The electrosorption valency of organic electrosorbates: Part I. Aliphatic compounds

Abstract

Thermodynamic experiments in electrosorption systems yield the electrosorption valency γ, the double-layer capacity C D and the maximum surface concentration Λ max as the most important results. These data are related to the geometric arrangement of the sorbate and its microscopic data, e.g. charge z, charge-transfer coefficient λ, dipole moment m, length l and area A of the molecule. Data are compiled and discussed for many aliphatic systems mainly on mercury and a few solid metal electrodes. For monosubstituted n-alkanes, the double-layer thickness is determined by l. Small positive values 0 < γ < 0.1 are obtained characterizing the substitution of two water molecules and a small dipole contribution of the functional group which is directed towards the solution. Small compounds with large dipole moments and bifunctional alkanes are adsorbed in a flat position mostly. Then the electrosorption valency is independent of m i and determined only by the water desorption. Aliphatic ions have larger positive or negative γ-values depending on the charge sign. The geometric penetration of the functional group into the double layer is dominating. Charge transfer and, consequently, bond formation seem to be important for some sulfur-containing compounds only, such as thiourea and dibutylsulfide. Then negative γ-values are obtained with a negative coefficient dγ/dϵ, indicating the potential-dependent charge transfer λ. For thiourea on various metals, λ can be correlated to the formation enthalpies of the corresponding sulfides.