Understanding the interaction and structural ordering of widely used molecules adsorbed on self-assembled monolayer (SAM) surfaces may have technological and biological implications such as the immobilization of biomolecules at an interface and the control of surface wettability and corrosion resistance. In this presentation, we report the structures of four common solvents—acetonitrile, ethanol, methanol, and water—on a SAM of <i>n</i>-alkanethiols at cryogenic temperatures, observed using reflection high-energy electron diffraction (RHEED). Each of these molecules exhibits a weaker interaction with the hydrophobic, nonpolar SAM surface compared to the intermolecular interaction and thus a preferential structural ordering with surface guidance is not anticipated. However, it is surprising that acetonitrile and ethanol both form a sustaining vertical order with long-range crystalline structures, whereas a polycrystalline film is observed for interfacial water and methanol. By comparing the interfacial and intermolecular interactions with a close examination of the crystal structures, we conclude that the presence of topographical steps on the SAM surface plays a critical role in promoting the crystalline growth of certain adsorbate molecules as a result of the lattice-matching template effect, as has been observed previously on highly-oriented pyrolytic graphite (HOPG). Such an effect is favored for acetonitrile and ethanol, whose assembly layers exhibit a lattice mismatch of ~1%. In contrast, the hydrogen-bonded methanol layers have a large lattice mismatch of ~10%. As for water, the strong hydrogen bonds, relatively fast crystallization kinetics, and the poor interaction with the hydrophobic surface are attributed to the growth of small randomly oriented crystallites even though the structural mismatch is slightly above 2%. Our study thus shows that SAM surfaces with molecularly tuned steps and different affinities may have the potential to be templates for crystal growth and interfacial studies.

Morphology-induced ordering of small molecules on hydrophobic self-assembled <i>n</i>-alkanethiols

ACS Fall 2020 Virtual Meeting

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Morphology-induced ordering of small molecules on hydrophobic self-assembled n-alkanethiols