Critical coagulation temperature

CFT Critical flocculation temperature. See Critical Coagulation Temperature. 

Critical Coagulation Temperature (CCT) The minimum temperature to which a dispersion must be raised to induce coagulation. See also Critical Coagulation Concentration. 

In most experiments the smallest amount of electrolyte needed to coagulate the sols measured after 2 hours standing was chosen as the CCC. When using HC1, this point is the critical coagulation pH. A constant temperature water bath was used for temperature different than 23°C. The pH values were measured with a Beckman Model 96A pH meter and a Fisher combination electrode. The electrophoretic mobility measurements were made with a Laser Doppler Electrophoresis apparatus. These experiments were performed by Mr. J. Klein of the Chemistry Department, Syracuse University. 

The sensitivity of the stability ratio to chemical or particle interaction factors can be illustrated by an examination of the model expression for Wn in Eq. 6.75. For example, if temperature and the particle interaction parameters are fixed, then Wn will vary with the concentration, c (also included in /c), of Z-Z electrolyte. At low values of c, k is also small, and the first equality in Eq. 6.75 indicates that Wu will take on its largest values. (Decreasing c also provokes an increase in dm because of Eq. 6.73, but this effect is dominated by that of k.40) Conversely, as c increases, the value of Wu will drop until it achieves its minimum, Wn = 1.0, when Z dm = 2 (Eq. 6.75). At this concentration, termed the critical coagulation concentration (ccc), or flocculation value, the flocculation process has become transport-controlled and therefore is rapid. Thus in general... 

Indeed, the presence of increasing amounts of bound Na" " must switch off the electrosteric contribution of the adsorbed polysilicate layer, and, for the pH 8.0 sol at its critical coagulation concentration, which is greater than O.lSAf, a secondary minimum well of >2kT must open up as a result of the compression of the thickness and screening length of the adsorbed layer (k, Boltzmann constant T, temperature). 

The coagulants (typically multivalent counterions) commonly used are the salts of aluminium and iron and salts or the bases of calcium and magnesium. The transition from stabilized to destabilized emulsions on changing the temperature is very sharp at the critical flocculation temperature (CFT) when nonionic surfactants are used. Generally, aqueous dispersions destabilize upon increasing temperature, while non-aqueous dispersions destabilize with decreasing temperature. 

The polymer used in the thermal inversion process of making membranes was initially dissolved and maintained at a temperature above the critical point of the binary system under consideration. Solutions of 10wt% polystyrene in cyclohexanol were prepared at 90°C and then placed in an oven at a temperature of 115-120°C. Note that the upper critical solution temperature of polystyrene/cyclohexanol system is about 82°C (Shultz and Flory, 1953). Also, the critical composition is at a polymer volume fraction of about 0.03, while the polymer-rich binodal composition is at a volume fraction of about 0.20. Thus, the cast solutions are expected to coagulate inside the spinodal curve, as we have verified using a diode-array time-resolved light scattering system similar to that used by Hashimoto and his coworkers (Inaba et al., 1986). Solid membranes were then made within 24 h, after the solutions were prepared. These membranes were then sputter-coated with gold-palladium and observed under the microscope. 

Beaded acrylamide resins (28) are generally produced by w/o inverse-suspension polymerization. This involves the dispersion of an aqueous solution of the monomer and an initiator (e.g., ammonium peroxodisulfates) with a droplet stabilizer such as carboxymethylcellulose or cellulose acetate butyrate in an immiscible liquid (the oil phase), such as 1,2-dichloroethane, toluene, or a liquid paraffin. A polymerization catalyst, usually tetramethylethylenediamine, may also be added to the monomer mixture. The polymerization of beaded acrylamide resin is carried out at relatively low temperatures (20-50°C), and the polymerization is complete within a relatively short period (1-5 hr). The polymerization of most acrylamides proceeds at a substantially faster rate than that of styrene in o/w suspension polymerization. The problem with droplet coagulation during the synthesis of beaded polyacrylamide by w/o suspension polymerization is usually less critical than that with a styrene-based resin. 

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