ABSTRACT
Emulsion systems still attract many researchers. This is because of their tremendous practical application in many fields of human activity and occurrence as natural systems [1].
Nevertheless, many aspects of emulsion properties (stability/instability) are not well known yet. Mostly emulsions are required to be stable, and their stability is dependent on their concentration and is also a function of the oil droplet size. The smaller the diameter the more stable the emulsion is [2]. Considering an oil-in-water (o/w) type emulsion, the Laplace pressure A P inside small oil droplets (say, below 1 pm) is sufficiently high to prevent their deformation in most practical conditions, so the droplets behave like rigid spheres:
where y is the interfacial oil/water tension and r is the droplet radius. It should be stressed that because thermodynamically the o/w system is unstable, as long as there are no factors that hinder decrease in the total surface of the droplets, it will tend to coalesce very fast. Emulsion instability can occur via creaming (sedimentation), aggregation, and coalescence [2, 3], and in the case of a pure oil phase dispersed in water these processes will cause separation of the phases. Therefore, to prolong the life of the dispersed state of the oil phase a third component is added, which is called an emulsifier. It collects at the oil/water interface, thus forming an adsorbed film on the oil droplet surface. This causes a decrease in the interfacial tension, and usually some changes in the electric potential (charge) take place at the interface. Applying the DLVO theory, the balance of attractive dispersion and repulsive electric interactions between the oil droplet-oil droplet and the water phase can be evaluated. However, at present it is known that the classical DLVO theory in many systems is not sufficient, so the extended theory is needed to describe the interactions correctly [4, 5].
