Charging of silver bromide aqueous interface: Evaluation of enthalpy and entropy of interfacial reactions from surface potential data

Dependence of surface potential (electrostatic potential at the inner Helmholtz plane, Ψ0) at the silver bromide aqueous electrolyte interface was measured as a function of the activities of Br− and Ag+ by using a single crystal silver bromide electrode (SCr–AgBr). Absolute values of surface potentials were obtained from electrode potentials of SCr–AgBr and isoelectric points. Measurements were performed at different temperatures in the range from 10 to 50 °C. Corresponding equilibrium constants of interfacial reactions were obtained using the surface complexation model and interpreted via the van’t Hoff equation. As a result of the interpretation for the binding of bromide ions leading to a negative surface charge, the thermodynamic parameters obtained were ΔnH∘=-33kJmol-1 and ΔnS∘=-31Jmol-1K-1; and for the binding of silver ions leading to a positive surface charge, ΔpH∘=-72kJmol-1 and ΔpS∘=-196Jmol-1K-1. Association of counterions (CI) with oppositely charged surface sites partially compensates the surface charge. Assuming approximately the same affinities for anions (NO3-) and cations (K+) thermodynamic parameters for their binding were obtained as ΔCIH∘≈7kJmol-1 and ΔCIS∘≈105Jmol-1K-1.

Graphical abstractA single crystal silver bromide electrode (SCr–AgBr) was used to measure the inner surface potential (Ψ0) at a silver bromide aqueous electrolyte interface in a broad temperature range. Reaction enthalpies and entropies were evaluated.Figure optionsDownload full-size imageDownload high-quality image (93 K)Download as PowerPoint slideResearch highlights► Bromide ions have a more pronounced tendency for binding at a silver bromide surface than silver ions Kn∘>Kp∘. ► The binding of bromide ions is significantly less exothermic ΔnH∘>ΔpH∘ΔnH∘>ΔpH∘. ► The binding of silver ions is less favorable due to a significantly higher negative entropy change ΔnS∘<ΔpS∘ΔnS∘<ΔpS∘.