Solvent dependence of the self-assembly process of an endgroup-modified alkanethiol

Adsorption of 12-(4-nitroanilino)dodecanethiol from ethanol and n-hexane onto polycrystalline gold substrates was studied in situ and in real time by second-harmonic generation. Depending on the fundamental wavelength used, the attachment of the thiol to the substrate or the behavior of the p-nitroaniline (pNA) endgroup can selectively be monitored. Striking solvent effects are shown for the process of film formation. The timescales on which changes related to the coverage and the orientation of the pNA moiety occur vary widely. In hexane the thiol coverage increases about four times faster than that in ethanol. The rearrangement of the pNA endgroups in ethanol occurs on a time scale that is larger by a factor of 2 to three compared to the adsorption. In contrast, hexane extends the process of reorientation by a factor of more than 30 relative to the time scale of adsorption. A qualitative model based on the differences in the solvent-thiol interactions is suggested.