Identify unmodeled Au(111) surface features affecting reconstructed NADG/NBDG monolayers
Determine the nature of the Au(111) surface feature(s) not captured by the computational modeling that required fixing the atoms surrounding the carboxyl groups at elevated positions in the reconstructed monolayer structures of 4-nitrophenyl-α-D-galacturonide (NADG) and 4-nitrophenyl-β-D-galacturonide (NBDG); specifically, ascertain whether Au adatoms, electrospray-deposited solvent or ionic contaminants (e.g., Na or Ca), or slight unit-cell packing differences due to neighboring adsorbates are responsible.
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Specifically, the atoms surrounding the carboxyl groups of the central monosaccharide units of both structures were fixed in a position higher than in the relaxed counterparts. This suggests that there could be some feature in the surface which we were unable to capture with our methods. This could for instance be adatoms, an assumption supported by the fact that the monolayers are formed close to step edges, potentially acting as a source of adatoms. A second possibility is that the electrospray deposition process itself introduces either solvent molecules or contaminants which could be incorporated into the monolayer structure, such as Na or Ca ions, known to leach from laboratory glassware. A final third option could be that the slightly elongated units cells of our model structures do not fully capture the packing effects due to neighbouring adsorbates, with the real systems being slightly more crowded than our models.