Half emulsion flocculation

I 70 Formulation of Liquid/Uquid Dispersions (Emulsions) Table 10.4 Half-life of emulsion flocculation. 

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. 

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. 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

According to W/O emulsion work by Lucassen-Reynders [34] droplet coverage is achieved only if the number of particles is much higher than the number of droplets. In her study, the number of tristearate crystals was 1000 times the number of droplets, indicating that half of the fat crystals flocculated in 1 s compared with 1000 s for the water droplets. Hence, the fat crystals were able to cover the droplets and form a network before any real droplet coalescence could occur, thereby stabilizing the emulsion. The network formation, however, also hindered the free diffusion of crystals to the interface. Without the presence of surfactants in the initial emulsion mix, no stabilization was observed. With added surfactant, however, crystal flocculation was reduced as the interparticle bond energy was lowered. Results showed that partial flocculation was best for W/O emulsion stabilization by tristearin. Lagaly et al. [114] also observed that surfactants aided the colloidal stabilization of emulsions. 

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