Formation of self-assembled monolayers with homogeneously mixed, loosely packed alkyl groups using unsymmetrical dialkyldithiophosphinic acids

We report the formation and characterization of self-assembled monolayers (SAMs) formed from unsymmetrical dialkyldithiophosphinic acid (R 1R2DTPA) adsorbates [CH3(CH2) n][CH3(CH2)15]P(S)SH (n = 5, 9) on gold substrates. These SAMs were characterized using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact angle goniometry, electrochemical impedance spectroscopy, and atomic force microscopy. Unsymmetrical R1R2DTPA SAMs contain mixtures of bidentate and monodentate adsorbates, similar to SAMs formed from analogous symmetrical R2DTPAs. In unsymmetrical R1R2DTPA SAMs, however, the short alkyl substituent of the R1R2DTPA adsorbates enforces spacing between the long hexadecyl substituents, which disrupts van der Waals interactions and causes the hexadecyl groups to be loosely packed and disordered. The structure of the SAM depends on the length of the short alkyl substituent: The hexyl chains in the C6C 16DTPA SAM are not long enough to stabilize the alkyl zone close to the substrate, leading to highly disordered SAMs with a low molecular packing density in which the hexadecyl chains lie down to fill the gaps between adjacent adsorbates. In contrast, the additional van der Waals interactions provided by the decyl chains of the C10C16DTPA SAM enable dense molecular packing in the alkyl zone close to the substrate. The structure of the SAM consists of a zone close to the substrate composed of a packed alkyl layer, with hexadecyl chains protruding above to form a loosely packed, disordered alkyl layer. Regardless of the structural differences between the C 6C16DTPA and C10C16DTPA SAMs, both SAMs exhibit homogeneous mixing of the alkyl chains within the SAM, demonstrating that binding two different chains to a single headgroup is an effective method to prevent phase segregation. © 2012 American Chemical Society