Phase Transition Entropy Research Articles

Organic alloy systems suitable for the investigation of regular binary and ternary eutectic growth

Transparent organic alloys showing a plastic crystal phase were investigated experimentally using differential scanning calorimetry and directional solidification with respect to find a suitable model system for regular ternary eutectic growth. The temperature, enthalpy and entropy of phase transitions have been determined for a number of pure substances. A distinction of substances with and without plastic crystal phases was made from their entropy of melting. Binary phase diagrams were determined for selected plastic crystal alloys with the aim to identify eutectic reactions. Examples for lamellar and rod-like eutectic solidification microstructures in binary systems are given. The system (d)Camphor–Neopentylglycol–Succinonitrile is identified as a system that exhibits, among others, univariant and a nonvariant eutectic reaction. The ternary eutectic alloy close to the nonvariant eutectic composition solidifies with a partially faceted solid–liquid interface. However, by adding a small amount of Amino-Methyl-Propanediol (AMPD), the temperature of the nonvariant eutectic reaction and of the solid state transformation from plastic to crystalline state are shifted such, that regular eutectic growth with three distinct nonfaceted phases is observed in univariant eutectic reaction for the first time. The ternary phase diagram and examples for eutectic microstructures in the ternary and the quaternary eutectic alloy are given.

Heat capacity and order–disorder phase transition in negative thermal expansion compound ZrW 2O 8

Heat capacity of ZrW 2O 8 was measured in the (1.8 to 483) K temperature range by using two adiabatic and a commercial relaxation calorimeters. Standard thermodynamic functions of ZrW 2O 8 were estimated from the measured molar heat capacity. A large λ-type anomaly of heat capacity due to an order–disorder phase transition was observed at T=(440±0.5) K. The enthalpy and entropy of phase transition were determined to be (1.56 ± 0.01) kJ · mol −1 and (4.09 ± 0.03) J · K −1 · mol −1, respectively. The magnitude of the entropy of transition supports the order–disorder mechanism previously suggested by the present authors.

Influence of Subphase Conditions on Interfacial Viscoelastic Properties of Synthetic Lipids with Gentiobiose Head Groups

The thermodynamic parameters and rheological properties of a synthetic glycolipid with a gentiobiose (O-(β-d-glucopyranosyl)−(1→6-β-d-glucopyranoside)) headgroup have been studied at the air/water interface. The phase transition entropy and latent heat were estimated by film balance experiments at various temperatures, whereas dynamic surface moduli (loss and storage modulus) were measured by an interfacial stress rheometer under well-defined thermodynamic conditions. The impact of glucosidic bonding was systematically studied by comparing the measured results to those from the lipid with a lactose (O-(β-d-galactopyranosyl)−(1→4-β-d-glucopyranoside)) headgroup, which possesses the similar monosaccharide composition. We found that the “bent” 1→6 junction of gentiobiose reduces the lateral intermolecular cooperativity and fluidizes the films in comparison to the lactose lipid monolayer.

SYNTHESIS AND THERMAL PROPERTIES OF TWIN DIMERS CONTAINING CYANOSTILBENE GROUPS AS A MESOGENIC GROUP

Twin dimers (TDs) with a different linking group (ester or ether) between the mesogenic group (cyanostilbene moiety) and the flexible spacer were synthesized to clarify the effect of the linking group on the mesomorphic properties. The sequence of the ester linking group and the position of the cyano group in the mesogenic core made an important contribution to the thermal properties of the twin dimers. A remarkable odd–even effect in the phase transition temperatures and entropy changes from a nematic phase to an isotropic fluid was observed for all the synthesized twin dimers.

Synthesis and Thermal Properties of Twin Dimers Containing Cyanostilbene Derivatives as a Mesogenic Group

We synthesized twin dimers (TDs) with different linking groups between a mesogenic group and a flexible spacer, different spacer length, and different terminal alkyl chain length to clarify the effect of these groups on the mesomorphic properties of the TDs. A remarkable odd–even effect on the phase transition temperatures and nematic–isotrophic phase transition entropy changes was observed for the TDs with different spacer length. Liquid crystallinity of TDs with ester linking group was superior to that of TDs with ether linking group. Length of terminal group played an important role in exhibiting a smectic phase for TDs with ester linking group.

Heat Capacity and Thermodynamic Properties of Inorganic Compounds Containing Tetrahedral Anions (BH-4, AlH-4, GaH-4, BF-4, ClO-4, BrO-4, and IO-4)

Adiabatic and differential scanning calorimetry data are presented on the heat capacity of inorganic compounds containing tetrahedral anions. The entropy and enthalpy of phase transitions in these compounds are evaluated, and the mechanisms of the transitions are discussed in terms of orientational disordering. The heat capacity data are used to estimate, by an additive scheme, the frequencies of lattice (translational and librational) vibrations. It is shown that the BH-4 anion experiences hindered rotation in lattice sites, while the anions in the other complex hydrides, perhalates, and fluoroborates undergo librational vibrations.

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N=1024 rigid banana-shaped molecules with bending angle φ =140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble (NpT). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.

Ferroelectric phase transitions in Pb2xSn2(1-x)P2Se6 system

Heat capacities of the Pb2xSn2(1-x)P2Se6 crystals (x=0, 0.098, 0.251, 0.402 and 1.0) were measured using an adiabatic calorimeter at temperatures between 10 and 350 K. In the crystal of x=0, two heat capacity anomalies corresponding to the ferroelectric commensurate - intermediate incommensurate(C-IC) phase transition temperature Ti, and the incommensurate - paraelectric (IC-N) phase transition temperature Tc, were observed at 193.24±0.10 and 220.07±0.15 K, respectively. The phase transition temperatures decreased with an increase in Pb2+ concentration. The anomaly at Ti disappeared at x=0.251 in the mixed systems of the Pb2xSn2(1-x)P2Se6. In the crystal of Pb2P2Se6 (x=1.0), no phase transition was observed. The normal heat capacities for the mixed crystals were determined by least squares fitting of the Debye and Einstein functions to the experimental data. The anomalous heat capacities gave the phase transition entropies of 8.5 and 1.5 J mol-1 K-1 for x=0. The large transition entropies are consistent with an order-disorder mechanism in the ferroelectric-paraelectric phase transitions in x=0.

Mixed Langmuir Monolayer of N-(1,1-Dihydroperfluorododecyl)-N,N,N-Trimethylammonium Chloride with Perfluorocarboxylic Acids

Surface pressure (π)–area (A), surface potential (ΔV)–area (A), and dipole moment (μ⊥)–area (A) isotherms for N-(1,1-dihydroperfluorododecyl)-N,N,N-trimethylammonium chloride (C12-TAC) and perfluorododecanoic acid (FC12) on the substrate of 0.01 M sodium chloride at pH 2.0 were investigated at the air–water interface by the Langmuir method and the ionizing electrode method. The temperature dependence of the transition pressure of each component was not larger than that of normal hydrogenated surfactant. The apparent molar entropy, enthalpy, and internal energy changes of phase transition from the disordered to the ordered state were calculated. Surface potentials (ΔV) were analyzed using the three-layer model proposed by R. J. Demchak and T. Fort (J. Colloid Interface Sci.46, 191–202, 1974) for FC12 and Gouy–Chapman treatment for C12-TAC. The contributions of the ω-CF3 group and the head group of the vertical component to the dipole moment, μ⊥, were estimated. The new finding was that the π–A curves are shifted to an area smaller than a molecular area of two pure components for the mole fraction (x) of perfluorododecanoic acid of x≥0.3. The molecular areas negatively deviate from the additivity rule at discrete surface pressures. Assuming a regular surface mixture, the Joos equation (Joos, P., and R. A. Demel, Biochim. Biophys. Acta183, 447, 1969) for analysis of the collapse pressure of mixed monolayer allowed estimation of the interaction parameter; ξ=−4.20 at x≤0.5 and ξ=−0.24 at x>0.5 between two fluorinated amphiphiles.

The Szilard engine revisited: Entropy, macroscopic randomness, and symmetry breaking phase transitions.

The role of symmetry breaking phase transitions in the Szilard engine is analyzed. It is shown that symmetry breaking is the only necessary ingredient for the engine to work. To support this idea, we show that the Ising model behaves exactly as the Szilard engine. We design a purely macroscopic Maxwell demon from an Ising model, demonstrating that a demon can operate with information about the macrostate of the system. We finally discuss some aspects of the definition of entropy and how thermodynamics should be modified to account for the variations of entropy in second-order phase transitions. (c) 2001 American Institute of Physics.

Determination of equilibrium pitch of cholesteric phases by isobaric–isothermal Monte Carlo simulation

In canonical Monte Carlo simulations of chiral liquid crystals, generally cholesteric phases with nonequilibrium pitch were formed under periodic boundary conditions. In order to overcome system-size effects the phase diagram of chiral calamitic liquid crystals, described here by the chiral Gay–Berne fluid, was now studied by simulations in the isothermal–isobaric ensemble (NpT). In the temperature–chirality parameter plane a rich polymorphism could be proven. In the cholesteric phase the ability of the simulation box to change the dimensions during the simulations enabled the determination of the equilibrium pitch in dependence on the chirality parameter describing the strength of the chiral interaction. Additionally, in dependence on temperature first-order phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. On heating a cholesteric phase (N*), for the first time by computer simulation of a many-particle system based on model intermolecular interactions a temperature-driven phase transition to a blue phase could be proven, a characteristic feature of many experimental and theoretical phase diagrams of chiral liquid crystals. Additionally, on cooling a cholesteric system a phase with a helical superstructure and smectic layers formed perpendicular to the helical axis, both characteristics of a twist grain boundary (TGB) phase, has been observed. This clear evidence shows that a chiral interaction proportional to the first pseudoscalar term of the expansion in rotational invariants is able to yield the observed phase sequence TGB-like phase, cholesteric phase, blue phase, and isotropic phase in dependence on one scalar parameter given here by the temperature, only.

Entropy of bcc → L, fcc → L, and fcc → bcc phase transitions of elemental substances as function of transition temperature

In the last two-three decades, the thermodynamic calculations of phase diagrams for both binary and multicomponent systems have been in general use (see, e.g., [1–3]). For most systems, a thermodynamic description of virtual phase transitions for pure components entering a system is necessary for performing these calculations. Thus, e.g., for calculating the Ni–Cr phase diagram, it is necessary to know the enthalpies and entropies of phase transitions for Ni and Cr both in the case of the actual phase transitions [fcc liquid melt (L) for Ni and bcc L for Cr] and virtual phase transitions of metastable phases of pure Ni (bcc phase) and Cr (fcc phase) to the liquid phase.

Thermal and biomechanical parameters of porcine cornea.

New methods in refractive surgery require a considerable understanding of the material "cornea" and are often studied by theoretical modeling in order to gain insight into the procedure and an optimized approach to the technique. The quality of these models is highly dependent on the preciseness of its input parameters. Porcine cornea often is used as a model in preclinical studies because of its similarity to man and its availability. The important physical parameters for biomechanical deformation, heat conduction, and collagen denaturation kinetics have been determined for porcine cornea. Experimental methods include densitometry, calorimetry, turbidimetry, tensile tests, stress relaxation, and hydrothermal isometric tension measurements. The density of porcine cornea was measured as p = 1062+/-5 kg/m3, the heat capacity gave c = 3.74+/-0.05 J/gK. The stress-strain relation for corneal strips is represented by a third order approximation where the secant modulus yields about Esec approximately equal to 0.4 MPa for small strains less than 2%. The normalized stress relaxation is described by an exponential fit over time. The denaturation process of cornea is characterized by specific temperatures which can be related to the change of the mechanical properties. Denaturation kinetics are described according to the model of Arrhenius yielding the activation energy deltaEa = 106 kJ/mol and the phase transition entropy deltaS = 39 J/(mol x K). The established set of parameters characterizes the porcine cornea in a reliable way that creates a basis for corneal models. It furthermore gives direct hints of how to treat cornea in certain refractive techniques.

Interdependence of liquid crystal phase transitions in mixed systems

From calorimetric measurements, the phase transition enthalpies and entropies in mixed systems of each of two cholesteryl esters with a nematogenic compound PCPB have been determined as a function of composition. The calorimetric data for the LC-I transition, as well as the slope (dp/dT)t of the phase boundary line LC-I, show a conspicuous discontinuity at the critical mole fraction of PCPB where the blue phases disappear. The enthalpy of the SmA-N* transition becomes zero at the same critical mole fraction. Possible reasons for the observed transition interdependence are discussed.

Phonon contributions to the entropies ofhP24and fccCo3V

Inelastic neutron-scattering spectra and neutron-diffraction patterns were measured on the alloy Co3V at temperatures from 1073-1513 K, where the hP24 (ordered hexagonal) and fee structures are the equilibrium states of the alloy. Phonon density of states (DOS) curves were calculated from the inelastic-scattering spectra, allowing estimates of the vibrational entropy in the harmonic and quasiharmonic approximations. The vibrational entropy of the hP24-fcc phase transition at 1323 K was found to be 0.07k(B)/atom. The anharmonic contributions to the entropy over a temperature range of 100 K were comparable to the vibrational entropy of this phase transition. The anharmonic softening of the phonon DOS was only slightly larger for the hP24 than the fee phase, however, so the anharmonic effects contribute only slightly to the difference in entropy of the two phases. The simple Gruneisen approximation was inadequate for predicting the thermal softening of the phonon DOS.

Frontier trends in the prediction of vapour pressure of metal-organic precursors

A quantitative approach to predictions of the volatility of metal-organic precursors has been developed. The proposed concept is based on up-to-date notions concerning the character of intermolecular interactions in molecular crystals allowing for the specific features of the molecular and electronic structure of metal-organic precursors. Using the data on the crystal and molecular structure of the metal-organic precursors within the frames of the methods of statistical thermodynamics and molecular physics, we have built the models for calculating the enthalpy and entropy of crystal-vapour phase transitions starting from the potentials of interatomic interactions. The temperature dependence of vapour pressure has been calculated for various precursors with known crystal and molecular structure. The calculated and experimental data on vapor pressure have been compared. The behaviour peculiarities of thermodynamic functions depending on the features of molecular structure of metal-organic precursors and their packing in crystals have been revealed. Information content and predictability of the proposed model notions for the forecast of vapour pressure for metal-organic precursors have been demonstrated.

Ferroelectric Phase Transitions in Sn2P2S6and Sn2P2Se6Crystals

Heat capacities of Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 single crystals were measured at temperatures between 10 and 380 K for the former and between 10 and 350 K for the latter using an adiabatic calorimeter. The heat capacity anomaly due to the ferroelectric-paraelectric phase transition occurred at 337.25 ±0.10 K in Sn 2 P 2 S 6 . The anomalies corresponding to the ferroelectric-incommensurate and incommensurate-paraelectric phase transitions occurred at 193.24 ±0.10 and 220.07 ±0.15 K in Sn 2 P 2 Se 6 . The normal heat capacities for the compounds were estimated by least squares fitting of the Debye and Einstein functions to the experimental data supplemented by spectroscopic normal mode frequencies. The anomalous heat capacities gave the phase transition entropies of 8.6 JK -1 mol -1 for Sn 2 P 2 S 6 and 8.5 and 1.5 JK -1 mol -1 for Sn 2 P 2 Se 6 . The large transition entropies are consistent with an order-disorder mechanism for the ferroelectric-paraelectric phase transitions in Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 .

Low-temperature heat capacity of 1-bromoperfluorooctane

The heat capacities of crystalline and liquid 1-bromoperfluorooctane, Br(CF2)7CF3, were measured by a full-automatic adiabatic calorimeter in the temperature range from 5K to 346K. The calorimeter, experimental procedure and results of testing the apparatus by standard substances (copper and heptane) were described. 1-Bromoperfluorooctane was found to be capable of existing in metastable and stable solid forms. Two solid-to-solid transitions at the temperatures 146.4K and 179.5K, and a fusion atT=278.90K have been discovered. The enthalpies and entropies of phase transitions, as well as the main thermodynamic functions {G°(T)−H°(0)}/T, {H°(T)−H°(0)}/Tand {S°(T)−S°(0)}, have been determined. The Gibbs free energy of formation of the substance under study was determined on the basis of these experimental data and the enthalpy of formation estimated by means of the Benson and Buss group additivity method.

DSC study of phase transition of anhydrous phospholipid DHPC and influence of water content

The phase transition behavior of 1, 2-di-n-heptadecanoyl phosphatidylcholine (DHPC) with and without water has been studied by use of differential scanning calorimetry. It was found by experiment that the glass transition occurred at first during the first heating of a sample of DHPC without water and then the sample underwent melting as an ordinary crystal. Therefore the sample of DHPC without water was a glassy crystal. However, the DHPC sample crystallizing from melt was an ordinary crystal. From the relationship between the total melting enthalpy Q1 of freezable water and the water contenth, it was concluded that the water contained in the DHPC samples might exist in three states recognizable thermodynamically. The water in the first state was an unfreezable water. It was the water bound directly with the head groups of the phospholipid, i.e. the primary hydration water. Every head group might bind seven such molecules of water. The water in the second state was the secondary hydration water, its melting point was close to that of pure water and its specific melting enthalpy dQr/dh was lower than that of pure water. Every molecule of DHPC might possess at most about six such molecules of water. With the further increasing of the water content, the bulk water appeared in the system, its melting point and specific melting enthalpy were close to those of pure water. The experiments demonstrated that the melting peak of DHPC shifted toward the side of lower temperature with the increasing of water content. Ath>16.2 (percent by weight), the temperature Ttr and the enthalpy Δtr of the gel-liquid crystal phase transition of the lipid were unchanged substantially with the increasing of water content. The experimental values of the quantities,T tr and Δtr, and phase transition entropy Δtr were in good agreement with those calculated by the set of Huang equations (1981).

The phase transition sequence in betaine calcium chloride dihydrate by adiabatic calorimetry

Abstract The specific heat of (C3)3NCH2COO.CaCl2.2H2O (BCCD) has been measured by adiabatic calorimetry from 20 K to 330 K. This experimental study refers to the rich variety of phase transitions showed by this crystal and has permitted a precise characterization of its calorimetric response. Most of the previously reported phase transitions show a clear specific heat anomaly. In some cases, however, these measurements do not permit one to confirm the occurrence of phase transitions when they are related to very small jumps of the modulation wave vector. The results presented in this work confirm a noticeable relationship between the wave-vector behaviour and the associated phase transition entropy. In fact, this correlation has permitted an unambiguous identification of some of the crystal stable phases.