Diffusional Surface Voltammetry as a Probe of Adsorption Energetics

Direct determination of the adsorption free energy for extremely low surface coverages (Henry limit) requires the use of a technique that must be highly sensitive to both the amount and the energetics of adsorbed molecules. Herein, we demonstrate that diffusional surface voltammetry (DSV), which embodies film and stripping voltammetries as two limiting cases, can be used to achieve this goal for electroactive adsorbates. To this end, a general analytical expression for the surface voltammetric peak potential of DSV is derived, which covers the full range of scan rates, bulk concentrations, and adsorptivity of the freely diffusing form of the redox couple, so that the surface redox conversion can be either equilibrated with or transport-isolated from the solution bulk. Strategies to get a quantitative insight into the energetics of electrosorption are outlined, and diagnostic criteria for their application are developed. In particular, it is demonstrated that DSV can be used in its stripping mode to determine group contributions to the adsorption free energy, avoiding possible interferences from intermolecular interactions or formation of oligomeric species. Application of this protocol to the reductive desorption of distinct homologous series of alkylthiolates adsorbed at mercury electrodes has allowed us to determine the contributions of the CHn groups (n = 0-3) to the free energy of adsorption of these molecules. These estimates are shown to correlate linearly with the corresponding group contributions to the octanol water partition coefficient, revealing that adsorption of individual hydrocarbon groups at the mercury/solution interface scales with their hydrophobicity. Overall, the present work enlarges the capability of surface voltarnmetry to probe adsorption energetics down to the micromolar level, and it represents a first step toward the development of a unified treatment of stripping and film voltammetries.