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 measurements123491052210532CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQM.S.F. and W.L. thank CNPq, respectively, for doctoral and research productivity grants. We also thank Dow Corning for the kind donation of the silicone compounds used in this study, and we are indebted to Prof. I. V. P. Yoshida for helpful discussions on chemical properties of these compounds. The authors are also indebted to the Brazilian Synchrotron Laboratory, LNLS, for allocation of SAXS beamtime and for the efficient beamline staff suppor