Self-assembled monolayers as a model for studying interfacial phenomena

Abstract

We reported self-assembled monolayers (SAMs) as a model system for understanding interfacial phenomena and investigating physicochemical properties of SAM-coated surfaces, such as water wettability. The studies reported herein have a great potential in the applications including microarrays, sensors, stimuli-responsive surfaces, and nanotechnology. Reactions that occur at interfaces often show different behaviors from their solution analogues. Herein, we demonstrated how proximity effect, one of the unique phenomena at interfaces, could control interfacial reactions. Terminal carboxylic acids in SAMs of 16-mercaptohexadecanoic acid on gold surfaces were treated with reagents (cyanuric fluoride and pyridine) that are used for forming acid fluorides from carboxylic acid in the solution reaction analogue. After the treatment, each of two different products, acid fluoride and interchain anhydride, was obtained on surfaces under different reaction conditions with keeping the reagents the same. Various factors, such as concentrations of reactants and reaction time, affected the product distribution (or the reaction pathway) on surfaces. The key factors for controlling the reaction pathway are the proximity effect of adjacent carboxylic acid chains in the SAMs and equilibrium shift. Products on surfaces and their reaction behaviors were characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltametry (CV). We also report the formation of SAMs presenting imidazolium ions at the tail ends on Au and $Si/SiO_2$, and the effects of counter anions and alkyl chain length on surface hydrophilicity/phobicity, i.e., wettability, to demonstrate the possible applications of ILs in surface sciences. The surface wettability of SAMs presenting imidazolium ions at the tail ends could be extrapolated to the water miscibility of the related ionic liquids. As a proof-of-principle, via direct ion exchange on a monolayer surface, changes in water contact ...