Emulsion gibbs phase rule

Due to this structure they tend to adsorb at the interfaces between a polar and a nonpolar fluid, for example water and oil. Emulsifiers reduce the surface tension and stabilize the surface by steric, electrostatic or hydrodynamic (Gibbs-Mar-angoni) effects [14], Droplet coalescence (flowing of one or more droplets together) can thus be reduced or prevented. Some emulsifiers can be characterized by their hydrophilic/lipophilic balance (HLB) value that provides information on the ratio of hydrophilic to lipophilic character of the surfactant molecule. The HLB value helps to determine the phase in which the emulsifier is soluble. Usually, the emulsifier used is soluble in the continuous phase (Bancroft rule). Furthermore, the HLB value gives a first hint whether the emulsifier is suitable for the production of an o/w (HLB value 8-18) or a w/o emulsion (HLB value 4—6) [15]. 

The Gibbs-Marangoni effect would also explain Bancroft s rule when making an emulsion of two phases, the one in which the surfactant is most soluble will become the continuous phase. If the surfactant is in the droplets, a y-gradient as depicted in Figure 2.12b would never develop, and the drops would be prone to coalescence. Hence, surfactants with an HLB value >7 tend to produce O/W emulsions those with HLB < 7, W/0 emulsions. 

Investigations of the effects of oil-soluble surfactants on the emulsification of paraffins in aqueous surfactant solutions led to the proposal that the formation of interfacial complexes at the oil-water interface could increase the ease with which emulsions could be formed and, possibly, explain the enhanced stability often found in such systems (Figure 9.9). By definition, an interfacial complex is an association of two or more amphiphilic molecules at an interface in a relationship that will not exist in either of the bulk phases. Each bulk phase must contain at least one component of the complex, although the presence of both in any one phase is not ruled out. The complex can be distinguished from such species as mixed micelles by the fact that micelles (and therefore mixed micelles) are not adsorbed at interfaces. According to the Le Chatelier principle, the formation of an interfacial complex will increase the Gibbs interfacial excess F/ [Eq. (9.2)] for each individual solute involved, and consequently, the interfacial tension of the system will decrease more rapidly with increasing concentration of either component. 

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