Interactions at complex interfaces

In this work, we discuss how improvements of an X-ray laboratory beamline and an optimal design of a microfluidic device allow reliable structural information to be obtained without the need for a synchrotron. We evaluate the potential of these new developments by probing several well known dispersions. These include dense inorganic gold and silica nanoparticles that scatter photons quite intensely, the bovine serum albumin (BSA) macromolecule, with moderate contrast, to highlight possible applications in biology, and latex nanospheres with only weak contrast with the solvent to show the limits of the setup. We established a proof of concept for a versatile setup that will open the way for more complex lab-on-a-chip devices suitable for in situ and operando structural analysis by small angle X-ray scattering analysis without the necessity for a synchrotron source.

Emulsion stabilization is most often achieved using simple surfactants, proteins, or particles. We investigated a complex natural stabilizer, gum arabic, widely used in food formulations and known for its remarkable metastability even at high dilution. Gum arabic is a heterogeneous mixture of partially conjugated protein–polysaccharide complexes, spanning a continuum of architectures revealed by chromatography and small-angle scattering. In solution, this diversity leads to hierarchical self-organization controlled by concentration and electrostatic repulsions.

At the oil–water interface, selective partitioning occurs: protein-rich species adsorb irreversibly and dominate the interfacial film. While film composition is weakly dependent on formulation parameters, interfacial density is strongly controlled by ionic repulsions and directly correlates with emulsion metastability. At high coverage, the adsorbed species aggregate into a percolated two-dimensional interfacial network. The interface can thus be described as a hydrophobic protein network anchored at the interface and coupled to a dense, water-swollen polysaccharide brush. This architecture explains the exceptional robustness of gum-arabic-stabilized emulsions and enabled, in practice, a tenfold reduction in stabilizer content.

Emulsions used in parenteral nutrition rely on phospholipid-stabilized nanoemulsions, selected for their biocompatibility. This choice is intrinsically paradoxical: phospholipids naturally favor near-zero curvature assemblies, such as lamellar phases, which are poorly suited to stabilizing highly curved oil droplets. In practice, small compositional changes often lead to catastrophic losses of metastability.

We addressed this problem using model lipid/oil systems and a combination of complementary approaches. Ternary phase diagrams were used to quantify the spontaneous curvature of amphiphilic films, neutron scattering to probe interfacial structure, and separation coupled to Raman spectroscopy to determine the effective localization of phospholipids. Emulsion metastability was quantified through kinetic measurements. Our results show that spontaneous curvature controls metastability at leading order, but that its precise value is critical. A slightly positive curvature is sufficient to stabilize nanodroplets via an adsorbed monolayer, whereas macrodroplets require interfacial multilayers. Conversely, excessively positive curvature promotes vesicle formation in the aqueous phase and depletes the interface. This work identifies a narrow formulation optimum, explaining the extreme industrial sensitivity while providing a rational framework for formulation design.