PHASE-BEHAVIOR OF POLYOXYETHYLENE TRISILOXANE SURFACTANT IN WATER ANDWATER-OIL

A phase diagram of the polyoxyethylene trisiloxane surfactant-water sy stem was constructed as a function of polyoxyethylene (EO) chain lengt h at 25 degrees C. The HLB (hydrophile-lipophile balance) of surfactan t corresponds to the volume ratio of the EO chain to that of surfactan t molecule (phi(EO)/phi(S)), where phi(S) and phi(EO) indicate the vol ume fractions of surfactant and hydrophilic moiety in the system, resp ectively. Aqueous micellar (Wm), hexagonal liquid crystalline (H-1), l amellar liquid crystalline (L alpha), and reverse micellar (Om) phases are formed with decreasing phi(EO)/phi(S). A sponge phase (D-2) is al so formed near the L alpha phase region. The effective cross sectional area per one surfactant molecule, as, in liquid crystals in the prese nt systems depends only on their EO chain lengths and are the same as that in ordinary linear hydrocarbon surfactant systems. Since the maxi mum length of the trisiloxane moiety in its extended form is short, th e L alpha phase is formed at phi(EO)/phi(S) between 0.45 and 0.6 in th e present system, whereas an ordinary linear-type nonionic surfactant forms the H-1 phase in the same range of phi(EO)/phi(S). The H-1 phase , which was observed in a narrow range of phi(EO)/phi(S), becomes stab le upon addition of oil. Although the H-1 phase in the absence of oil is considered to have the hexagonal structure confirmed by polarized o ptical microscopy, the calculated radius of the cylinder is much longe r than the hydrophobic chain length in its extended form. Perhaps, the present H-1 phase is different from an ordinary hexagonal liquid crys talline structure. The three-phase behaviors of microemulsions in the present systems were also examined.