Molecular structure and dynamics in monolayers of long chain alkanes and alkyl-derivatives

Long chain alkanes (C₃₄H₇₀ and C₅₀H₁₀₂), a fatty acid (C₁₇H₃₅COOH) and an alkyl-substituted triiodobenzoate (I₃H₂C₆COOC₁₈H₃₇) have been adsorbed at the interface between organic solutions and the basal plane of graphite. In-situ scanning tunneling microscopy (STM) has been employed to investigate their structure and dynamics on the scale of 10 pm and 1 ms or longer. All adsorbates form two-dimensional polycrystals. The molecules tend to organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The n-alkane chains pack in a lattice commensurate with the graphite lattice and the carbon skeleton planes approximately perpendicular to the substrate. Due to the additional space required by a carboxyl end group the alkyl lattice in the fatty acid is incommensurate with the substrate and the carbon skeleton planes lie approximately parallel to the surface. In the triiodobenzoate the headgroup takes the space of about two alkyl chains resulting in an interdigitated packing.