Self-Assembled Monolayers of Nitrile-Functionalized Alkanethiols on Gold and Silver Substrates

摘要

Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH_2)_(16)SH (NC-C16), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles in NC-C16/Ag and NC-C16/Au were estimated to be 29.5 °±5° and 42.5°± 5° from the surface normal, respectively, while the data suggest lower ordering for NC-C16Au. The -C≡N bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65°± 7° for both NC-C16/Au and NC-C16/Ag. Comparison with previous data on CH_3-terminated SAMs reveals that substitution of weakly interacting CH_3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by ～7.5° and ～17.5° in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile groups is assumed to underlie the structural behavior by controlling a balance between the headgroup-substrate and interchain interactions. The near-parallel orientation of the nitrile groups to the surface in both of these SAMs can be explained on the basis of minimization of the unfavorable CN-CN dipole-dipole interactions.