Interaction of 2-mercaptoethanesulfonate monolayers on silver with sodium cations

Interactions of 2-mercaptoethanesulfonate (MES) monolayers formed on silver surfaces with Na+ cations of the surrounding electrolytes were investigated by surface-enhanced Raman scattering (SERS). The structures and wavenumbers of the MES anion and the anion-sodium cation ion pair were calculated at the SCF (self-consistent field) level to facilitate assignment of the bands and for a better interpretation of the cation-induced effects. The SERS results demonstrate that the interaction of sulfonic groups of immobilized MES molecules with sodium cations is significantly different to the respective interactions observed for 'free' MES molecules in aqueous solution. In the case of surface-attached MES molecules, the contact ion pairs with metal cations are easily formed. The local symmetry of a large part of the sulfonic groups forming contact ion pairs with the metal cations is not lowered, in contrast to what one can observe in crystals of MES sodium salt and to what was predicted theoretically for an isolated MES molecule interacting with Na+. It is worth emphasizing that such interactions could significantly modify the structure of organic monolayer/electrolyte interface. Copyright (C) 2002 John Wiley Sons, Ltd.