Experimental XR curves from a 2 ml bulk NP solution injected onto a 2-inch-diameter sapphire substrate (bulk electron density = 1.175 e− Å−3) and kept at constant = 60 mK for ∼20 h. The solution contains the NPs necessary to assemble a close-packed NP bilayer covering the entire sapphire wafer after complete solvent evaporation. Up-triangles: NP solution surface 5 h after injection. The oscillatory XR character indicates surface NP accumulation. Squares: (×10-2) same NP solution 8 h after injection. The smaller oscillation amplitude suggests partial NP dissolution into the bulk sub-phase. Diamonds: (×10-4) same NP solution 18 h after injection. The absence of XR oscillations indicates the complete disappearance of the transient surface NP layer. Down-triangles: (×10-8) example of XR measurement at the buried substrate–NP-solution interface. The monotonic XR character indicates the absence of NP accumulation at the buried solid–liquid interface. The solid lines are physical model fits obtained by applying the Parratt recursive method to the electron density profiles reported in inset (a). Inset (b): schematic representation of two core-shell NPs at the toluene–vapour interface, with the parameters involved in the physically motivated model described in §4.