Effect of SPAN80 on the structure of emulsified aqueous suspensions

The influence of an oil-soluble surfactant (SPAN80) on water in oil emulsions is investigated. The aqueous phase is a nanofluid consisting in a suspension of silica nanoparticles modified by CTAB. This aqueous solution, when emulsified in paraffin oil, is known to produce droplets with structural properties that depend upon the [CTAB]/[SiO2] mixing ratio. For large mixing ratios, droplets have the usual spherical shape whereas for small ones, they are deformed and behave like stiff polymorphous objects. The present work focuses on the study of the robustness of this phenomenon when adding SPAN80 into the paraffin oil phase in a broad range of concentrations. Optical tomography microcopy is used to describe the structure of the emulsions. SPAN80 actually comes in addition to the stabilizing role of CTAB. It contributes in the decrease of interfacial tension and to a larger dispersity. For sufficiently large concentrations, emulsions are shown to always contain spherically shaped droplets. A similar phenomenology is observed for SDS/alumina aqueous suspensions. This suggests that the change from polymorphous to spherically shaped droplets is a general property of emulsified nanofluids in SPAN80/paraffin oil solutions. (C) 2016 Elsevier B.V. All rights reserved.