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| - Structure and Molecular Dynamics of Alkane Monolayers Self-Assembled on Mica Platelets
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| - Monolayers of dialkyldimethylammonium (C10−C18) cations have been electrostatically bound to the surfaceof mica platelets. The structure and chain dynamics of these well defined self-assembled alkane monolayershave been probed by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy, nuclear magneticresonance, and differential scanning calorimetry (DSC). All results indicate that at low temperatures the alkylchains 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 andgauche conformers. Although the translational freedom of the chains is limited by the electrostatic binding ofthe headgroups to the substrate, the conformational transformation destroys the two-dimensional lattice leadingto disordered molecules with a liquidlike conformation. This phase transition manifests itself in IR-absorptionfrequency and carbon resonance frequency shifts as well as in an enthalpy change. The methylene carbonresonance frequencies of the trans and gauche conformers are better resolved than their IR-absorptionfrequencies and show clearly that the change in conformation starts at a considerably lower temperature thanthat indicated by IR spectroscopy and extends over a wide temperature range. The relatively sharp transitionsseen in IR-absorption maxima plots are due to poor resolution of the heterogeneously broadened lines andconsequent sluggish detection of the onset of the trans/gauche transformation. The phase transitions observedby 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 transitiontemperatures of SAMs electrostatically bound to a planar mica surface are higher than those assembled onplanar or curved (nanoparticles) gold surfaces. XRD revealed that the average molecular axis of a dioctadecylmonolayer is inclined to the mica surface by ca. 50°. The thickness of this ultrathin film increased by ca.5.5% across the transition temperature indicating a decrease in density.
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