Interfacial rheology of graft-type polymeric siloxane surfactants

An interfacial stress rheometer has been used to study the rheological properties of a silicone oil/water interface in the presence of siloxane surfactants that are used in the personal care industry as water-in-silicone-oil emulsifiers. The surfactants are graft-type poly(dimethylsiloxane)-poly(oxyalkylene) copolymers. They appear to stabilize water-in-silicone-oil emulsions in a fashion similar to that of Pickering emulsions, in which solid particles, such as silica or clay, accumulate in the oil/water interface forming a solidlike "eggshell" that resists coalescence. These oil-soluble copolymers, when dissolved in silicone oil and contacted with water, spontaneously form small particles of a copolymer-rich phase dispersed in the oil phase. These particles appear to accumulate in the interface and act analogously to the solid particles in Pickering emulsions by forming a solidlike network in the silicone oil/water interface that can resist coalescence. The rheology of the interface is influenced by the formation and aggregation of these surfactant-rich particles in the silicone oil/water interface. Consequently, measuring the effects of different processing parameters on interfacial rheology can be used to study their effect on the formation of these surfactant-rich particles. Furthermore, the wetting behavior of the particle surface becomes more hydrophilic when exposed to water as evidenced by changes in contact angle measurements. Presumably, when these particles are in the silicone oil/water interface they present their hydrophilic poly(oxyethylene) domains toward water, resulting in a nonuniformity of surface wettability. This nonuniform surface wettability enhances these particles' ability to reside in the silicone oil/water interface.