  Alkylsilane self-assembled monolayers (SAMs) are often used as model
substrates for their ease of preparation and hydrophobic properties. We have
observed that these atomically smooth monolayers also provide a slip boundary
condition for dewetting films composed of unentangled polymers. This slip
length, an indirect measure of the friction between a given liquid and
different solids, is switchable and can be increased [Fetzer et al., Phys. Rev.
Lett. 2005; B\"aumchen et al. J. Phys.: Cond. Matt. 2012] if the alkyl chain
length is changed from 18 to 12 backbone carbons, for example. Typically, this
change in boundary condition is affected in a quantized way, using one or the
other alkyl chain length, thus obtaining one or the other slip length. Here, we
present results in which this SAM structure is changed in a continuous way. We
prepare SAMs containing bidisperse mixed SAMs of alkyl silanes, with the
composition as a control parameter. We find that all the mixed SAMs we
investigated show an enhanced slip boundary condition as compared to the
single-component SAMs. The slip boundary condition is accessed using optical
and atomic force microscopy, and we describe these observations in the context
of X-ray reflectivity measurements. The slip length, varying over nearly two
orders of magnitude, of identical polymer melts on chemically similar SAMs is
found to correlate with the density of exposed alkyl chains. Our results
demonstrate the importance of a well characterized solid/liquid pair, down to
the angstrom level, when discussing friction between a liquid and a solid.
