Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

A new method of semigrand canonical ensemble to calculate first-order phase transitions for binary mixtures

First-order phase transitions of binary mixtures at the given pressure (P) and temperature (T) are studied by taking into account the composition fluctuations. Isothermal-isobaric semigrand canonical ensemble is adopted to find the relations among the total number of molecules, the composition fluctuations and Gibbs free energy density. By combining two identical subsystems of mixtures successively, the free energy density is transformed until being stable and its linear segments represent phase transitions. A new method is developed to calculate the phase equilibriums of binary mixtures. The method handles multiple types and number of phase equilibriums at single time and its solutions are physically justified. One example is shown for calculating the phase diagram of binary Lennard-Jones mixture. It demonstrates that the fluctuations of the total number of molecules in mixtures are fundamental behind phase transitions and the van der Waals loops in Gibbs free energy are reasonable.     

Bulk and interfacial wetting behavior of binary mixtures induced by associating between unlike-pair molecules

The statistical associating fluid theory of Wertheim is applied to describe binary mixtures with associating between unlike-pair molecules. The phase behavior of this binary mixture would fall into five different types (I, II, III, V, and VI) of the classification scheme of van Konynenburg and Scott by varying the associating strength and the energy parameters. Both interfacial wetting behavior and wetting transitions are carefully examined in all the vapor-liquid-liquid (gamma-beta-alpha) three-phase-coexisting regions of the binary mixtures. The global wetting behavior and wetting transitions are delineated by scanning the parameter space. In certain regions, the middle beta phase exhibits interfacial phase transitions sequentially, nonwetting --> partial-wetting --> nonwetting, at the interface separating lower alpha and upper gamma phases along with increasing temperature.     

A study of the nematic-smectic A-smectic C point in a binary liquid-crystalline mixture by adiabatic scanning calorimetry

An adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A-smectic C (NAC) multicritical point in mixtures of 4-n-pentyl-oxyphenyl-4'-n-octyloxybenzoate and 4-n-hexyloxyphenyl-4'-n-octyloxybenzoate. The NC transitions are weakly first order and the latent heat becomes zero at or very near the NAC point. Near the NAC point no thermal feature associated with the NA transitions could be observed. The heat capacity anomalies along the second order AC transition line and along the first order NC line indicate the presence of a tricritical point at or very near the NAC multicritical point. No thermal indications for a biaxial nematic phase were observed.     

Phase equilibria in phospholipid-water systems

This work is a critical account of phase studies of phospholipid-water mixtures. The Phase Rule and equations of thermodynamics of heterogeneous systems are applied in the analysis of calorimetric and dilatometric results for these systems. It is inferred that the approach in which a lipid-water mixture of low lipid content is regarded as a one-component system is misleading and unhelpful. A mixture containing water and a pure synthetic phospholipid is a binary system, and the Phase Rule can be applied to it in a straightforward manner. Non-isothermal transitions and invariant three-phase reactions of the eutectic, peritectic and polytectic type can be encountered in lipid-water systems, like in other binary mixtures. The concentration dependence of the enthalpy of the isothermal three-phase transition yields a well-known Tammann triangle. For the volume change at such transition an analog of the Tammann triangle can be drafted which can be useful in interpretation of dilatometric and densitometric data for heterogeneous binary systems.The literature thermodynamic data for the aqueous lipids are analyzed on the basis of the phase diagram of the DPPC-water system. The measured values of the transition enthalpy and volume change at the pre- and main transitions refer to invariant phase reactions. The measured heat capacities and thermal coefficients of expansion actually characterize the biphasic lipid-water mixtures and may reflect phenomena specific for heterogeneous binary systems only. A reinterpretation of the literature DSC data for the DPPC-water system at temperatures below zero is suggested.Experience gained in the study of equilibrium phase diagram, kinetic and colloidal aspects of phase behaviour of dioctadecyldimethylammonium chloride-water system indicates that the common procedures of the sample preparation of lipid-water mixtures may lead to the states which are non-equilibrium in two respects: the system contains a metastable phase, and the phase is dispersed to such an extent that an irreversible colloidal structure is formed. Definitive phase studies of the phase behaviour of phospholipid-water systems in the whole concentration range are urgently needed.     

Hydration of dimethyldodecylamine-N-oxide: enthalpy and entropy driven processes

Dimethyldodecylamine-N-oxide (DDAO) has only one polar atom that is able to interact with water. Still, this surfactant shows very hydrophilic properties: in mixtures with water, it forms normal liquid crystalline phases and micelles. Moreover, there is data in the literature indicating that the hydration of this surfactant is driven by enthalpy while other studies show that hydration of surfactants and lipids typically is driven by entropy. Sorption calorimetry allows resolving enthalpic and entropic contributions to the free energy of hydration at constant temperature and thus directly determines the driving forces of hydration. The results of the present sorption calorimetric study show that the hydration of liquid crystalline phases of DDAO is driven by entropy, except for the hydration of the liquid crystalline lamellar phase which is co-driven by enthalpy. The exothermic heat effect of the hydration of the lamellar phase arises from formation of strong hydrogen bonds between DDAO and water. Another issue is the driving forces of the phase transitions caused by the hydration. The sorption calorimetric results show that the transitions from the lamellar to cubic and from the cubic to the hexagonal phase are driven by enthalpy. Transitions from solid phases to the liquid crystalline lamellar phase are entropically driven, while the formation of the monohydrate from the dry surfactant is driven by enthalpy. The driving forces of the transition from the hexagonal phase to the isotropic solution are close to zero. These sorption calorimetric results are in good agreement with the analysis of the binary phase diagram based on the van der Waals differential equation. The phase diagram of the DDAO-water system determined using DSC and sorption calorimetry is presented.     

A study of the nematic-smectic A-smectic C point in a binary liquid-crystalline mixture by adiabatic scanning calorimetry

An adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A-smectic C (NAC) multicritical point in mixtures of 4-n-pentyl-oxyphenyl-4′-n-octyloxybenzoate and 4-n-hexyloxyphenyl-4′-n-octyloxybenzoate. The NC transitions are weakly first order and the latent heat becomes zero at or very near the NAC point. Near the NAC point no thermal feature associated with the NA transitions could be observed. The heat capacity anomalies along the second order AC transition line and along the first order NC line indicate the presence of a tricritical point at or very near the NAC multicritical point. No thermal indications for a biaxial nematic phase were observed.     

Thermodynamic characteristics and the temperatures of relaxation transitions of poly(methacrylic acid)

The heat capacity of poly(methacrylic acid) containing 2.5 wt % water was measured in a vacuum adiabatic calorimeter at temperatures between 80 and 325 K. The heat capacity of anhydrous poly(methacrylic acid) was calculated, and its standard enthalpies of combustion and formation were determined. On the basis of the enthalpy of melting of the “free”-water phase, the limit of water solubility in the polymer was found calorimetrically at 273 K. The temperatures of relaxation transitions (the glass transition and the β and γ transitions) of poly(methacrylic acid) mixtures with water were determined via differential thermal analysis in the region 80–550 K. In addition, the determination of the temperatures of transitions of anhydrous poly(methacrylic acid) was performed via extrapolation to zero water content of the concentration dependences of the relaxation-transition temperatures.     

Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) the use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) the peculiar properties of natural surfactants saponins, which form extremely viscoelastic surface layers; and (3) the main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to fully explore these opportunities.     

Determination and measurement of solid–liquid equilibrium for binary fatty acid mixtures based on NRTL and UNIQUAC activity models

The estimation of solid–liquid phase equilibrium is important for the design, development, and operation of many industrial processes because of application in many manufacturing fields such as cosmetic, pharmaceutic, and biotechnology industries. In this work, we measured solid–liquid phase equilibrium of six fatty acid binary mixtures using the DSC technique and developed thermodynamic approaches for binary fatty acid mixtures to estimate melting temperatures as a function of mole fraction in solid–liquid phase equilibrium. Derivation of NRTL and UNIQUAC activity models was developed to predict melting temperatures and latent heat to achieve eutectic points of undecylic acid, pentadecylic acid, margaric acid, and stearic acid six pairwise binary mixtures. The fatty acids eutectic mixtures are appropriate for heat water systems, phase clothes, concrete, and other similar applications. The results showed low deviations from experimental data measured in this study.

     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Dielectric relaxation studies of 6OCB/8OCB mixtures with the nematic-smectic A-nematic re-entrant phase sequence

The low frequency relaxation process was studied for 6OCB/8OCB mixtures with three concentrations (27.0, 27.3 and 27.5 wt %) exhibiting the isotropic-nematic-smectic A-nematic re-entrant-crystalline phase sequence and four mixtures (28.5, 30.0, 35.0 and 40.0 wt %) with the isotropic-nematic-crystalline phase sequence. In the liquid crystalline phases, all dielectric spectra could be excellently described by the Debye equation. The relaxation time tau passes smoothly through the phase transitions separating the liquid crystalline phases. The activation barriers hindering the molecular rotations around the short axes are practically the same in the nematic and smectic phases and become larger in the re-entrant nematic phase. Smaller values of the barrier in the nematic phase of mixtures in comparison with those obtained recently for pure 6OCB and 8OCB are explained as an effect of weakening of the molecular interactions caused by increased dipole-dipole associations between molecules in mixtures in relation to pure substances. The slightly larger activation barrier in the nematic re-entrant phase indicates stronger molecular associations in this phase. d     

Effect of water on physical transitions of human serum albumin

Differential thermal analysis is performed for native human serum albumin, denatured human serum albumin, and their mixtures with water in the range 80–550 K. The temperatures of phase transitions of the protein are determined, and the effect of water on these transitions is ascertained. The limit of water solubility in the native protein is estimated from the calorimetry data on the enthalpy of melting of the excess-water phase. The phase diagram of the albumin-water system is plotted and analyzed in wide temperature and concentration ranges.     

Plasma: The Fourth State of Matter

The plasma state is frequently referred to as the fourth state of matter in the sequence: solid, liquid, gas, and plasma. The statement implies that plasma is another phase. Each state is achieved by adding heat to the previous state. The first three states are the three common phases achieved via phase transitions. The statement that plasmas are the fourth state of matter is examined considering phase transitions. It is shown that the transition from gas to plasma is not a phase transition similar to the other phase transitions at which transitions the differential of the Gibbs free energy equals zero. Therefore, strictly speaking, plasmas are better not called the fourth state of matter.     

Structural phase transitions in pentacosane C25H52

The pentacosane undergoes a series of structural phase transitions in addition to the melting transition according to an x-ray scattering study. A phenomenological theory is developed to describe the structural phase transitions between different phases observed in pentacosane. We present a detailed analysis of the different phases and analyze the temperature anomalies of the elastic constants and heat capacity. The thermodynamic anomalies on the different phase transitions are described by the coupling between the order parameters and the elastic strains. The theoretical predictions are found to be in good qualitative agreement with experimental results.     

Surface phase transitions in athermal mixtures of hard rods and excluded volume polymers investigated using a density functional approach

Using fundamental measures' density functional framework based on Wertheim's first order perturbation theory [J. Chem. Phys. 87, 7323 (1987)] we study the surface phase transitions in athermal polymer-needle mixtures, which demix in bulk into the isotropic polymer-rich (rod-poor) and polymer-poor (rod-rich) phases. We find that the polymer-rich (rod-poor) phase wets the hard wall at coexistence and the wetting transition is of first order. In the partial wetting regime we find a sequence of layerings but these transitions are gradually suppressed as the chain length increases. For long enough chains we detect the prewetting line. Rods exhibit pronounced ordering at the wall in the polymer-rich phases. Our results imply that experiments on the (isotropic) wetting transition for colloidal rod-polymer mixtures should be easier to carry out than those for the colloidal rod-sphere mixtures because the wetting transition occurs at lower rod densities. On the other hand, layerings in sphere-needle mixtures may turn out to be difficult to observe experimentally because some of them will be metastable with respect to the freezing transition, whereas the remaining ones are located very close to the binodal.     

Analysis by calorimetry

The last years have seen the development of differential calorimetry into a scanning technique for routine analysis. In order of ease of determination heats of fusion, heats of reaction, phase diagrams, purity analysis, heat capacity and similar heat effects are added to the list of quickly measurable quantities. Accuracies of the order of 1–5% of heats of fusion and heat capacity are obtainable under favorable conditions. Special topics which have been discussed are instrumentation, transition temperatures, heat capacities, glass transitions, heats of transitions, and phase diagrams.     

Alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.     

Specific heat study of the complex phase transitions in magnetism and dielectricity on triangular lattice antiferromagnets

The complex phase transitions involving dielectricity and magnetism on distorted triangular lattice antiferromagnets, "KNiC_l3-family" crystals, which have the crystal structures derived from the prototype CsNiCl₃, were studied through the heat capacity measurements. These crystals are classified into characteristic three groups from the viewpoint of magnetic and structural properties clarified so far. The results of the dielectric constant measurement as a function of temperature are also presented in detail. The present calorimetric study reveaed that the structural successive phase transitions rather than the magnetic transitions in these crystals are recognized more distinctly as the presence of specific heat anomalies at the respective phase transition points. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic appearance of first-order gas-liquid expanded and liquid expanded-liquid condensed phase transitions below the triple point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (pi-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 degrees C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the pi-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.