Effect of added salt on three-phase behavior in a sucrose monoalkanoate system

Effect of added salt on the three-phase behavior of microemulsion in water/octaethylene glycol dodecyl ether (C12EO8)hexanol/decane and water/sucrose monododecanoate (SMD)/hexanol/decane systems was investigated at 25 degrees C. The three-phase body is shifted to higher hexanol mixing ratio upon addition of KSCN, whereas the opposite tendency is observed upon addition of NaCl in the C12EO8 system. These phenomena can be explained by salting-out and salting-in effects, respectively. On the other hand, the three-phase body is shifted to higher hexanol mixing ratio upon addition of both inorganic salts in the SMD system. In the absence of oil, however, the effect of added salt on the clouding phenomena in the SMD system is similar to that in the C12EO8 system. The mixing fraction of hexanol in the total surfactant (nonionic surfactant + hexanol) in the interface of micro-oil and water domains inside the middle-phase microemulsion is estimated according to the geometrical relation of a three-phase body in the space of compositions. As st result, the effect of added inorganic salt on the mixing fraction in the interface is opposite to that on the apparent mixing fraction of hexanol in the total surfactant to form the three-phase body in the SMD system. Consequently, this novel phenomenon can be explained by the change in the nature of excess oil phase or micro-oil domain due to the high solubility of hexanol in oil.