Critical flocculation density

Figure 8.9. Critical flocculation density of a 0.04 wt % PEHA emulsion stabilized with 0.03 wt % PS-6-PFOA as determined by DLS from the average hydrodynamic radius (Yates et al., 1997).

A variety of organic colloids including emulsions and polymer latexes have been dispersed in carbon dioxide in the presence of surfactants (3,13). In most cases, owing to the lower interfacial tension of the former as explained shortly it is easier to form organic-in-C02 emulsions than water-in-C02, emulsions. Sterically stabilized colloids are stable above the critical flocculation density (CFD) and precipitate below this density. In some cases the CFD occurs at the upper critical solution density of the steric stabilizer, that is, the density at which the stabilizer phase separates from CO2, as has been shown by theory (14,15) and experiment (16). So-called ambidextrous surfactants have been designed to allow polymer latexes produced in CO2 to be transferred to an aqueous solution to form a dispersion (17,18). 

DIC Dickson, J.L., Ortiz-Estrada, C., Alvarado, J.F.J., Hwang, H.S., Sanchez, I.C., Lnna-Barcenas, G., Lim, K.T., and Johnston, K.P., Critical flocculation density of dilute water-in-CO2 emulsions stabilized with block copolymers, J. Colloid Interface Sci., Ill, 444, 2004. 

Free energies of interaction for two surfaces coated with adsorbed block copolymer, given in Figure 4b, were used to identify the critical flocculation density (CFD)[72]. As solvent density is decreased from the good solvent limit, the free... 

TABLE 1. Critical flocculation density both with and without the bare surface Hamaker interaction for various stabilizer architectures and properties. Ref. [72]. Note and Vg arc the... 

System Upper Critical Density of Stabilizerin Bulk Solution (P/Pc) Critical Flocculation Density (P/Pc)... 

It is important to note that the UCSD above is for the actual finite molecular weight stabilizer, not an infinite molecular weight stabilizer (theta density). Many experimental stability studies of emulsions and latexes in liquid solvents indicate a correlation between the critical flocculation temperature and the theta temperature, 7 0.[68]. However, the difference between Tq and the UCST is often only a few degrees... 

Figure 7. (a) Schematic illustrating the increase in concentration and expulsion of solvent as surfaces are compressed in the simulations, (b) Pressure-polymer concentration phase diagram. Flocculation of surfaces occurs at the upper critical solution density when the concentration between the surfaces reaches an unstable value (the critical concentration). 

The principle of this method is that the initial slope (time = zero) of the optical density-time curve is proportional to the rate of flocculation. This initial slope increases with increasing electrolyte concentration until it reaches a limiting value. The stability ratio W is defined as reciprocal ratio of the limiting initial slope to the initial slope measured at lower electrolyte concentration. A log W-log electrolyte concentration plot shows a sharp inflection at the critical coagulation concentration (W = 1), which is a measure of the stability to added electrolyte. Reerink and Overbeek (12) have shown that the value of W is determined mainly by the height of the primary repulsion maximum in the potential energy-distance curve. 

Thickener control philosophies are usually based on the idea that the underflow density obtained is the most important performance criterion. The overflow clarity is also a consideration, but this is generally not as critical. Additional factors which must be considered are optimization of flocculant usage and protection of the raking mechanism. 

Stability of the microlatexes obtained depends critically on formulation. A good match (in terms of solubility parameters and molar volume) between oil and lipophiles can lead to very stable latexes. A poor match means unstable latexes. It has been confirmed that certain reverse microlatexes are stable for periods of several years without change in particle size. This implies a total absence of flocculation, despite the big difference in density between polymer particles 1.4) and the continuous phase ( 0.8). 

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