Model Study of Penetration of Cl− Ions from Solution into Organic Self-Assembled-Monolayer on Metal Substrate: Trends and Modeling Aspects

Abstract

Organic molecules that form self-assembled-monolayers on metal substrates may provide efficient corrosion protection. Herein we study how such self-assembled-monolayers hinder the penetration of Cl − ions from aqueous solution toward the metal substrate. We first elucidate some aspects that are relevant for modeling charged ions near surfaces with slab models that utilize periodic-boundary-conditions, in particular: (i) solvation effects, (ii) inter-ion electrostatics, (iii) depolarization effects, and (iv) effects of periodic-boundary-conditions along lateral directions and, for multi-slab models, also along the surface normal direction. The last two effects are artifacts hence they can be avoided or at least minimized by proper modeling. We further present a simple scheme that describes the activation barrier for Cl − penetration into self-assembled-monolayer as a function of the electrode potential and show that the activation barrier decreases as the electrode potential increases, as would be intuitively expected, however, for thick self-assembled-monolayers the barrier remains sizable even at rather positive potentials, which may be one of the reasons why dense and sufficiently thick self-assembled-monolayers can efficiently inhibit corrosion. By utilizing a simple model where metal substrate, organic layer, and aqueous solvent are described implicitly by dielectric continuum slabs, we analyze two important effects by which self-assembled-monolayers hinder the penetration of Cl − ions toward the metal substrate. The first effect is due to inferior solvation of ions in organic layer compared to that in aqueous solvent and the estimated difference is larger than 1 eV. This effect is independent of the thickness of the organic layer, provided that the layer is sufficiently thick (≳10 Å). The second effect is due to electric field at the electrochemical interface and the extent by which it affects the penetration of Cl − depends on the electrode potential and on the thickness of the organic layer. Other effects, such as local deformation of organic layer during Cl − penetration, cannot be described by current simple models and will be considered in our next publication. Finally, calculations indicate that due to stronger solvation of Na+ counter-ions their penetration into organic layer is inferior to that of Cl − . Energetically the most favorable way for Na+ to penetrate is in the form of Na+/ Cl − ion-pairs, but it is inferior to penetration of Cl − alone.