CHEMICAL FUNCTIONALITY IN SELF-ASSEMBLED MONOLAYERS - STRUCTURAL AND ELECTROCHEMICAL PROPERTIES

We examine the defectiveness and permeability of monolayers of long-chain organic molecules bound to gold electrodes via a sulfur attachment. The adsorbates, which were chosen to present a range of chemical groups at the outer surface of the monolayer, are a fluorinated alkanethiol (CF3(CF2)7-(CH2)2SH), an unsubstituted alkanethiol (CH3(CH2)9SH), an alcohol-thiol (HOCH2(CH2)10SH), a nitrile-thiol (NC(CH2)10SH), and a carboxylic acid-thiol (H02C(CH2)10SH). Both the defectiveness of the monolayers, as determined by electron transfer to acceptors in a contacting aqueous solution, and the permeability of the monolayers to aqueous ions, as determined by electrochemical capacitance measurements, correlate reasonably well with the quality of the packing of the carbon chains, which was determined by infrared spectroscopy, and with the relative cross sectional area of the chains compared to the other parts of the adsorbate. On the other hand, neither the permeability nor the defectiveness of the monolayers correlates well with the wettability of their surfaces. Thus, the fluorinated alkanethiol monolayer, with larger area chains, is the least permeable and the least defective, followed in order by the hydrophobic alkanethiol monolayer, the hydrophilic alcohol-thiol monolayer, the hydrophobic nitrile-thiol monolayer, and finally by the hydrophilic acid-thiol monolayer. © 1990, American Chemical Society. All rights reserved.