The Phase Diagram
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The phase diagram delineates the potential working area for a particular two-phase system and is a “fingerprint” unique to that system under set conditions of, for example, pH, temperature and salt concentration. Information that can be generated from such a diagram ( see Fig. 1 ) includes: the concentration of phase-forming components necessary to form a system with two phases that are in equilibrium, the subsequent concentration of phase components in the top and bottom phases, and the ratio of phase volumes. Present on the diagram is a binodal curve, which divides a region of component concentrations that will form two immiscible aqueous phases (i.e., above the curve) from those that will form one phase (i.e., at and below the curve). Coordinates for all “potential” systems will lie on a tie-line; the tie-line connects two nodes on the binodal, which represent the final concentration of phase components in the top and bottom phases. Moving along the tie-line coordinates denote systems with differing total compositions and volume ratios, but with the same final concentration of phase components in the top and bottom phases. Also present on the binodal is a critical point ; just above this point the composition and volume of both phases are almost equal (as is partitioned material). A theoretical account of the phase diagram can be found in refs. 1-3 .
Fig. 1. Illustration of the phase diagram. Bottom phase polymer/salt X (% w/w) is plotted on the abscissa and top phase polymer Y (% w/w) is plotted on the ordinate. A1, A2, and A3 represent the total compositions (●) of three systems lying on the same tie-line with different volume ratios. The final composition of the top and bottom phase is represented by nodes T and B (■), respectively. The ratio of the segments AB (top phase) and AT (bottom phase) can be approximated graphically by the volume ratio of the two phases. The critical point, Cp (◯) is determined by extrapolation (------) through the midpoints of a number of tie-lines. The difference in concentration of component X and Y between the two phases is represented by AY and AX.
