Influence of the Molecular Ordering on the Wetting ofSiO2/Air Interfaces by Alkanes

Alkanes from 16 to 50 C atoms show three wetting topologies at ${\mathrm{SiO}}_2/\mathrm{air}$ interfaces. Below the bulk freezing temperature $T_{\mathrm{b}}$, small crystallites (frozen droplets which can be annealed into mesas of uniform height) are observed. A monolayer of homeotropically oriented molecules covers the air/substrate interface in between. At $T_{\mathrm{b}}$, the crystallites melt into droplets. The ordered monolayer remains up to $T_{\mathrm{S}}^{\mathrm{SV}}\approx T_{\mathrm{b}}+3\mathrm{}°\mathrm{C}$, probably due to fluctuation-stabilized “surface freezing.” Above $T_{\mathrm{S}}^{\mathrm{SV}}$, a liquid alkane film wets the surface. The decoupling of $T_{\mathrm{b}}$ and $T_{\mathrm{S}}^{\mathrm{SV}}$, and the difference between the ordered and liquid surface energies, causes the droplet-to-film transition.