Structure and molecular dynamics of alkane monolayers self-assembled on mica platelets

Monolayers; of dialkyldimethylarnmonium (C-10-C-18) cations have been electrostatically bound to the surface of mica platelets. The structure and chain dynamics of these well defined self-assembled alkane monolayers have been probed by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy,. nuclear magnetic resonance, and differential scanning calorimetry (DSC). All results indicate that at low temperatures the alkyl chains preferentially assume an all-trans conformation, leading to a highly ordered two-dimensional lattice. With increasing temperature, the all-trans conformation is gradually transformed to a mixture of trans and gauche conformers. Although the translational freedom of the chains is limited by the electrostatic binding of the headgroups to the substrate, the conformational transformation destroys the two-dimensional lattice leading, to disordered molecules with a liquidlike conformation. This phase transition manifests itself in IR-absorption frequency and carbon resonance frequency shifts as well as in an enthalpy change. The methylene carbon resonance frequencies of the trans and gauche conformers are better resolved than their IR-absorption frequencies and show clearly that the change in conformation starts at a considerably lower temperature than that indicated by IR spectroscopy and extends over a wide temperature range. The relatively sharp transitions seen in IR-absorption maxima plots are due to poor resolution of the heterogeneously broadened lines and consequent sluggish detection of the onset of the trans/gauche transformation. The phase transitions observed by DSC also seem to be relatively sharp but are really broad if the onset of the enthalpy change is considered. The phase transition temperatures depend on the length of the alkyl chains and their packing. The transition temperatures of SAMs electrostatically bound to a planar mica surface are higher than those assembled on planar or curved (nanoparticles) gold surfaces. XRD revealed that the average molecular axis of a dioctadecyl monolayer is inclined to the mica surface by ca. 50degrees. The thickness of this ultrathin film increased by ca. 5.5% across the transition temperature indicating a decrease in density.