The degree of interfacial structuring of $n$-hexadecane and octamethylcyclotetrasiloxane (OMCTS) was measured within a nanometer boundary regime to silicon surfaces. Boundary-layer effects on lubricating sliding (in terms of a thermodynamic stress activation parameter) and the layer thickness were determined by scanning force microscopy. A $2.0\pm 0.3\mathrm{nm}$ thick, entropically cooled layer was found for $n$-hexadecane. Measurements on spherically shaped OMCTS molecules exhibited only an interfacial “monolayer,” and identified the molecular shape of $n$-hexadecane responsible for augmented interfacial structuring. Interfacial liquid structuring was found to reduce friction.

• Received 11 September 2000

DOI:https://doi.org/10.1103/PhysRevLett.88.154302