Nanodomains within bicontinuous silicone/water microemulsions retard TiO 2 nanoparticle aggregation

Phase Diagrams and Structural Characterization of Mixtures of Silicone Surfactants + Silicone Oils + Water

Silicone surfactants display unique properties and are widely employed in pharmaceutical and cosmetic products. In this work, we study water incorporation into silicone oils using silicone surfactants. Despite their importance, there are only a few studies reporting their phase equilibrium and structural characterization. Here, we determined the phase diagram of systems containing silicone oils, silicone surfactants, and water. In particular, we investigated the self-assembly behavior of two siloxane surfactants with the different hydrophilic-lipophilic balance: M(D'E₇OH)M and MD₁₈(D'₃E₁₈OAc)M and two silicone oils (cyclic oil-D₄ and linear oil-MD₁₅M). The phase behavior of the mixtures was investigated through optical inspection and structural characterization of aggregated states (microemulsions and mesophases) using small angle X-ray scattering (SAXS). These water-in-oil microemulsions or bicontinuous microemulsions incorporated a maximum amount of approximately 20 wt % water for the two surfactants with cyclic oil. A similar behavior was also identified with linear silicone oil, though with smaller water contents. We also observed the formation of anisotropic states, with a predominance of lamellar phases and a small region of a hexagonal phase. A quantitative analysis of the SAXS curves in the lamellar region reveals that this mesophase swells continuously after the addition of water lamellar periods ranging from 50 Å (with 18 wt % water) to 64 Å (with 40 wt % water). Our results confirm and expand the earlier literature on similar compounds, indicating their potential in incorporating water into silicone mixtures and forming interesting mesophases. Accompanying this characterization, we also report a comprehensive and systematic set of structural details for the different systems (microemulsions, bicontinuous phases and mesophases) formed by these mixtures, derived from the SAXS measurements.

Silicone Microemulsion Structures Are Maintained During Polymerization with Reactive Surfactants

Bicontinuous microemulsions exhibit domain structures on the nanoscale (<20 nm). Normally, such fine details are lost during the conversion from a fluid microemulsion to solid elastomeric materials, as a consequence of interfacial destabilization via polymerization of either the oil phase or monomers in the aqueous phase. Very little is known about the polymerization of silicone microemulsions and the morphological changes that occur upon transition from a nanostructured liquid to a solid matrix. Silicone microemulsions polymerized by free radical (aqueous phase) and condensation (silicone phase) processes, respectively, were characterized by small-angle X-ray scattering and transmission electron microscopy. It was found that cross-linking of the silicone phase alone led, over time, to large increase of the size of the microemulsion nanodomains. By contrast, photoinduced polymerization of a reactive surfactant and acrylic monomers in the aqueous phase was effective at retaining bicontinuous nanomorphology, irrespective of the degree of cross-linking of the silicone phase.