Abstract
A-B-type silicone copolymer (or surfactant), Me3SiO(Me2SiO)12-Me2SiCH2CH2CH2-O-(CH2CH2O)33.1H(Si14C3EO33.1), forms lamellar liquid crystal (Lα) in a pure state. In a binary water-Si14C3EO33.1 system, the Lα phase coexists with excess water in a dilute region, whereas the Lα phase changes to an isotropic solution (Wm) by replacing water with ethylene glycol. Although water enhances the segregation between hydrophilic and lipophilic chains of copolymer, it does not change the layer curvature of long lipophilic-chain copolymer to be positive. Since ethylene glycol is more soluble in the hydrophilic chain than water, it changes the curvature from zero to positive, where positive curvature means the copolymer-layer curvature is convex toward water or polar solvent. When ethylene glycol is replaced with PEG 300 (polyethylene glycol, Mn ca=300), the Lα-H1(normal hexagonal phase)-Wm transition takes place. With the further increase in the molecular weight of PEG, the Lα phase coexists with an excess PEG as well as the water-copolymer system and the solubilization of PEG in the Lα phase decreases. This change in the phase behavior may be attributed to the solubilization part of PEG in the hydrophilic chain of the silicone copolymer. On the other hand, non-polar solvent, octamethylcyclotetrasiloxane, D4, is soluble in the lipophilic chain, and the Lα phase changes to a reverse micellar solution (Om) via reverse hexagonal (H2) phase. With the increase in the molecular weight of oil, the Lα phase retreats to the concentrated region and the solubilization of oil (poly(dimethylsiloxane)) in the Lα phase becomes very low. As a result, PEG and silicone oil change the surfactant (or copolymer) layer curvature in the opposite way, but, the swelling of high-molecular-weight solvents or homopolymers in the Lα phase is very restricted. In ternary water/ Si14C3EO33.1/D4, ethylene glycol/ Si14C3EO33.1/D4, and PEG/Si14C3EO33.1/poly (dimethylsiloxane) systems, isotropic microemulsions are formed at an equal polar/non-polar solvent ratio. The relationship between microemulsions with water and oil or with hydrophilic and lipophilic homopolymers is also discussed.
