Phase transition temperatures pressure effects

Figure 12 Effect of pressure on the three phase-transition temperatures of TTF-TCNQ. (From Ref. 80.)...

In summary, we calculate that the low clinoenstatite is not stable under hydrostatic conditions and enstatite has a comparatively small stability field. The energy differences are so small that they are within the reliability of the simulations and thus the precise positions of the phase boundaries are not well located. The primary reason for this problem is the reliability of the potential models. Hence, calculating phase relationships represents the most difficult challenge for free energy minimization techniques. However, the simulations do provide valuable insights into the mechanisms of phase transitions and the effect of pressure and/or temperature on the crystal structures and the relative phase stabilities. 

At 1 atm (Figure 2.4a), an increase of up to 1 1 B/BN ratio does not change the while it significantly increases the degree of conversion. This surprising effect is again related to the dissociation of the product. Also, at some critical amount of dilution (x = 1.2), one can reach full conversion even at low gas pressure. At 100 atm, three characteristic regents can be outlined (i) /<0.1 - dissociation temperature defines combustion (ii) 0.2 < 2 < 0.7 - another phase transition temperature, that is, BN m.p., equal to (iii) 7 > 0-7 - increase of x leads... 

The effect of pressure on the phase-transition temperature can be evaluated using the Clausius-Clapeyron equation ... 

Plot the enthalpy, entropy, and chemical potential of water as a function of temperature at some pressure P using the Steam Tables, and identify the discontinuities occuring at the phase transition. Neglect the effect of pressure on the enthalpy and entropy of the liquid. 

It has been shown that ab initio total energy DFT approach is a suitable tool for studies of phase equilibria at low temperatures and high pressures even when small energy differences of the order of 0.01 eV/mol are involved. The constant pressure optimization algorithm that has been developed here allows for the calculation of the equation of state for complex structures and for the study of precursor effects related to phase transitions. 

Ruckenstein and Li proposed a relatively simple surface pressure-area equation of state for phospholipid monolayers at a water-oil interface [39]. The equation accounted for the clustering of the surfactant molecules, and led to second-order phase transitions. The monolayer was described as a 2D regular solution with three components singly dispersed phospholipid molecules, clusters of these molecules, and sites occupied by water and oil molecules. The effect of clusterng on the theoretical surface pressure-area isotherm was found to be crucial for the prediction of phase transitions. The model calculations fitted surprisingly well to the data of Taylor et al. [19] in the whole range of surface areas and the temperatures (Fig. 3). The number of molecules in a cluster was taken to be 150 due to an excellent agreement with an isotherm of DSPC when this... 

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. 

---