Control of access to surfaces with self-assembling surfactants bearing fluorocarbon chains

Cyclic voltammetry and microgravimetry via the electrochemical quartz crystal microbalance have been used to examine ion and solvent transport in self-assembled viologen monolayers on gold metal surfaces. Fluorocarbon chains have been incorporated into these viologen monolayers as outer spacers in order to vary systematically the steric bulk of the monolayer. Ions can be electrochemically driven into and out of the monolayer by virtue of the redox chemistry of the viologen group embedded within the monolayer. The transport of these ions provides a straightforward way of evaluating the barrier properties of the fluorocarbon chain region of the monolayer. We find that the larger size of these fluorocarbon chains compared with alkyl chains makes a more formidable barrier to ion transport than hydrocarbon outer spacers of viologens in previous studies. Trends in ion-pairing strength and the simultaneous transport of water during ion transport have been evaluated for several counter-anions. Relative transport rates for anions of various sizes and hydration numbers have also been ascertained. The structure and orientation of the monolayers were also investigated using advancing contact angle measurements and IR reflectance spectroscopy.