Desorption and exchange of self-assembled monolayers (SAMs) on gold generated from chelating alkanedithiols

The kinetics of thermal desorption and displacement of self-assembled monol ayers (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, CH 3(CH2)(16)SH) were explored. The kinetics were monitored by optical ellipso metry, contact angle goniometry, and polarization modulation infrared refle ction absorption spectroscopy (PM-IRRAS). Thermal desorption studies of the SAMs in decalin at elevated temperatures demonstrated an enhanced stabilit y for films generated from d-C17 and m-C17 relative to that for the film ge nerated from n-C17. These studies further demonstrated that SAMs adsorbed a t elevated temperatures (e.g., 50 degrees C) are more stable than those ads orbed at room temperature, Upon exposure to ambient laboratory conditions f or 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 ch ains, led to structural change(s), as indicated by a progressive decrease i n the values of the hexadecane contact angles. The data from the displaceme nt 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 o f crystallinity of the alkyl chains of the SAMs failed to correlate with th e 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 decr eased tendency toward desorption as a disulfide. The slight thermodynamic p reference for m-C17 over d-C17 probably originates from an enhanced conform ational flexibility for m-C17 that permits enhanced binding of this adsorba te to the surface of gold.