Flocculation energy barrier preventing

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

These are dispersions of liquid drops in an immiscible liquid medium. The most common systems are oil-in-water (O/W) and water-in-oil (W/O). It is also possible to disperse a polar liquid into an immiscible nonpolar liquid, and vice versa these are referred to as oil-in-oil (0/0) emulsions. In order to disperse a liquid into another immiscible liquid, a third component is needed that is referred to as the emulsifier. Emulsifiers are surface-active molecules (surfactants) that adsorb at the liquid/liquid interface, thus lowering the interfacial tension and hence the energy required for emulsification is reduced. The emulsifier plays several other roles (i) it prevents coalescence during emulsification (ii) it enhances the deformation and break-up of the drops into smaller units (iii) it prevents flocculation of the emulsion by providing a repulsive barrier that prevents close approach of the droplets to prevent van der Waals attraction (iv) it reduces or prevents Ostwald ripening (disproportionation) (v) it prevents coalescence of the drops and (vi) it prevents phase inversion. 

The surfactants used for the preparation of disperse systems are seldom effective in maintaining the long-term physical stabihty (absence of flocculation and/or coalescence) of the formulation. This is due to their weak and reversible adsorption and lack of the presence of a high-energy barrier that prevents flocculation as a result of van der Waals attractions. For this reason, dispersants and emulsifiers of polymeric nature that are strongly and irreversibly adsorbed at the interface are required. In addition, these polymeric dispersant provide effective repulsive forces (referred to as steric repulsion) that overcomes the van der Waals attractions. The criteria for an effective dispersant are [1, 2] ... 

The above thermodynamic argument implies that to stabilize an emulsion against flocculation and coalescence, one needs to create an energy barrier between the droplets to prevent their close approach (whereby the van der Waals attraction is strong). Several... 

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