A-B-type silicone copolymer (or surfactant), Me
3SiO(Me
2SiO)
12-Me
2SiCH
2CH
2CH
2-O-(CH
2CH
2O)
33.1H(Si
14C
3EO
33.1), forms lamellar liquid crystal (L
α) in a pure state. In a binary water-Si
14C
3EO
33.1 system, the L
α phase coexists with excess water in a dilute region, whereas the L
α phase changes to an isotropic solution (W
m) 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, M
n ca=300), the L
α-H
1(normal hexagonal phase)-W
m transition takes place. With the further increase in the molecular weight of PEG, the L
α phase coexists with an excess PEG as well as the water-copolymer system and the solubilization of PEG in the L
α 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, D
4, is soluble in the lipophilic chain, and the L
α phase changes to a reverse micellar solution (O
m) via reverse hexagonal (H
2) phase. With the increase in the molecular weight of oil, the L
α phase retreats to the concentrated region and the solubilization of oil (poly(dimethylsiloxane)) in the L
α 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 L
α phase is very restricted. In ternary water/ Si
14C
3EO
33.1/D
4, ethylene glycol/ Si
14C
3EO
33.1/D
4, and PEG/Si
14C
3EO
33.1/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.
View full abstract