Experimental Measurements of the Adhesive Energy

Observations of large contact angles in emulsions were first reported by Aronson and Princen [105,106]. The authors have studied oil-in-water droplets stabilized by anionic surfactant in the presence of various salts. Similar systems were studied by Poulin [110]. Anionic surfactants such as sulfate, sulfonate, or carboxylate surfactants [106,110] exhibit a good stability and a strong adhesion in the presence 

The energy of adhesion between hexadecane droplets stabilized in water by SDS in the presence of NaCl is shown in Fig. 2.35. It is observed that the adhesion depends strongly on the temperature and on the salt concentration. For a given salt concentration, there is a well defined temperature, T, above which there is no adhesion. As the behavior of the surface energy changes at T, this temperature can be referred to as a wetting transition temperature [109]. The dependence of T versns the salt concentration is plotted on Fig. 2.36. 

Moreover, the adhesion depends strongly on the nature of the salt [105]. A pronounced difference is noted in Fig. 2.37, where the energy of adhesion with KCl, NaCl and LiCl are compared [110]. The system is much more adhesive with KCl. This is the contrary with a carboxylate surfactant [106,110]. This behavior is important because it suggests that the adhesion is linked to the solubility of the 

Rgure 2.36. Onset of adhesion, T, between hexadecane droplets stabilized in water by SDS as a function of the NaCl concentration. (Adapted from [111].) 

The study of inverse adhesive emulsions has revealed the same features as direct emulsions [112,113]. Here again, it was shown that adhesion is favored when the surfactant becomes less soluble in the continuous phase [113]. This can be tested experimentally by using binary mixtures of oils, one in which the surfactant is soluble and another one in which the surfactant is insoluble. For example, water droplets can be stabilized in mineral oil by sorbitan monooleate (Span 80). This surfactant is soluble in dodecane whereas it is not in silicon oil. The affinity of the surfactant for the organic solvent can be tuned by mixing dodecane and silicon oil. As shown in Fig. 2.38, the energy of adhesion between water droplets strongly varies as the ratio of the mixture is changed. A sharp rise is noted as the surfactant 

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