Patent GB-1377478-A (GB)
Title: Method and apparatus for determination of spinodal and critical  points or loci associated with phase transition
IPC: G01N25/02
Filed: 1971-01-20
Abstract: 1377478 Photo-electric phase investigation M GORDON J M K G COWIE and B W READY J GOLDSBROUGH 13 Dec 1971 [18 Dec 1970 20 Jan 1971(3)] 60171/70 2753/71 2754/71 and 2755/71 Heading G1A Phase transitions of chemical compounds in solution are investigated by subjecting a sample to (positive or negative) thermal pulses which cause the sample to undergo increasingly different temperature changes and measuring photoelectrically the different intensities of light or other radiation which has been scattered by the sample as a result of phase transitions occurring during the thermal pulses and producing distributions of particles. The investigation involves the determination of: spinodal points (the temperature T unst  between the metastable and unstable states of a sample solution) for a number of concentrations to develop a spinodal curve; cloud point temperature T met  e.g. by extrapolation from the same graph used to determine spinodal temperature; to give a cloud point locus; and the critical locus (a point for binary solutions, a curve for a ternary system, being the border between stable and unstable conditions without any intervening state of metastability) from the point(s) of tangency between the locus of Tmet (the cloud point) and the spinodal locus. A typical application is in determining statistical parameters relating to the molecular structure of macromolecules in solution e.g. polystyrene in cyclohexane. Apparatus. A sample is carried in a transparent capillary cell 1 in a temperature controlled environment which is under the control of a fluidflow system. In one arrangement, Fig.3, two fluidcontaining baths 7, 8 are provided, one maintained at constant temperature T 1  and the other capable of variation to a series of different temperatures T 2 . Switching of flow from the baths by valves 12, 13, in alternation, provides the required thermal pulses. In a second arrangement, a single bath 17 is provided which contains a heater at the  top and a refrigerator at the bottom, by which the temperature can be progressively changed (lowered). A heater 26 is also provided next to the sample cell, and thermal pulses of varying temperature drop are effected by switching this on and off. The sample is illuminated from one end by a laser 4 or a collimated light source of e.g. U.V. wavelengths. Scattered light occurring during each thermal pulse is sensed direct or via a light guide by a detector 5 and recorded at 6 with the actual temperature of the sample. Graphs are given illustrating in Fig. 1 (not shown) the light intensity pulses produced as negative thermal pulses are provided to cool the sample by 1 degree in 10 minutes, each pulse comprising a change from a fixed higher temperature T 1  to a progressively lower temperature T 2 ; in Fig.2 (not shown) extra polation technique to determine spinodal and cloud points; in Fig.7 (not shown) determination of cloud point using the apparatus of Fig.4 by a technique involving first maintaining the sample at constant T 1  and then switching off the heater, followed by a gradual increase in the fluid-flow temperature; in Fig.8 (not shown) determination of the critical locus; in Fig.9 (not shown) examples of spinodal curves. It is stated that X-ray scattering may also be measured.