Esterification of Self-Assembled Carboxylic-Acid-Terminated Thiol Monolayers in Acid Environment: A Time-Dependent Study

This contribution reports on the influence of acids on the quality of carboxylic-acid-terminated self-assembled monolayers (SAMs) on gold prepared from ethanolic solution of HS−(CH2)15−COOH and HS−(CH2)11CONH−(EG)6CH2−COOH. Null ellipsometry, contact angle goniometry, and infrared reflection−absorption spectroscopy are used to monitor the physical and chemical changes occurring within the SAMs upon acid post treatment; after incubation with acids present in the solution; and after incubation in aged acid containing solutions. The presence of acid has a positive effect on the crystallinity, packing, and orientation of the supporting alkyl and ethylene glycol subunits of the SAM. Our studies also confirm previous findings stating that the carboxylic groups are rapidly converted into ethyl ester groups in the presence of hydrochloric acid in the incubation solution. It is also evident that the conversion occurs in the presence of the weaker acid, acetic acid, although at a much slower rate than that for hydr...