Flocculation electrolyte effect

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

Fig. XIV-9. Effects of electrolyte on the rate of flocculation of Aerosol MA-stabilized emulsions. (From Ref. 35.)...

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

A comparatively minute concentration of an electrolyte results in flocculation. The change is, in general, irreversible water has no effect upon the flocculated solid. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

Gmin.in the free energy-particle separation curves. Since AS g is reduced in concentrated dispersions,the flocculation of the dispersion occurs at relatively lower Gm- n than that observed with dilute dispersions. Thus, this effect would result in a reduction of the CFC for concentrated dispersions. However, the net result of reduction of CFC may be due to a combination of this effect and depletion of electrolyte from the dense region of the adsorbed layers. 

These data require extension but in a tentative manner the conclusions can be summarized in Figure 8 where the domains of coagulation and flocculation are represented. Moreover, these ideas have only been applied to sodium chloride. With higher valency electrolytes more specific effects may occur which could dominate the phenomena. 

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... 

The effect of Na+ on the stability of water-in-oil emulsions is exercised mainly through its influence on sodium caseinate. It has been shown that as the surface concentration of casein on oil droplets is increased, the oil-in-water emulsion becomes less susceptible to flocculation/coalescence in the presence of electrolyte. Added NaCl broadens the droplet size distribution at a low casein content (0.25%) but causes this effect at a high casein content (0.5%) only when CaCl2 is added (Dickinson et al., 1984). 

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