Formation and Stability of Hexadecanethiolate SAMs Prepared in Aqueous Micellar Solutions of C12E6
By Yan, Dong; Saunders, Jeremy A. & Jennings, G. Kane
Published in Langmuir
2003
Abstract
Self-assembled monolayers (SAMs) prepared by the adsorption of hexadecanethiol (C16SH) onto gold exhibit infrared and impedance spectra that are consistent with a greater overall chain density when prepared in aqueous micellar solutions of hexaethylene glycol monododecyl ether (C12E6) than when prepared in ethanol or isooctane, although the initial step of the assembly process is much slower in the micellar solution. On the basis of ex situ infrared spectra taken at different adsorption times, SAMs prepared in C12E6(aq) undergo a more thorough structural transformation to likely minimize collapsed chains and other defects within the film. These water-borne SAMs are greatly resistant to structural changes when immersed in blank solvents (C12E6(aq), ethanol, and isooctane), but changes in the IR spectra suggest that a small fraction of thiolate molecules desorb from the surface when exposed to a solution containing C16SH in ethanol. Conversely, when ethanolic SAMs are exposed to a solution of C16SH in C12E6(aq), changes in the IR spectra are consistent with a net adsorption of thiols to increase the adsorbate density. SAMs prepared in isooctane exhibit significantly less stable structures when exposed to blank solvents as compared to SAMs prepared in C12E6(aq) and ethanol. Combined, these results reveal that solvent-adsorbate and solvent-film interactions play an important role in defining the structure of a SAM and that the use of weaker solvents to prepare the SAMs yields more densely packed films with fewer defects that are more stable against desorption.