Phase transition temperatures and enthalpies

The influence of substituent size, polarity, and location on the thermotropic properties of synthetic phosphatidylcholines has been studied by Menger et al. [18], The effect of increasing membrane curvature on the phase transition has been investigated by DSC and FTIR [19]. In addition, a data bank, LIPIDAT, on lipid phase transition temperatures and enthalpy changes is available [20, 21],... 

Abstract. Gas interstitial fullerenes was produced by precipitation of C6o from the solution in 1,2 dichlorobenzene saturated by O2, N2, or Ar. The structure and chemical composition of the fullerenes was characterized by X-ray powder diffraction analysis, FTIR spectroscopy, thermal desorption mass spectrometry, differential scanning calorimetric and chemical analysis. The images of fullerene microcrystals were analyzed by SEM equipped with energy dispersive X-ray spectroscopy (EDS) attachment. Thermal desorption mass spectroscopy and EDS analysis confirmed the presence of Ar, N and O in C60 specimens. From the diffraction data it has been shown that fullerite with face centered cubic lattice was formed as a result of precipitation. The lattice parameter a was found to enhance for precipitated fullerene microcrystals (a = 14.19 -14.25 A) in comparison with that for pure C60 (a = 14.15 A) due to the occupation of octahedral interstices by nitrogen, oxygen or argon molecules. The phase transition temperature and enthalpy of transition for the precipitated fullerene microcrystals decreased in comparison with pure Cgo- Low temperature wet procedure described in the paper opens a new possibility to incorporate chemically active molecules like oxygen to the fullerene microcrystals. 

Portugall et al. (1982) noticed that the phase transition temperature and enthalpy of the following side-group type liquid crystal polymer, (3.13), were dependent on the degree of polymerization n. The polymer was noncrystalline. If n = 13, the glass transition temperature was 53 °C, the clearing point of the liquid crystal phase was 100 °C, the enthalpy of... 

An enormous number of phase transitions are known to occur in common solid compounds. For example, silver nitrate undergoes a displacive phase transition from an orthorhombic form to a hexagonal form at a temperature of approximately 162°C that has a enthalpy of 1.85 kj/mol. In many cases, the nature of these transitions are known, but in other cases there is some uncertainty. Moreover, there is frequently disagreement among the values reported for the transition temperatures and enthalpies. Even fewer phase transitions have been studied from the standpoint of kinetics, although it is known that a large number of these transformations follow an Avrami rate law. There is another complicating feature of phase transitions that we will now consider. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

The fact that internal energy and enthalpy are state properties means that any conv enient process path from a reference state to a process state may be chosen, even if the actual process proceeds by a different path. As a rule, you would choose a path that allows ou to make use of heat capacities, phase transition temperatures, and latent heats tabulated in an avtulable reference (like this text). 

Table II. Phase Transition Temperatures and Phase Transition Enthalpies AH of the Homopolymer and Block Copolymers...

The polymorphism of calcium carbonate is still not completely understood despite its very long history and the appearance of many published studies. In addition to calcite, aragonite and vaterite, non-crystalline forms of CaCOs of biolo cal origin exist. An account of the relationships between these solid phases has appeared recently [39] together with a summary of thermodynamic and kinetic data for the transformations of the metastable polymorphs aragonite and vaterite to the stable calcite. These authors describe the preparation of non-crystalline calcium carbonate and report preliminary values of the transition temperature and enthalpy change for its crystallization to calcite. The DSC method, siq)ported by TG and PXRD, was used in this study. 

If the copolymer is based on monomers whose homopolymers exhibit identical me-sophases, its phase diagram follows ideal solution behavior with a continuous, linear or slightly curved dependence of the transition temperature and enthalpy of at least the highest temperature mesophase on copolymer composition [132, 134,136-140,364]. For example. Fig. 24 a demonstrates that the two structural units ofpoly 3-[(4 -(4"-cya-nophenyl)phenoxy)propyl]vinyl ether -co- 5-[(4 -(4"-cyanophenyl)phenoxy)pentyl] vinyl ether are isomorphic within the nematic mesophase, although they are not iso-... 

A phase diagram is a graphical representation of the relationship between a given set of experimental parameters and the phase changes occurring in a material. Sample volume, transition temperature and enthalpy, pressure and composition of the material are commonly used parameters in phase diagrams. Transition temperatures measured by TA are not equilibrium values and vary with the experimental conditions, particularly the scanning rate. Therefore,... 

Thus, to calculate the composition of the azeotrope point and the difference in the regular solution parameters, only and pure component transition temperatures and enthalpies are required. That so much can be accomplished with only knowledge of T n is very convenient, since the commonly used contact method for determining binary phase diagrams of mesogenic mixtures will only provide temperature information. 

Effect of Polymers on Thermotropic Phase Behavior. In order to determine if the polymers had any direct effect on the bilayers, the gel-to-liquid crystalline phase transition temperature was measured for the samples. The gel-to-liquid crystalline phase transition temperature has been found to be a very sensitive measure of bilayer packing and order. If the polymers were directly interacting with the bilayer, it should be observed in the transition temperature and enthalpy. 

The basic methods for the determination of phase transition temperatures (7 ) are DSC (differential scanning calorimetry), DTA (differential thermal analysis), and polarization microscopy. Every method has its advantages and restrictions. DSC allows one to determine the enthalpies of phase transitions (A// ). Microscopy allows one both to determine the phase transition temperatures and to identify the t)fpe of mesophase. DTA gives reliable results for melting temperatures of LCs that show solid-state polymorphism, and of LC mixtures. The differences in that can be found in publications by different authors arise from different measurement techniques and the presence of impurities. We have selected the data with the higher 7 values in such cases. 

Differential scanning calorimetry (DSC) is widely used for studying binary and multicomponent systems containing surfactants. Transition temperatures and enthalpies are often determined and used to draw the limits of existence of the different phases of surfactant-based systems [1-6], The state of the surfactant molecules in these phases is studied by means of thermal analysis [7,8], DSC is also a useful technique to obtain information about the phase diagrams of surfactant-based systems and the various microstructures formed in these systems. The properties that can be obtained from DSC for binary systems are the following [6] ... 

The enthalpy over a temperature range that includes phase transitions, melting, and vaporization, is represented by ... 

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