Delayering of Intermediate-Length Alkanes Adsorbed on Solid Surfaces

We have recently discovered that a film of the intermediate-length alkane, dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) does not completely wet SiO$_{2}$ and highly oriented graphite surfaces on a nanometer length scale.\footnote{M. Bai \textit{et al}., cond-mat/0611497.} In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of C32 molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report noncontact Atomic Force Microscopy and synchrotron x-ray measurements indicating that a similar delayering transition occurs for films of other intermediate-length alkanes: C24, C25, C30, and C36 deposited from solution onto a SiO$_{2}$ surface. These results raise a number of interesting questions including whether the delayering transition is driven by conformational changes in the molecules and what implications the nonwetting behavior may have for lubricating nanoscale devices.