Structure, reactivity and microrheology in self-assembled monolayers

Abstract

Self-assembled monolayers of thiols on gold surfaces provide convenient systems for modeling adsorption and adhesion. A transverse shear mode (TSM) device was used to determine how octadecanethiol, octadecylamine and tetradecanol mechanically couple to gold. An order—disorder transition was detected for the first time in the chemisorbed thiol monolayer and thin film. No such transition is observed for the physisorbed amine or alcohol films, although melt transitions are observed for both chemisorbed and physisorbed films. These results illustrate both an important property of self-assembling systems, and also the utility of the TSM as a probe of adhesive interactions, interfacial shear and viscosity of very thin films. Surface-enhanced Raman spectra of thiol and disulfide monolayers show definitively that aromatic thiols adsorb on gold with cleavage of the SH bond, and that the SS bond in symmetric aromatic disulfides also cleaves upon adsorption. Aromatic thiol monolayers are apparently more stable than their alkyl counterparts towards oxidation. The first deliberate preparations of oxidized thiol monolayers (sulfinates, sulfonates) are described. The relative adsorptivities of thiolates, sulfinates and sulfonates are explained by frontier orbital theory. Charge transfer interactions are shown to be more important that Coulombic attractions in determining the relative adsorptivities, and ultimately the stability of the monolayers. These experimental and theoretical results contribute to our fundamental understanding of adhesive interactions, and provide a rational basis for designing new adsorbates and adhesives with prescribed surface interactions.