Cryo-TEM imaging of a novel microemulsion system of silicone oil with an anionic/nonionic surfactant mixture

Abstract

We report the nanostructures in a novel microemulsion system of silicone oil, water and a surfactant mixture of an anionic and a nonionic surfactant. The phase diagram of the investigated system exhibits two isotropic single-phase channels for constant temperature. The upper channel, that is the channel with the higher mass fraction of nonionic surfactant, starts with an L₃-phase at the water side, and passes through a minimum continuously to the oil side. The channel with the lower mass fraction of nonionic surfactant starts with an L₁-phase at the water side, and passes with increasing oil content and increasing mass fraction of nonionic surfactant to the middle of the phase diagram and ends there. No connection between the two channels was detected at a surfactant concentration of 15%. The two channels are separated by a single L_α-phase and multiphase regions. In contrast to the results from microemulsions with nonionic surfactants, cryo-TEM micrographs on this system show that the upper phase channel has a bicontinuous structure from zero to only about 35% of oil. At higher oil content the channel contains water droplets in a continuous oil phase. At a water/oil ratio of 1 : 1, the structure looks like a polyhedral foam structure or a high internal phase emulsion (HIPE) structure, and not like the usual bicontinuous structure, as generally assumed. Nevertheless, the dimensions of the imaged bicontinuous and water-in-oil-structures were consistent with the theoretical consideration for nanostructures in microemulsions. The lower channel with its o/w-structure could not well be imaged with the cryo-TEM method. Instead of small droplets, small vesicles were imaged, that obviously were formed by the loss of oil in the thin film during the specimen preparation process for cryo-TEM.