Asao Harashima

Solubilization Enhancing Effect of A-B-Type Silicone Surfactants in Microemulsions

We report a solubilization enhancing effect of A-B-type silicone surfactants in microemulsions. The effect of added long-chain silicone surfactants, Si25C3EO51.6 (extended length≈21.8 nm) and Si14C3EO15.8 (extended length≈8.5 nm) on the solubilization capacity of C12EO5 (extended length≈3 nm)/water/dodecane microemulsion was investigated at the hydrophile-lipophile balance temperature at which a microemulsion (surfactant) phase containing equal weights of oil and water touches the three-phase body. The addition of silicone surfactants exhibits an enormous increase of the swelling of the middle phase primarily with an associated increase in the structural length scale of the microemulsion. The solubilization power increases with increasing x2 (mole fraction of silicone surfactants to the total surfactant) and going through a maximum it decreases, since a lamellar liquid crystal introduces in the multiphase region at low surfactant concentrations. The solubilization capacity reaches at the maximum to an almost equal level for different x2 values, 0.02 for Si25C3EO51.6 and 0.09 for Si14C3EO15.8. The solubilization power of the lamellar phase shows a similar trend with lower magnitude.

Effect of mixing lipophilic and hydrophilic silicone surfactant systems

The phase behavior of a hydrophilic A—B-type silicone surfactant, (CH3)3SiO-[(CH3)2SiO]3.8-(CH3) 2SiCH2CH2CH2-O-(CH2CH2O)51.6H, Si5.8C3EO51.6, was investigated by phase study and small-angle X-ray scattering Si5.8C3EO51.6 forms a micellar cubic phase and a hexagonal phase in aqueous mixtures. The structure of the cubic phase seems to be face-centered type. When lipophilic surfactant (Si14C3EO7.8 or Si25C3EO7.8) is added to Si5.8C3EO51.6/water systems, a transition from the hexagonal phase to the lamellar phase takes place, owing to a change in the hydrophile—lipophile balance of the system. The change in the surface area per surfactant molecule is larger as the polydimethylsiloxane chain is longer, even if the EO number remains constant. This fact is attributed to the coiling of the long lipophilic chain in order to reduce the entropy loss.