Temperature-induced phase transitions pressure effects

For instance, Raman spectroscopy was used to study the effect of pressure and temperature on the phase composition of fluoranil crystals. Figure 2 shows the Raman spectra obtained at a series of increasing pressures, where the changes in band frequency indicate the existence of pressure-induced phase transitions. It was deduced from sharp discontinuities in the Raman spectra that a phase transition took place at a temperature of around 180 K if the pressure was 1 atm, but that this transition shifted to 300 K if the pressure was increased to 0.8 GPa. Other work indicates that this particular phase transition does not entail a change in the crystal space group, but involves displacement within the unit cell. 

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed," and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

The effect of s—d hybridization on the relative stability of the structures of metallic Ca and Sr has been studied as a function of temperature and pressure within the context of the pseudopotential theory for metals. The inclusion of hybridization is found to favour the f.c.c. structure at all pressures and, in particular, is necessary to explain the observed f.c.c. structure in these metals at zero temperature and pressure. Phase boundaries are calculated by equating the free energy of the f.c.c. structure to that of the b.c.c. structure. Temperature-induced phase transitions are predicted to occur at 555 K in Ca and 625 K in Sr as compared with the actual temperatures of 721 and 830 K. From the changes in free energy of the reactions of the alkaline-earth metals with gases as a function of temperature, it is confirmed that at 298— 1400 K almost all of the residual gases in electrovacuum devices combine with the metals to form stable compounds. The dehydration of calcium... 

The peak height ratio between the antisymmetric and symmetric CHj stretching modes H-2880/H-2850 is plotted as a function of pressure in Figure 7. This parameter has been shown to reflect the magnitude of interchain interactions in systems that contain polymethylene chains [20]. An abrupt increase in this ratio, and thus an increase in the interchain interactions, is observed at the pressure-induced transition from the micellar to the coagel phase. Furthermore, it is also evident from Figure 7 that the transition has a considerable pressure hysteresis. Interestingly, pressure [76] and temperature [79] have different effects on the H-2880/H-2850 ratio in the micellar phase of... 

Local anesthetics. Anesthetics interact with membranes and increase the gel to liquid-crystalline transition of fully hydrated bilayers. They induce a volume expansion which has the opposite effect of HHP and so they antagonize the effect of HHP on membranes fluidity and volume, making membranes more fluid and expanded. The application of HHP to membrane-anesthetic systems may even result in the expulsion to the aqueous environment. The local anesthetic tetracaine (TTC) can be viewed as a model system for a large group of amphiphilic molecules. From volumetric measiuements on a sample containing e.g. 3 mol% TTC, it has been found that the main tansition at ambient pressure shifts to a lower temperature. The expansion coefficient a drastically increases relative to that of the pure lipid system in the gel phase, and the incorporation of the anesthetic into the DMPC bilayer causes an about 15 % decrease of relative to that of the pure lipid system. The addition of 3 mol% TTC shifts the pressure-induced liquid-crystalline to gel phase transition towards somewhat higher pressures. Larger values for the compressibilities are found for both lipid phases by addition of 3 mol% TTC, and there is no apparent difference in the coefficient of compressibility between the gel and liquid-crystalline phases. Comparison of the IR spectra of DMPC and DMPC/TTC mixtures at pH 5.5 as a function of pressure shows an abrupt... 

Polymer phase transitions are traditionally probed by exploiting the temperature dependence of x to induce segregation. The location of the transition can be readily changed by adjusting the component volume fractions, ( > or N. Recently the effects of liquid organic solvents , homopolymer diluents and pressure have been studied in efforts to assess the phase behavior of blends and copolymers under realistic processing conditions. 

Recently, the effect of pressure-induced luminescence rigidochromism on the emission behavior of 4a has also been reported [58], In ambient pressure, the emission of the complex occurs at 695 nm in benzene. However, at a pressure higher than that required to induce the solvent to undergo the room temperature phase transition from fluid to solid, the emission band shifts to 575 nm, which is similar to the solid state emission wavelength (580 nm) of the complex. 

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