Self-organization, phase behavior, and microstructure of poly(oxyethylene) poly(dimethylsiloxane) surfactants in nonpolar oil

Linear, long poly(oxyethylene) poly(dimethylsiloxane) surfactants, formula Me3SiO-(Me2SiO)(m-2)-Me2SiCH2CH2CH2-O-(CH2CH2O)(n)H (SimC3EOn), form reverse micelles in oil such as poly(dimethylsiloxane) and hydrocarbons. The critical micellar concentration (CMC) decreases dramatically with increasing the hydrophilic-chain length of the surfactant, whereas the difference in hydrophobic chain length has less influence on the CMC. Hence, the segregation of the poly(oxyethylene) (EO) chain from nonpolar medium is a main factor to form aggregates in oil. Since the lipophilic surfactants used in this study have very long hydrophilic and hydrophobic chains compared to conventional nonionic surfactants, they also form liquid crystals in nonpolar medium such as discontinuous reverse micellar cubic and reverse hexagonal phases at a high surfactant concentration and even in the absence of solvent. Judging from SAXS data, oil penetrates in the palisade layer of surfactant, increasing the preferred negative curvature and relaxing the packing restriction of the hydrophobic chain. Although a normal micellar cubic phase is always changed to a micellar solution upon dilution with water, the present reverse micellar phase coexists with oil in a wide range of composition in the squalane system.