Where N is the atomic number density, b the coherent scattering length. The Nb of a medium can be directly calculated from its composition. To carry out neutron reflectometry at oil-water (PBS) interfaces, we applied the "spin-freeze-thaw" method . Note that as the melting temperature of Novec 7500 was too low, it was not possible to carry out this procedure with this fluorinated oil, and hexadecane (~18.0 °C) was utilized as an alternative. The oil layer (hexadecane) was contrast matched to a hydrophobic Si substrate by spin coating. The oil layer was then frozen and sandwiched between the substrate and the aqueous phase. After ensuring the absence of trapped air within the system, the temperature of the cell was raised to 25 ± 1°C, causing the oil layer to melt. For a relatively thin oil film, the reflections from both the Solid substrate-oil (R1) and oilwater (R2) interfaces contribute to the measured reflectivity. The reflectivity can be calculated using the thick-film approximation :