Phase transitions reversible

An interesting example is available of inequivalent nuclei becoming equivalent on raising the temperature and going through a phase transition, reversibly. This occurs in squaric acid C4(0)2(0H)2, studied by single-crystal 13C NMR,29 and later (2004) also by 170 NMR. Here, MAS was used to narrow the lines sufficiently to observe the changes in the isotropic parts of the chemical-shift matrices. The phase transition occurs at 373 K. 

This is reliable and fairly accurate, if tedious. It was used, for example, by Hoover [92] to locate the melting parameters for soft-sphere systems. The only point to watch out for is that one should not cross any phase transitions in taking the path from 1 to 2 it must be reversible. 

Texter J, Antalek B and Williams A J 1997 Reverse micelle to sponge phase transition J. Chem. Phys. 106 7869-72... 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

Both K2TaF7 and K2NbF7 exhibit a reversible phase transition at about 200°C, which is characterized by a significant change in density [148]. 

The appearance of spontaneous polarization in the case of LuTaO is related to volumetric irregularities and ordering of the Li+ - Ta5+ dipoles, as is in the case of the similar niobium-containing compound Li4Nb04F. It can be assumed that the main difference between the two compounds is that the irregularities and the Li+ - Ta5+ dipoles are thermally more stable compared to the niobium-containing system. This increased stability of the dipoles leads to the reversible phase transition at 660°C. 

Such changes indicate the occurrence of a reversible phase transition. No other appreciable anomalies were observed at temperatures of up to 650K that do not correspond to the phase transition at about 595K described in [439]. 

Phase transition irreversible, 225 order - disorder, 224-228 reversible, 225, 229, Physicochemical properties of ammonium hydrofluoride, 39 deviations from ideal, 149 ideal system, 148 NbF5 and TaFs, 25 niobium containing melts, 150 tantalum containing melts, 151 M5Nb3OFlg, 234-235 Piezoelectric properties, 245-247 Plasma chemical decomposition equipment, 311... 

Minol II will exhibit a reversible phase transition beginning at 32°, producing volume changes of about 3.8%. These volume changes could causfe microcrystalline cracks and pores which could reduce detonation velocity and mechanical strength (Ref 36)... 

While non-isothermal measurements can provide a rapid and useful qualitative indication of the occurrence of one or more reactions and the main features of behaviour (such as reaction temperatures, phase transitions, melting etc.), the method cannot be recommended as providing the most accurate kinetic data, particularly when the reaction is reversible. 

At the temperature of a phase transition, the transfer of heat is reversible. 

For most tissues, cells and organs, the effects of cold on the cellular membrane are fully reversible. Cells cooled to 1 °C to 4 °C for short periods of time (about four hours) can regain normal cellular functions, including membrane-linked functions, when rewarmed. This seems to suggest that the phase transition in the membrane-bound phospholipids is reversible when the temperature is elevated to normothermia. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

After expression of poly(VPGXG) genes, the biopolymer can easily be purified from a cellular lysate via a simple centrifugation procedure, because of the inverse temperature transition behavior. This causes the ELPs to undergo a reversible phase transition from being soluble to insoluble upon raising the temperature above the and then back to soluble by lowering the temperature below Tt (Fig. 9). The insoluble form can be induced via addition of salt [27]. The inverse transition can... 

Fig. 9 Purification of ELPs by ITC is based on the reversible inverse phase transition. Le/i Protein purification via direct ELP fusions. A soluble ELP fused to a target protein becomes reversibly insoluble upon increasing temperature above 7,. Center Protein purification via ELP coaggregation. An excess of free ELPs enhances the aggregation of trace quantities of ELP-fusions. Right Purification via ELP-mediated affinity capture (EMAC). ELPs are fused to capture proteins, which bind specifically and reversibly to a target protein. This target protein can then be aggregated at temperatures above the T,. Adapted from [38] with permission from Elsevier, copyright 2010...

Figure 12.12 Kinetics of the (2 x 2) —3CO - ( /l9 xvT9)R23.4° — 13CO phase transition on a Pt( 111) electrode in a CO-saturated 0.1M H2SO4 electrolyte, observed via SFG of atop CO. The frequency shift data in (b) and (e) indicate that a new potential is estabhshed on the electrode within 0.2 s. The forward transformation is much slower than the reverse. There are minimal differences between the first and second cycles, indicating minimal change in electrolyte composition during kinetic measurements.

If a modification is unstable at every temperature and every pressure, then its conversion into another modification is irreversible such phase transitions are called monotropic. Enantiotropic phase transitions are reversible they proceed under equilibrium conditions (AG = 0). The following considerations are valid for enantiotropic phase transitions that are induced by a variation of temperature or pressure. 

---