Formation and Reaction of Interchain Carboxylic Anhydride Groups on Self-Assembled Monolayers on Gold

This paper demonstrates a new method for the synthesis of mixed self-assembled monolayers (mixed SAMs) by reaction of a reactive intermediatean interchain carboxylic anhydridewith alkylamines. The interchain anhydride was prepared in high yield from a SAM of 16-mercaptohexadecanoic acid (HS(CH2)15COOH) on gold by treatment with trifluoroacetic anhydride. Alkylamines reacted cleanly with the interchain anhydride to generate a mixed SAM, which comprised a mixture of acids and amides in approximately 1:1 ratio on the surface. The SAMs of the interchain anhydride and the mixed SAM were characterized using polarized infrared external reflectance spectroscopy, X-ray photoelectron spectroscopy, contact angle, and ellipsometry. Control of the wettability of the SAMs was demonstrated by allowing the interchain anhydride to react with alkylamines having different alkyl groups; this model system gave wetting data consistent with earlier studies of mixed monolayers and verified the ability of this method to manipulate interfacial physical properties. In certain circumstances, this method is experimentally simpler as a method to produce mixed SAMs than are conventional methods involving coadsorption of two thiols from a mixture in solution. It also assures two other characteristics of the mixed SAM: that the composition of the SAM is roughly 1:1 in carboxylic acid and amide groups, and that these two groups are well mixed on the surface.