Nonionic surfactant mixtures. I. Phase equilibria in C10E4–H2O and closed-loop coexistence

A detailed phase diagram for mixtures of the homogeneous nonionic surfactant 3,6,9,12-tetraoxadocosanol (C10E4) with water is presented. Closed-loop coexistence is shown to be a prominent feature in this system as well as in three other aqueous mixtures of nonionic surfactant to which it is compared: 3,6,9,12,15-pentaoxapentacosanol (C10E5), decyldimethylphosphine oxide (C10PO), and dodecyldimethylphosphine oxide (C12PO). The sizes and locations of these curves are discussed in terms of a decorated lattice-gas model of closed-loop coexistence. The shape of one loop (C10E4) in the vicinity of its lower critical point is nonclassical; β is found to be 0.36±0.02. Sources of complexity in the diagram for C10E4–water are discussed.