Observation of a Vanishing Miscibility Loop in the Phase Diagram of Solutions of Mixed Nonionic Ionic Amphiphile

Aqueous solutions of dodecyl hexaoxyethylene ether (the non-ionic amphiphile C12E6) with added sodium dodecyl sulfate (SDS) show a closed solubility loop which shrinks progressively as the SDS/C12E6 molar ratio r increases and disappears when r is larger than 0.013. The results are explained by noting that the interaction potential between two mixed micelles contains, besides an attractive part typical of non-ionic micelles, an electrostatic repulsive contribution.