Mapping the ionic fingerprints of molecular monolayers.

We have previously proposed, and experimentally resolved, an ionic charge relaxation model for redox inactive self-assembled monolayers (SAMs) on metallic electrodes in contact with a liquid electrolyte. Here we analyse, by capacitance spectroscopy, the resistance and capacitance terms presented by a range of thiolated molecular films. Molecular dynamics simulations support a SAM-specific energy barrier to solution-phase ions. Once surmounted, the entrapped ions support a film embedded ionic capacitance and non-faradaic relaxation, which can be assigned as a particular case of general electrochemical capacitance.