Orthobaric conditions

Fig. 13.2 Vapor pressures (PV,T) and liquid molar volumes (Vm,T) along the orthobaric LV equilibrium line for reference isotopomer pairs, showing their distinctly different behaviors (schematic). The thicker lines label the heavier isotopomers. The curves begin at the triple points and end at the critical points (except for He which has no triple point under orthobaric conditions). Not to scale, isotopic differences are exaggerated (Reprinted from Van Hook, W. A., Rebelo, L. P. N. and Wolfsberg, M., Fluid Phase Equilib. 257, 35 (2007), copyright 2007, with permission from Elsevier)...

Figure 2. FIR spectra of liquid methyl fluoride. Temperatures and densities follow orthobaric conditions from 133 K (right) up to 293 K (left).

On figure 2, from reference (6), a shift towards low frequencies is observed as the temperature of the liquid methyl fluoride is increased gradually along the liquid-gas coexistence curve (i.e. under orthobaric conditions). Here the density effect is essentially responsible for the frequency shift following the loosening of local structures. 

Actually, values ranging from 1 to 3 has been found in different liquids. Moreover, varying continuously the orthobaric conditions of the liquid state for one selected compound may allow to observe a definite change, so that assignments to different mechanisms can be made. As an example the comparison of with for liquid OCS under orthobaric conditions is shown on figure 8,... 

We have applied the model over an extended range of external conditions from the triple point up to the critical point of water. Figure 2.24 compares the experimental " (steam tables) vapor pressures with the calculated ones. The two sets of values are practically identical. Figure 2.25 compares the corresponding values for the orthobaric densities of water. As observed, the density is well described over the full range. 

---