stems from their utility in nanotechnology and biosensing. In particular, supported lipid vesicles (SLVs) have become popular in both fundamental biophysical studies and pharmaceutical screening applications. It is thus essential to gain information on the structural properties and phase behavior of SLVs. Here we report on a study on the influence of cholesterol on the phase behavior of SLVs of saturated phospholipids by using quartz crystal microbalance with dissipation monitoring, a label-free and nonintrusive surface-sensitive technique. Two complementary approaches have been used, a Voigt-based viscoelastic model yielding shear viscosity temperature profiles and the first-order derivative of the frequency (mass-sensitive) shifts. Anomalies in the shear viscosity and extrema in the first-order derivative frequency curves stand as a token of the main phase transition and provide information on its gradual suppression upon addition of cholesterol. This method proves convenient for its small sample volume needed, its short temperature equilibration time and the non-necessity of external labels. This work can be regarded as a starting point for further studies on more rare lipid systems and different geometries, such as tethered SLVs or biologically relevant vesicles produced by living cells.

Effect of the liquid crystal solute on the rotator phase transitions of n-alkanes

Recent experimental studies have shown that the liquid crystal substance plays an important role in determining the structures and the phase transitions of the different rotator phases in binary mixtures of n-alkane and liquid crystals.

The heat-transfer method: a versatile low-cost, label-free, fast, and user-friendly readout platform for biosensor applications

In recent years, biosensors have become increasingly important in various scientific domains including medicine, biology, and pharmacology, resulting in an increased demand for fast and effective readout techniques. In this Spotlight on Applications, we report on the recently developed heat-transfer method (HTM) and illustrate the use of the technique by zooming in on four established bio(mimetic) sensor applications: (i) mutation analysis in DNA sequences, (ii) cancer cell identification through surface-imprinted polymers, (iii) detection of neurotransmitters with molecularly imprinted polymers, and (iv) phase-transition analysis in lipid vesicle layers. The methodology is based on changes in heat-transfer resistance at a functionalized solid-liquid interface. To this extent, the device applies a temperature gradient over this interface and monitors the temperature underneath and above the functionalized chip in time. The heat-transfer resistance can be obtained by dividing this temperature gradient by the power needed to achieve a programmed temperature. The low-cost, fast, label-free and user-friendly nature of the technology in combination with a high degree of specificity, selectivity, and sensitivity makes HTM a promising sensor technology.

Study of the smecticA-hexaticB phase transition in homeotropic single domain samples of 65OBC liquid crystal by photopyroelectric calorimetry

The smecticA-hexaticB phase transition was studied in a homeotropic single domain sample and in a non-aligned sample of n-hexyl-4'-n-pentyloxybiphenyl-4-carboxylate liquid crystal compound to probe the effect of different amount of defects on the phase transition. The specific heat, the thermal diffusivity and the enthalpy exchange were monitored over the transition and, at the same time, polarization microscopy observations could be carried out. The transition during the first cooling run was found to be accompanied by a considerably larger defect annealing in the non-aligned sample than in the homeotropic one, but the critical behaviour of the specific heat remained substantially the same.

Invited review article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

The study of thermophysical properties is of great importance in several scientific fields. Among them, the heat capacity, for example, is related to the microscopic structure of condensed matter and plays an important role in monitoring the changes in the energy content of a system. Calorimetric techniques are thus of fundamental importance for characterizing physical systems, particularly in the vicinity of phase transitions where energy fluctuations can play an important role. In this work, the ability of the Photopyroelctric calorimetry to study the versus temperature behaviour of the specific heat and of the other thermal parameters in the vicinity of phase transitions is outlined. The working principle, the theoretical basis, the experimental configurations, and the advantages of this technique, with respect to the more conventional ones, have been described and discussed in detail. The integrations in the calorimetric setup giving the possibility to perform, simultaneously with the calorimetric studies, complementary kind of characterizations of optical, structural, and electrical properties are also described. A review of the results obtained with this technique, in all its possible configurations, for the high temperature resolution studies of the thermal parameters over several kinds of phase transitions occurring in different systems is presented and discussed.

Renormalization-group analysis of the R(I)-R(V) rotator phase transition

A model for coupled tilt angle and lattice distortion parameter is proposed to describe the R(I)-R(V) transition in n-alkane. The model is treated in the framework of a Landau mean-field theory and renormalization-group theory. The influence of gauche conformations and molecular flexibility on the R(I)-R(V) transition is discussed within the mean-field theory. The fluctuations on the R(I)-R(V) transition are discussed by the renormalization-group theory. Renormalization-group theory predicts that the R(I)-R(V) transition can be driven first order by fluctuations and becomes second order at a tricritical point. Available experimental data are consistent with our model.

Critical behavior near the Lifshitz point in Sn(2)P(2)(S(1 - x)Se(x))(6) ferroelectric semiconductors from thermal diffusivity measurements

The thermal diffusivity of the ferroelectric family Sn(2)P(2)(Se(x)S(1 - x))(6) (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals, with the aim of studying the evolution of the ferroelectric transition with Se doping. Its change from second order character to first order while passing the Lifshitz point (x approximately 0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations. The critical behavior of the ferroelectric transition in terms of the different universality classes and their underlying physical dominant effects (tricriticality, long-range dipole interactions, Lifshitz point) has been discussed using thermal diffusivity measurements in the very close vicinity of the critical temperature. This study reveals that for Se concentrations around the Lifshitz point, long-range dipole interactions do not play a significant role and that the critical parameters are close to those predicted for the Lifshitz universality class.

Confinement-driven weakening of the rotator phase transitions in an alkane through a possible tricritical point

We report calorimetric and X-ray measurements in the R1, R2, and R5 rotator phases of a long chain alkane (n-tetracosane) in bulk and confined to porous matrices (PTFE and Anopore) of two different length scales. Probing the order within and normal to the layers, in the Anopore case having a mesoscopic length scale (200 nm), drastic weakening of the R2-R1 and R1-R5 transitions is seen. The effect on the latter is to such an extent that it results in the first observation of a confinement-driven second order transition in these systems. A significant reduction of the temperature range of the R1 phase is also seen in the Anopore case, a feature argued to cause the change in the order of the transition. Comparisons are also made on the recent prediction of such a point in a Landau model. These findings, while paving a new means of realizing a tricritical point, will lead to better understanding of finite size effects in alkanes.