Phase Behavior of Polyoxyethylene Trisiloxane Surfactant in Water and Water−Oil

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Abstract

A phase diagram of the polyoxyethylene trisiloxane surfactant−water system was constructed as a function of polyoxyethylene (EO) chain length at 25 °C. The HLB (hydrophile−lipophile balance) of surfactant corresponds to the volume ratio of the EO chain to that of surfactant molecule (φEO/φS), where φS and φEO indicate the volume fractions of surfactant and hydrophilic moiety in the system, respectively. Aqueous micellar (Wm), hexagonal liquid crystalline (H1), lamellar liquid crystalline (Lα), and reverse micellar (Om) phases are formed with decreasing φEO/φS. A sponge phase (D2) is also formed near the Lα phase region. The effective cross sectional area per one surfactant molecule, aS, in liquid crystals in the present systems depends only on their EO chain lengths and are the same as that in ordinary linear hydrocarbon surfactant systems. Since the maximum length of the trisiloxane moiety in its extended form is short, the Lα phase is formed at φEO/φS between 0.45 and 0.6 in the present system, whereas an ordinary linear-type nonionic surfactant forms the H1 phase in the same range of φEO/φS. The H1 phase, which was observed in a narrow range of φEO/φS, becomes stable upon addition of oil. Although the H1 phase in the absence of oil is considered to have the hexagonal structure confirmed by polarized optical microscopy, the calculated radius of the cylinder is much longer than the hydrophobic chain length in its extended form. Perhaps, the present H1 phase is different from an ordinary hexagonal liquid crystalline structure. The three-phase behaviors of microemulsions in the present systems were also examined.

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