Observation of thickness quantization in liquid films confined to molecular dimension

Liquid octamethylcyclotetrasiloxane (OMCTS) films have been prepared between two flat silicon substrates with varying nanometer-sized gaps which correspond to two to three molecular diameters of OMCTS. These extremely confined liquid films have been investigated using X-ray scattering methods as a function of applied pressure, in particular by observing the specular reflectivity and the diffuse scattering. For the first time, scattering methods show that the gap size cannot be changed continuously with changing pressure. Instead, the gap is quantized due to ordering effects of the OMCTS molecules. The use of scattering methods has enormous future implications as they allow very detailed investigations of confined liquids at a molecular level.