Substrate-induced freezing of alkane monolayers adsorbed on Au(1 1 1) dependant on the alkane/gold misfit

The molecular structure of the interfaces between pure CnH2n+2 (n=10, 12, 14, 16) and Au(1 1 1) surface has been studied by scanning tunneling microscopy. At 291 K, self-organized monolayers with a lamella structure are formed with these alkanes. The ordered monolayers are melting at temperature higher by 46, 28, 15, 5 K than the melting point of bulk CnH2n+2 crystals with n=10, 12, 14, 16, respectively. Two kinds of melting process were observed: (i) a direct solid/liquid phase transition within the monolayer for C10H22, C12H26 and C14H30 molecules, (ii) an intermediate phase for C16H34 molecules. This mesophase corresponds to a two-dimensional liquid crystal formed by molecules moving along their axis and along Au〈1 1 0〉. These results agree well with calculations using a geometric model taking in account the misfit between the CH2–CH2–CH2 period along alkyl chain and the gold lattice along 〈1 1 0〉 direction.