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The properties of butane (C4H10) lubricants confined between two approachingsolids are investigated by a model that accounts for the curvature and elasticproperties of the solid surfaces. We consider the linear n-butane and thebranched iso-butane. For the linear molecule, well defined molecular layersdevelop in the lubricant film when the width is of the order of a few atomicdiameters. The branched iso-butane forms more disordered structures whichpermit it to stay liquid-like at smaller surface separations. During squeezingthe solvation forces show oscillations corresponding to the width of amolecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezedout before those of n-butane. Since the (interfacial) squeezing velocity inmost practical applications is very low when the lubricant layer has molecularthickness, one expects n-butane to be a better boundary lubricant thaniso-butane. N-butane possessing a slightly lower viscosity at high pressures,our result refutes the view that squeeze out should be harder for higherviscosities, on the other hand our results are consistent with wear experimentsin which n-butane were shown to protect steel surfaces better than iso-butane.Comment: 7 pages, 10 figures, format revtex. Submitted to J. Chem. Phy

Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity