Solvates phase diagram

Figure 17.4 The phase diagrams of the systems (a) HF/H2O and (b) HCI/H2O. Note that for hydrofluoric acid all the solvates contain >1HF per H2O, whereas for hydrochloric acid they contain <1HC1 per H2O. This is because the H bonds F-H F and F-H O are stronger than O-H O, whereas C1-H---C1 and C1-H---0 are weaker than 0-H---0. Accordingly the solvates in the former system have the crystal structures [HsOJ+F , [H30]+[HF2] and [H30] [H3F4], whereas the latter are [H30]+C1 , [H502]" C1 and [H502]" CP. H2O. The structures of HCI.6H2O and the metastable HCI.4H2O are not known.

Fig. 8 Phase diagram showing the triple point and the critical point. The supercritical zone exists at temperatures and pressures above the critical point. In the supercritical zone, the compound has the density and solvating power of a liquid, but the diffusivity and viscosity of a gas, and exists in a single homogeneous phase. Below the critical point, and along the liquid/gas coexistence line, a liquid and gas phase split can be observed visually. At the triple point, solid, liquid, and gas coexist. At temperatures and pressures below the triple point, solid can sublime directly to gas, for example, by freeze-drying.

In the case of solvates, binary phase diagrams of temperature versus concentration of the solvent (or water) at a given pressure are useful for the understanding of the phase transitions. The characterization of solvates and hydrates need the use of both DSC and TG. Desolvatation can be complex melting of the solvate followed by exothermic recrystallization into the anhydrous form or solid-state transformation with... 

The DSC, TG curves of solvates and hydrates are related to the phase diagrams between substance and solvent (or water). Eutectic are observed. Fusion or decomposition of the solvate may occur during heating. Therefore, one may observe the melting of the solvate followed by recrystallization into the anhydrous form or the endothermic desolvatation in the solid state. In certain cases both phenomena may over-lapp. Details about experimental factors and examples can be found in Ref. If the anhydrous form is metastable, further phase transitions follow the desolvatation. If several solvates or hydrates exist, the transitions observed depend on the pressure, as demonstrated by Soustelle in the case of copper sulfate pentahydrate. Depending on the pressure, the direct dehydration into the anhydrous or the dehydration via the monohydrate, or the three dehydration steps trihydrate, monohydrate and anhydrous forms may be obtained. Hydrates have been the subject of... 

Often the solvates (hydrates) are not detected since, according the corresponding phase diagram, at ambient temperature, they can be partly or completely dissociated. Suspensions of hydrates in water should shift the equilibrium toward the formation of the stable hydrated form. The ability of DSC measurements at subambient temperatures allow to determine phase transitions. Giron et al. proposed to use the melting peak of freezable water for the analysis of suspensions of drug substances in water in combination with TG for the determination of the number of molecules of water bounded as hydrates. 

Setting aside any consideration of solvate species or considerations of chemical reaction, systems of polymorphic interest consist of only one component. The complete phase diagram of a polymorphic system would provide the boundary conditions for the vapor state, the liquid phase, and for each and every true polymorph possible. From the phase rule, it is concluded that the maximum amount of variance (two degrees of freedom) is only possible when the component is present in a single phase. All systems of one component can therefore be perfectly defined by assigning values to a maximum of two variable factors. However, this bivariant system is not of interest to our discussion. 

When substances are capable of forming multiple solvated forms, it is observed that the different solvates will exhibit different regions of stability, and the pressure-temperature phase diagram becomes much more complicated. Each solvate will be characterized by its own dissociation curve, and these families of curves mutually terminate at points of intersection. Each dissociation curve will exhibit first an initial increase and then a plateau as conversion to another solvation state begins, and then a decrease as the vapor pressure of the solvate product becomes established. At temperature values slightly above the intersection point of two dissociation curves, the solvate product would have a higher vapor pressure than the solvate reactant and would therefore be metastable with respect to the higher solvate. However, once the temperature is allowed to rise beyond the plateau value, the solvate product becomes the stable phase. 

A Partial Phase Diagram and Crystal Solvate for the Poly(p-Phenyleneterephthalamide)/SuUiiric Acid System... 

Although the presence of solvate phases has been established and qualitative phase diagrams have been published, to our knowledge, a detailed model for a pol3nner solvate and its phase behavior has not been presented. At this time we would like to present a partial phase diagram for the poly(p-phenylene terephthalamide) (PPTA)/sulfuric acid system and a model for the crystal solvate formed. In addition the structure of a model complex will be described. 

Section A-B in Fig. 2 shows that the solubility falls as the contaminant is diluted by the fluid. The rapid rise in solubility in Sec. B-C occurs at pressures quite higher than the critical pressure because of the rapid rise in density, and therefore solvating power, of the SCF at around this pressure. This r on has been defined by King as the threshold pressure which is the pressure at which the solute begins to dissolve in the SCF. l Obviously, this pressure is technique dependent and varies with the analytical method sensitivity used to measure the solute concentration in the SCF. A decrease in solubility, as shown in r on C-A may occur at higher pressures due to r ulsive forces that may squeeze the solute out of solution. For moderately volatile solutes, a rise in solubility, as shown in section i>- , can occur if there is a critical line in the mixture phase diagram at higher pressures. 

Fig. 1-39. Phase diagram for a binary solvent/ dissolved substance system with eutectic point without solvate formation.

Fig. 1.4 Phase diagrams of (AN) -LiX mixtures with LiPFj, LiTFSI, LiC104, UBF4, and LiC02CF3 [134, 135] and ion/solvent coordination within the solvate crystal structures (a) (AN)6 UPFe [137], (b) (AN)5 LiPF6 [138], (c) (AN)i LiTFSI [139], (d) (AN)4 LiC104 [140], (e) (AN)2 LiBF4 [141], and (f) (AN)i LiBF4 [142] (the sample values are indicated by an x for fuUy amorphous samples and by a triangle for partially crystalline samples)...

D. Brouillette, D. E. Irish, N. J. Taylor, G. Perron, M. Odziemkowski, J. E. Desnoyers, Phys. Chem. Chem. Phys. 2002, 4, 6063-6071. Stable solvates in solution of lithium bis(trifluoromethylsulfone)iniide in gjymes and other aprotic solvents Phase diagrams, crystaUogiaphy and Ramtm spectroscopy. 

---