In a three-phase equilibrium of H2O - oil - amphiphile mixtures, the middle amphiphile rich phase may or may not wet the water/oil interface. For nonwetting middle phases, theory predicts a nonwetting → wetting transition upon approaching either one of the two critical endpoints. With respect to an experimental confirmation of this prediction, the situation appears to be controversial. In this paper, we have, therefore, studied the wetting behavior of the middle phase as it depends on the amphiphilicity of nonionic amphiphiles. We find that in mixtures with short-chain amphiphiles, the middle phase wets the water/oil interface in the entire three-phase interval, whereas with long-chain amphiphiles it (apparently) never wets. For medium-chain amphiphiles, however, one does find a nonwetting → wetting transition. On the basis of this result, we suggest that there exist four cases for the wetting behavior as a consequence of the dependence of the relations between the three interfacial tensions on amphiphilicity. The wetting behavior can be correlated with the evolution of the three-phase bodies from a tricritical point. Upon increasing amphiphilicity, their characteristic properties pass through maxima in the range of medium-chain amphiphiles, coinciding with the transition from always wetting to never wetting.
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