The non-linearity of the evolution of interfacial shear storage moduli as a function of feed solution composition suggested that protein adsorption at corresponding liquid-liquid interfaces also varied non-linearly with feed concentrations. To test this hypothesis, we labelled BSA and b-casein with complementary Alexa Fluor TM dyes, and quantified their relative surface concentrations at the surface of oil-in-water emulsion droplets, after replacement of the aqueous phase with successive dilution/aspirations (Figures ). As the fluorescence intensities measured from corresponding microscopy images do not allow quantitative surface densities to be extracted, we quantified the surface densities of monolayers of BSA and b-casein at fluorinated interfaces (monolayers of perfluorinated alkanes) using surface plasmon resonance (SPR; Figures ). Although this corresponds to an approximation due to the differences in chemistry of the linear perfluorinated alkyl thiol selected and that of the branched Novec 7500, SPR allows precise quantification of surface densities. The surface coverage of b-casein was found to be approximately double that of BSA, near 200 ng/cm 2 . Although values found in the literature vary significantly depending on the oil or air interface characterised, the surface densities quantified from our SPR data are in good agreement with those reported for both proteins at air-liquid interfaces (based on radiolabelling and radioactivity monitoring) and comparable, although slightly below, values reported from neutron reflectometry measurements . Therefore our work confirms that denser layers of b-casein form at Novec 7500 interfaces compared to assemblies formed with BSA. This suggests that the less structured b-casein proteins are able to pack more densely, perhaps similarly to polymer surfactants displaying a short hydrophobic head group with a long hydrophilic tail chain. Analysis of the evolution of the protein composition at the interface evidenced a clear nonlinearity, with significant levels of adsorption of b-casein even at low feed concentrations (e.g. 5%