The solubilisation of a water insoluble functionalized silicone oil in an aqueous surfactant solution: A novel mechanism for the solubilisation process

Few hydrophilic side groups turn a silicon oil molecule (Mr 15000) into a surfactant: mixed aggregates with “classic” surfactant are not microemulsion droplets, but micelles [Display omitted] ► High molecular weight silicone oil solubilisation enabled by hydrophilic side groups ► Amino functionalisation of oil vital for “solubility” in aqueous surfactant solutions ► Oil–surfactant mixed aggregates are not microemulsion droplets but polymer micelles ► “Oil” acts as extremely lipophilic amphiphile, not as classic oil The solubilisation of the functionalized silicone oil aminoethyl aminopropyl methylpolysiloxane WR 1300 has been investigated in aqueous solutions of the surfactant pentaethoxy-iso-tridecanol (iC 13E 5). The silicone oil consists of a linear backbone with an average of 200 dimethylsiloxane units and 2–3 functionalized side groups. In various applications the compound is considered a silicone oil. The surfactant iC 13E 5 forms a L α-phase in water, and with decane a microemulsion can be obtained. The solubilisation experiments started with lamellar iC 13E 5 phases. With proper amounts of WR 1300 transparent, optically isotropic but highly viscous single phases are formed. These phases cannot be considered true microemulsions with an oil core and a surrounding surfactant layer. Cryo-TEM micrographs show micelles with irregular shapes and about 25 nm diameter. Their size is independent of the oil/surfactant ratio. It can be explained on the basis of a model where the amino groups of the oil are all found at the surface of the micelles, limiting the radius of the aggregates to the largest length of the polydimethylsiloxane backbone between two such groups or between one end of the oil and one functionalized group. The micellar structures contain only a few siloxane molecules exposing their hydrophilic groups to the water. The functionalized silicone oil molecules can thus be considered as surfactant molecules with a few polar groups and a large hydrophobic chain from dimethyl siloxane.