New Dihexadecyldithiophosphate SAMs on Gold Provide Insight into the Unusual Dependence of Adsorbate Chelation on Substrate Morphology in SAMs of Dialkyldithiophosphinic Acids

We report the formation and characterization of new self-assembled monolayers (SAMs) formed from dihexadecyldithiophosphate (C<sub>16</sub>)<sub>2</sub>DDP and compare their properties with those of SAMs formed from the structurally similar adsorbate dihexadecyldithiophosphinic acid (C<sub>16</sub>)<sub>2</sub>DTPA. The new (C<sub>16</sub>)<sub>2</sub>DDP SAMs were characterized using X-ray photoelectron spectroscopy, reflection–absorption infrared spectroscopy, contact angle measurements, and electrochemical impedance spectroscopy. The data indicate that (C<sub>16</sub>)<sub>2</sub>DDP forms SAMs on gold films formed by e-beam evaporation in which all adsorbates chelate to gold, in contrast to (C<sub>16</sub>)<sub>2</sub>DTPA SAMs, in which 40% of the adsorbates are monodentate. The alkyl chains of the (C<sub>16</sub>)<sub>2</sub>DDP SAM are also less densely packed and ordered than those of the (C<sub>16</sub>)<sub>2</sub>DTPA SAM. To understand these differences, we present density functional theory calculations that show that there are only minimal differences between the geometric and electronic structures of the two adsorbates and that the energetic difference between monodentate and bidentate binding of a gold­(I) ion are surprisingly small for both adsorbates. This study leads to the conclusion that differences in intermolecular interactions within the SAM are the driving force for the difference in chelation between the two adsorbates.