  The properties of butane (C4H10) lubricants confined between two approaching
solids are investigated by a model that accounts for the curvature and elastic
properties of the solid surfaces. We consider the linear n-butane and the
branched iso-butane. For the linear molecule, well defined molecular layers
develop in the lubricant film when the width is of the order of a few atomic
diameters. The branched iso-butane forms more disordered structures which
permit it to stay liquid-like at smaller surface separations. During squeezing
the solvation forces show oscillations corresponding to the width of a
molecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezed
out before those of n-butane. Since the (interfacial) squeezing velocity in
most practical applications is very low when the lubricant layer has molecular
thickness, one expects n-butane to be a better boundary lubricant than
iso-butane. N-butane possessing a slightly lower viscosity at high pressures,
our result refutes the view that squeeze out should be harder for higher
viscosities, on the other hand our results are consistent with wear experiments
in which n-butane were shown to protect steel surfaces better than iso-butane.
