Flocculation reversible

A further prediction of the DLVQ theory is that under certain conditions the total potential-energy curves may exhibit a secondary minimum. It is then expected that reversible flocculation may occur, and discussion of this follows in the next section. 

The above is only a very brief account of the DLVO theory, since its full development involves rather elaborate mathematics and some necessary approximations which arc probably of limited validity. Nevertheless, the general principles upon which it is based are valuable guides to an understanding of lyophobic colloids. 

It is interesting to note that, if the formation of a double layer is regarded as the adsorption of counter-ions by the charged particle, then the principles concerning the effect of adsorption on interparticle forces set out in Chapter 5 ean be applied. Thus, as two double layers overlap at constant potential, counter-ions are rejected from the space between the particles, the adsorption of counter-ions decreases, and, according to the general principles discussed in Chapter 5, the particles repel one another. 

The problem is closely analogous to that we discussed earlier in connection with nucleation in molecular systems (Chapter 4). In both instances we are concerned with the factors which determine whether an aggregate of molecules or particles grows in size or tends to disintegrate. In this chapter we shall put the theory into a more precise form by considering the aggregation process as a sequence of addition reactions involving colloidal particles  

Each step will be governed by an equilibrium constant, K(i, i + 1), which, assuming that the particles behave as ideal solutes, is given by 

This technique is useful when it is desired to flocculate an aqueous dispersion temporarily (e.g., for ease in filtration, handling, or storage), but in a condition where it can readily be deflocculated. The particles are first treated with an ionic surfactant that imparts a potential to the particles sufficiently high to disperse them. Then the dispersion is treated with sufficient easily soluble electrolyte to compress the electrical double layer surrounding the particles to the point where flocculation 

Reversible flocculation of aqueous dispersions stabilized sterically with POE nonionics can be accomplished by increasing the temperature. With increase in temperature, the hydrogen bonding of the POE chains to water is reduced and the chains tend to aggregate, flocculating the dispersion. Upon reducing the temperature, the chains hydrate again and the particles redisperse. 

THE RELATION OF SURFACTANT CHEMICAL STRUCTURE TO DISPERSING PROPERTIES 

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GC Jeffrey, RH Ottewill. Reversible aggregation Part reversible flocculation monitored by turbidity measurements. Colloids Polymer Sci 266 173-179, 1988. 

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, ... 

With regard to reversible flocculation kinetics, the problem is even more challenging- Detailed models for the deflocculation process as well as the flocculation process are required computer simulation is probably going to be the only way forward here ... 

This model was introduced by Neville and Hunter (13,14) for the case of sterically stabilized dispersions which have undergone reversible flocculation. It is assumed that the major contribution to the excess energy dissipation in such pseudoplastic systems comes from the need to provide energy from the shear field to separate contacting particles. Under these conditions, the extrapolated yield value is given by the expression (13,32,33),... 

We first consider emulsion droplets submitted to attractive interactions of the order of ks T. Reversible flocculation may be simply produced by adding excess surfactant in the continuous phase of emulsions. As already mentioned in Chapter 2, micelles may induce an attractive depletion interaction between the dispersed droplets. For equal spheres of radius a at center-to-center separation r, the depletion... 

When Of a, the potential energy of interaction between an elementary particle and a hoc is equivalent to the potcntiul energy 4(h) between a particle and a fiat plate thus 4(h) used by Huckenstcin and Prieve (1976) still applies. Furthermore, if the conditions necessary for Equation (13) of that paper apply, then (berate constants for reversible flocculation inay be calculated as K/ = 4 / Kf and Kr — Aoaf K where X/ and K, are given by Equation (13). 

In case of reversible flocculation, the temporary existence of monomers and dimers is assumed and the degree of flocculation (D) was calculated from the equation ... 

State (g) represents the case of weak and reversible flocculation. This occurs when the secondary minimum in the energy distance curve is deep enough to cause flocculation. This situation can occur at moderate electrolyte concentrations, in particular with larger particles the same occurs with sterically and electrosterically stabihsed suspensions. It also occurs when the adsorbed layer thickness is not very large, particularly with large particles. The minimum depth required to cause... 

Jeffrey GC, Ottewill RH. Reversible aggregation. Part I. Reversible flocculation monitored by turbidity measurements. Colloid Polym Sci 1998 266 173-179. 

In summary the depth of the secondary minimum increases with electrolyte concentration for particles of the same size and also increases with particle size. The possibility of reversible flocculation has been examined using a series of polystyrene latexes containing particles of increasing diameter namely, 210 nm, 340 nm, 781 nm and 1.01 pm [46]. The ccc was determined optically for each sample by the addition of sodium chloride. The sample was then dialysed against distilled water and, after 20 h, it was examined optically. The results are shown in Figure 3.21. In the case of the smallest particles the turbidity ratio (turbidity measured at two wavelengths) was either the same or lower than that obtained immediately before dialysis. This indicated that for this system the... 

The kinetic equation for reversible flocculation in a dilute monodisperse o/w emulsion when neglecting coalescence is [52—54]... 

Application of the microslide preparative technique combined with video microscopy is promising and has allowed the measurement of flie coupling of reversible flocculation and coalescence (27, 29). However, some experimental difficulties were encountered droplets could sometimes be seen sticking to flic glass surface of the microslide. 

If the depth of the primary minimum (that on the left from the maximum in Fig. 6a) is not so great, i.e., the attractive force which keeps the drops together is weaker, then the floes formed are labile and can disassemble into smaller aggregates. This is the case of reversible flocculation (3). For example, a floe composed of i+j drops can be split into two floes containing i and j drops. We denote the rate eonstant of this reverse process by (see Fig. 20a). In the present case bofli the straight process of flocculation (Fig. 19) and the reverse process (Fig. 20a) take simultaneously plaee. The kinetics of aggregation in this more general and eomplex case is described by the Smoluchowski set of equations, Eq. (96), where one is to substitute ... 

Figure 21 Plot of the inverse dimensionless concentration of floes, n n, vs. the dimensionless time, t = ajn ri, for A/ = 4 and various values of the dimensionless ratio u = 2afJ(nQa ar and afare the rate constants for the reverse and straight processes. Theoretical curves for reversible flocculation from Ref. 62.

Next, Professor Dukhin et al. contribute a ehapter dealing with fundamental processes in dilute 0/W emulsions. A basic problem is to couple the processes of coalescence and flocculation by introducing a reversible flocculation, i.e., a process whereby the floe is disintegrated into individual droplets. The authors have utilized video-enhanced microscopy (VEM) to study the emulsified systems and to determine critical time eonstants for a stepwise flocculation/deflocculation and coalescence. 

Kan KH, Li J, Wijesekera K, Cranston ED (2013) Polymer-grafted cellulose nanocrystals as pH-responsive reversible flocculants. Biomacromolecules 14 3130-3139... 

An additional important prediction of the DLVO theory is that under certain conditions, a coUoid may undergo a form of reversible flocculation brought about by the existence of a so-called secondary minimum in the potential-energy curve. The existence of the secondary minimum has been confirmed experimentally and since it represents a potentially important theoretical and practical aspect of the DLVO theory, it will be discussed briefly below. 

FIGURE 10.16. In many colloidal systems, the interaction energy curve will have a small minimum, the secondary minimum, M , that allows the particles to undergo a lose, reversible flocculation. In some systems of relatively large, monodisperse particles, the secondary minimmn may lead to an optical phenomenon called opalescence in which a very regular structure is developed (similar to a crystal structure) that produces beautiful and interesting patterns with incident light. 

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