Lipid-stabilized water-oil interfaces studied by microfocusing small-angle X-ray scattering

Water-in-oil (w/o) simple emulsions are dispersed microconfined systems that find applications in many areas of advanced materials and biotechnology, such as the food industry, drug delivery, and cosmetics, to name but a few. In these systems, the structural and chemical properties of the boundary layer at the w/o interface are of paramount importance in determining functionality and stability. Recently, microfluidic methods have emerged as a valuable tool for fabricating monodisperse emulsion droplets. Because of the intrinsic flexibility of microfluidics, different interfaces can be obtained, and general principles governing their stability are needed to guide the experimental approach. Herein, we investigate the structural characteristics of the region encompassing the liquid/liquid (L/L) interface of w/o emulsions generated by a microfluidic device in the presence of phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and other intercalating amphiphiles (dopants) using microfocused small-angle X-rays scattering (μ-SAXS). We show that phospholipids provide a stable and versatile boundary film of ∼100 μm whose basic units are swollen and uncorrelated DMPC bilayers. The internal arrangement of this interfacial film can be tuned by adding molecules with a different packing parameter, such as cholesterol, which is able to increase the stiffness of the lipid membranes and trigger interbilayer correlation.