Observation of a liquid-to-layered transition in thin liquid films when surface and interface regions overlap

We have used x-ray reflectivity to study the coupling of surface and interface layering in a molecularly thin normal liquid [tetrakis(2-ethylhexoxy) silane (TEHOS)], as a function of temperature and film thickness. The best fits to the data were obtained with an electron density model that consists of a uniform density component superimposed upon molecular-scale density oscillations (layers). The two types of layer profiles were observed to vary with temperature from 187-286 K. The amount of material in the molecular layers increases as that in the uniform density layer decreases, with the onset of liquid-to-layered transition occurring at a total film thickness of ∼40 (about twice the bulk correlation length of TEHOS).