Structural gradient in bicontinuous microemulsions
Urban, V. S., D. G. Hayes, S. V. Pingali, H. M. O'Neill, and R. Ye.  2018.  XVII International Small Angle Scattering Conference - SAS201, Traverse City, MI, October 7-12, 2018.

Winsor-III microemulsion systems, consisting of a middle, bicontinuous microemulsion (BμE) phase in equilibrium with aqueous and oil excess phases, are employed for the extractive purification of proteins or removal of heavy metals and pollutants from wastewater and for hosting (bio-) chemical reactions, while BEs are employed in tertiary oil recovery, delivery systems for drugs and nutraceuticals, and as templating media for preparation of nanomaterials. Winsor-III systems are useful for solubilization of biomolecules into BEs. We demonstrate here for the first time via small-angle neutron scattering (SANS) that the BμE Winsor-III phase undergoes gradual change of structure and composition in the vertical direction, contrary to the commonly held belief of uniform structure and composition. A vertical stage was deployed to enable precise alignment of a custom-designed rectangular cell containing the WIII system with respect to the neutron beam, allowing for several different vertical positions to be analyzed. For two different Winsor-III systems, results reflect an increase of surfactant concentration in the downward direction, particularly near the lower liquid-liquid interface, consistent with the ultralow interfacial tension that often occurs at that interface for many WIII systems. For a Winsor-III system containing cosurfactant, trends suggest a decrease of cosurfactant concentration in the downward direction. Non-uniformity in the vertical direction have implications in the transport of solutes between WIII phases during the extractive purifications and for the preparation of nanomaterials.