Phase transition of n-alkane layers adsorbed on mica

Thin (thickness h approximate to 3 nm) films of n-octadecane and n-hexadeca ne adsorbed on mica surfaces from vapor close to their bulk melting points (T-m) have been studied in a surface force apparatus. Using data on the gro wth rate of capillary condensates between the mica surfaces in contact and measurements of h, we have identified a transition in the structure of the adsorbed films a few degrees above T-m. As T decreases the alkane layers ap pear to undergo a transition to a more ordered structure, akin to the postu lated "surface freezing'' of long-chain liquid n-alkanes.