Irreversibility flocculation

The foundations of the theory of flocculation kinetics were laid down early in this century by von Smoluchowski (33). He considered the rate of (irreversible) flocculation of a system of hard-sphere particles, i.e. in the absence of other interactions. With dispersions containing polymers, as we have seen, one is frequently dealing with reversible flocculation this is a much more difficult situation to analyse theoretically. Cowell and Vincent (34) have recently proposed the following semi-empirical equation for the effective flocculation rate constant, kg, ... 

As an irreversible flocculation occurred with a drop-wise approach for both cationic and anionic PEC particles when the mixing ratio was close to unity, a more versatile and simpler method was found to be the one-shot addition of one solution to the other. Characterisation of particles obtained via this path revealed very similar properties to those obtained by a slow drop-wise approach [155]. 

In Fig. 1.1 the DLVO interaction potential Wdlvo is schematically sketched together with its two contributions. If the maximum of Wdlvo is sufficiently high (larger than a few kT), flocculation is prevented. Flocculation does occur when the particles can get very close into the socalled primary minimum this minimum is usually deep enough for irreversible flocculation. 

Addition of soluble homopolymer can lead to reversible and irreversible flocculation by depletion and bridging flocculations (41). 

The lyophobic systems are also called suspensoids. They easily precipitate out forming irreversible flocculates. But if they are left undisturbed, despite their intrinsic instability they remain unchanged for long periods (see Box 3.1). Research has shown that it is the electrical potential difference between the surface and the solution far away from the particles that determines the dluturnity (stability) of the lyophobic system. This electrical potential difference arises due to the charge carried by the particles of a sol, and the charge carried by the ions in the surrounding medium. 

The main criteria for pharmaceutical suspensions are [35-37] Colloid and physical stability, with acceptable shelf-life under storage conditions colloid stability indicates the lack of any strong irreversible flocculation. On application, any aggregates should be broken down to single particles (weak flocks are broken under shear). 

Figure 1.13 Proportion of lattice sites occupied as a function of height after 20 minutes, simuiating an emulsion undergoing irreversible flocculation. The numbers in the key refer to the break-up probability, fbre k. and the bonding probability, / bond, respectively...

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