Phase behavior of binary water-trimethylsilane surfactant systems: Origin of the dilute lamellar phase

We have used the highly purified, homogeneous trimethylsilane surfactant CH3Si(CH2)(6)(OCH2CH2)(5)OCH3 to determine the full binary phase diagram with water. The trimethylsilane surfactants display similar properties to those of the well-known superspreading trisiloxane surfactants but have higher hydrolytic stability. Although the richness and sequence of phases found for the present system resemble that known from water-nonionic surfactant systems of the alkylpolyglycolether type, a peculiarity is observed: there are two disconnected lamellar phases, a concentrated one and a dilute one. Furthermore, an L-3 phase is found located at lower concentrations than the dilute lamellar phase. The position, the extent, and the phase sequence around the L-3 and dilute lamellar phases allows a clear assignment of seven two-phase regions, which require the existence of four triple lines. Each two-phase region has been identified, and all, triple lines have been located to within a few hundredths of a degree.