Peculiarities of surfactants selection for emulsion stabilisation

The problem of selecting a surfactant for an effective emulsion stabilisation has already been intensively discussed for more than 80 years after the publication of the classic work by Bancroft [33]. This rule which in some papers [52] is referred to as a thumb rule gives a 

Using the HLB system for the characterisation of surfactants, a minimum interfacial tension is observed when the HLBr is reached. Fig. 6.7 shows, as an example, the interfacial tension and the droplet size in emulsions of decane and sunflower oil, respectively, as a function of the HLB. For decane, the minimum y value and the minimum size of emulsion droplets are observed in the region of HLB 9 and for sunflower oil around 11, which corresponds to the required HLB values. Apparently, one of the reasons leading to an increase in emulsion stability when reaching HLBtp is the increase of emulsion dispersity under conditions of the maximum decrease in interfacial energy. 

Two processes take place simultaneously in the emulsion formation a) dispersion of one liquid in another b) coalescence, i.e. irreversible fusion of two or more droplets and formation of a larger one. The coalescence proceeds in several subsequent stages [54] formation of a thin film as result of droplet contact, thinning of the film, and its rupture when achieving a critical thickness. The role of the surfactant concentration in the formation of emulsions can be demonstrated using as an example sunflower oil emulsions stabilized by potassium monododecyl phosphate (see Fig. 6.8). 

The emulsions were prepared on an ultrasonic disperser, coalescence stability was determined from the volume of the separated oil phase after 24 hours. As it is seen from Fig. 6.8, a complete coalescence stability of the emulsions is achieved at surfactant concentrations beyond the CMC. At the same time, the concentration at which the maximum coalescence stability is achieved is not sufficient to provide sedimentation stability of the emulsions. 

The maximum sedimentation stability is achieved at C CMC. After the CMC, the emulsion droplet size decreases in accordance with the rate of interfacial tension decrease. Increase in sedimentation stability is observed in accordance with increasing degree of dispersity. 

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