Phase transition temperatures definition

On the other hand, despite the information about long chain sulfates, sulfonates, phosphates, and carboxylates that indicates stronger interaction with Ca2+ than with Mg2+ (i.e., in apparent harmony with the sequence of the Hofmeister (44) series), several difficulties remain. For example, while Miyamoto s data for DS (10) indicate the interaction sequence Mg < Ca < Sr < Ba from solubility measurements (as well as from temperature/CMC measurements if one accepts the Mg—Ca sequence of the present paper), this sequence, with the exception of the position of Mg and Ca, is the opposite of that found by Deamer et al. (33) from condensation effects on the force/area curves of ionized fatty acids. At the same time, the ion sequence obtained by these authors from phase transition temperatures of spread fatty acids (33) differs from that deduced from the above-mentioned condensation effects, and the latter depended strongly on pH. Lastly, definite differences in ion sequence effects exist for the alkaline earth metals in their interaction with long... 

In scanning calorimeters, the calibration of the temperature scale involves a somewhat more laborious procedure because a thermometer usually caimot be used for this purpose because there is not enough room for it. Such instruments are calibrated by means of reference substances, that is, highly pure substances that undergo phase transitions at definite temperatures. The test runs should be carried out with different substances, melting at different temperatures, using a wide variety of heating rates because the temperature correction may depend not only on... 

Another type of transition is the second-order phase transition in which the first derivative of the chemical potential is continuous while the second derivative is not. This means that enthalpy, volume, and entropy vary continuously with temperature through a second-order phase transition temperature. This behavior is qualitatively different from that of a first-order phase transition, as illustrated in Figure 4.5. Whereas first-order phase transitions occur at a definite temperature for a given pressure, and with separation of the phases, second-order transitions do not exhibit a separation of phases and occur over a range of temperatures. The transition from superfluid helium to normal liquid helium and the transition from being a superconducting metal to being an ordinary conductor are examples of second-order transitions. 

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

The martensite - austenite transition temperatures we find are for all systems in accordance with the previously published ones . Some minor deviations can be attributed to the fact that we are simulating an overheated first order phase transition. Therefore, for our limited system sizes, one cannot expect a definite transition temperature. 

The transition from one phase to another is called phase-transition it takes place at a definite temperature (under a specified pressure) with the absorption of an amount of heat A for each gram-molecule, or mol, passing over. When the various phases of a system can exist side by side for an indefinite period they are said to be in equilibrium. 

In 1968, an international agreement was reached about the definition of an official (practical) scale of temperature for T> 14 K. This temperature scale IPTS-68, corrected in 1975 [11], was defined by reference fixed points given by transitions of pure substances. To extend the low-temperature range of IPTS-68, the EPT 76 [12-13] gave nine reference temperatures defined by phase transition of pure substances in particular the superconductive transition (between 0.5 and 9K) of five pure metals was introduced. Moreover,... 

Navard and Haudin studied the thermal behavior of HPC mesophases (87.88) as did Werbowyj and Gray (2), Seurin et al. (Sp and, as noted above, Conio et al. (43). In summary, HjPC in H2O exhibits a unique phase behavior characterized by reversible transitions at constant temperatures above 40 C and at constant compositions when the HPC concentration is above ca. 40%. A definitive paper has been recently published by Fortin and Charlet ( who studied the phase-separation temperatures for aqueous solutions of HPC using carefully fractionated HPC samples. They showed the polymer-solvent interaction differs in tiie cholesteric phase (ordered molecular arrangement) from that in the isotropic phase (random molecular arrangement). 

It is appropriate at this point to make a few comments about the importance of the observed thermal anomalies in connection with the theories of water structure mentioned above. If the reality of the thermal anomalies is accepted, the ultimate theory of water structure must be able to allow for the existence of these anomalies and, hopefully, eventually predict their existence. If the thermal anomalies do indeed manifest higher-order phase transitions, structured elements of a certain size must be present in water. In other words, the uniformists , average structural models must definitely be ruled out. Furthermore, noting that the anomalies tend to center around discrete temperatures and apparently are completed over a few degrees, we concluded that if they do manifest... 

Entropy is a measure of the degree of randomness in a system. The change in entropy occurring with a phase transition is defined as the change in the system s enthalpy divided by its temperature. This thermodynamic definition, however, does not correlate entropy with molecular structure. For an interpretation of entropy at the molecular level, a statistical definition is useful. Boltzmann (1896) defined entropy in terms of the number of mechanical states that the atoms (or molecules) in a system can achieve. He combined the thermodynamic expression for a change in entropy with the expression for the distribution of energies in a system (i.e., the Boltzman distribution function). The result for one mole is ... 

Both spin-crossover transitions (HS < LS, FO LS) are first order accompanied by definite jumps of populations, while the cooperative Jahn-Teller transition (HS FO) is weak first-order (very close to a second-order transition). It suggests a possibility of observation of hidden cooperative Jahn-Teller transition (the broken line in Fig. 7) between the metastable HS and FO phases, if the HS phase could be supercooled enough below the spin-crossover transition temperature Tc by a rapid cooling. 

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