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)](2)C[CH2SH](2)), 2-pentadecylpropane-1,3-dithiol (m-C17, CH3(CH2)(14)CH[CH2SH](2)), and heptadecanethiol (n-C17, CH3(CH2)(16)SH) 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 degrees 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 much greater than 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.