Self-assembled monolayers of nitrile-functionalized alkanethiols on gold and silver substrates

Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH2)(16)SH (NC-C16), on (I 11) 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 29.5degrees +/- 5degrees and 42.5degrees +/- 5degrees from the surface normal, respectively, while the data suggest lower ordering for NC-C16/Au. The -CdropN bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65degrees +/- 7degrees for both NC-C16/Au and NC-C16/Ag. Comparison with previous data on CH3-terminated SAMs reveals that substitution of weakly interacting CH3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by similar to7.5degrees and similar to17.5degrees 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.