Temperature-salt concentration phase diagram

FIGURE 1.16 Temperature-salt concentration (T-c) phase diagram of n-butylammonium vermiculite. 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

A number of publications have appeared recently on super-lattice complexes which have enhanced conductivity, eg. "nazirpsio NaaPOif 2Zr02 2Si02 whose conductivity at room temperature is of the same order as that of an aqueous salt solution. Most of the super-lattices are unstable thermodynamically, and can be expected to collapse under chemical attack by the anodic and cathodic reactants. However, there may exist some thermodynamically stable structures, and the search should concentrate on the complicated phase-diagram studies of selected quatemarys. 

The phase diagram was mapped out on the LOQ instrument at ISIS, Didcot, U.K. [16], just as for the butylammonium vermiculite system [6]. Typical scattering patterns from an r = 0.01, c = 0.25 M propylammonium vermiculite gel are shown at T = 36, 38, 40 and 42°C in Figure 9.6. The story is by now a familiar one a gel with a well-defined d-value at low temperatures (in this case d = 60 A) collapses as the temperature increases at a well-defined phase transition temperature (in this case Tc = 39°C). Note that the butylammonium system will not swell at c > 0.2 M, whereas here we have colloidal swelling at c = 0.25 M. The complete c, T phase diagrams at low r for both the propylammonium and butylammonium systems, taken in contiguous experiments, are shown in Figure 9.7. It is clear that the propylammonium vermiculites will swell in salt concentrations up to about 0.5 M in cold water. In these circumstances, their -values decrease below 50 A, and so could be measured on LAD. 

The phase behaviour of a mixed biopolymer solution is quantitatively characterized by a phase diagram, which graphically describes the boundary conditions of phase separation, the partitioning of the components between the phases and the effects of different variables (temperature, pH, salt concentration, etc.) on the phase behaviour of biopolymers. Conventionally, phase diagrams of three-component (ternary) systems are presented in triangular coordinates. However, an excess of solvent water... 

Hence, fixing the temperature and one of the concentration parameters (for example, the salt concentration) suffices to define the system. The phase diagrams were experimentally derived by using fractions of potato starch... 

Proteins, like any other molecule, have a certain solubility limit, above which an aggregated state (either amorphous or crystalline) is thermodynamically favored. The solubility of a protein as a function of some variable like temperature, pH, or the concentration of a precipitant such as salt or polyethylene glycol (PEG) may be represented with a phase diagram (Figure 4). 

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