Three-phase behavior and the roles of C4E1 in the water/C12E5/C4E1/decane system

Abstract The three-phase behavior and the roles of 2-butoxyethanol (C 4 E 1 ) in the water/pentaethyleneglycol dodecyl ether (C 12 E 5 )/C 4 E 1 /decane system were studied with variation of temperature ( T ). The fish-shaped three-phase region consisting of microemulsion, excess aqueous, and excess oil phases contacted the single microemulsion region at an optimum temperature ( T opt ) at each weight fraction ( W 1 ) of C 4 E 1 /(C 4 E 1  + C 12 E 5 ) (water/oil = 1/1 (w/w)). When W 1 was varied, T opt and the highest T ( T H ) for the three-phase region went through a relative minimum and a relative maximum, respectively. In the quaternary composition tetrahedron, a three-phase region appeared in the water–C 4 E 1 –decane face at 24.5 °C, expanded with increasing T , and reached the water–C 12 E 5 –decane face at 36.8 °C. This region separated into two at 39.2 °C. One section took on a cone-like shape above 41 °C, and disappeared into a critical double tie line at 42.5 °C (the relative maximum of T H ). The other section shrank with increasing T , and disappeared into the critical tie line in the water–C 4 E 1 –decane face at 46.5 °C. C 4 E 1 plays dual roles, as a surfactant and as a cosolvent. C 4 E 1 became more monomerically oil-soluble with increasing T .