Molecular Layering in Nanometer-Confined Lubricants

Molecular layering of liquids in nanometer-scale confinement is demonstrated for typical lubricant constituents such as polyalphaolefins (PAO) and an ester by means of atomic force microscopy. Layering is observed in force vs. distance curves for poly-(1-decene) tetramers (PAO6) and undecamers (PAO40) and for a 2-ethylhexyl monoester on graphite, mica, and polished steel surfaces and is compared to the layering of hexadecane and 1-hexadecene. On graphite surfaces, the confined molecules are oriented parallel to the surfaces for all liquids, resulting in layers with a thickness comparable to the diameter of the alkyl chains. On mica, confined hexadecane molecules also lie parallel to the surface, while the molecules in the first layer of 1-hexadecene and PAOs take a more upright orientation. Confinement on the oxidized polished steel surfaces results in a molecular layering which most often resembles the layering on graphite and differs significantly from layering on the ionic oxide mica.