Desorption and Exchange of Self-Assembled Monolayers (SAMs) on Gold Generated from Chelating Alkanedithiols

The kinetics of thermal desorption and displacement of self-assembled monolayers (SAMs) on gold derived from the adsorption of 2,2-dipentadecylpropane-1,3-dithiol (d-C17, [CH3(CH2)14]2C[CH2SH]2), 2-pentadecylpropane-1,3-dithiol (m-C17, CH3(CH2)14CH[CH2SH]2), and heptadecanethiol (n-C17, CH3(CH2)16SH) were explored. The kinetics were monitored by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Thermal desorption studies of the SAMs in decalin at elevated temperatures demonstrated an enhanced stability for films generated from d-C17 and m-C17 relative to that for the film generated from n-C17. These studies further demonstrated that SAMs adsorbed at elevated temperatures (e.g., 50 °C) are more stable than those adsorbed at room temperature. Upon exposure to ambient laboratory conditions for one month, densely packed SAMs generated from n-C17 and d-C17 underwent no detectable structural changes. In contrast, similar treatment of the SAM generated from m-C17, which possesses a relatively low density of alkyl chains, led to structural change(s), as indicated by a progressive decrease in the values of the hexadecane contact angles. The data from the displacement studies suggest the following trend in the thermodynamic stabilities of the thiol-derived SAMs: m-C17 > d-C17 . n-C17. The degree of crystallinity of the alkyl chains of the SAMs failed to correlate with the observed trend in stabilities. The strong thermodynamic preference for m-C17 and d-C17 over n-C17 probably originates from the unique ability of the former adsorbates to chelate to the surface of gold and perhaps their decreased tendency toward desorption as a disulfide. The slight thermodynamic preference for m-C17 over d-C17 probably originates from an enhanced conformational flexibility for m-C17 that permits enhanced binding of this adsorbate to the surface of gold.