Derivation of an extended Bjerrum equation for the activity coefficient of ions based on an analysis of Coulombic forces

Abstract: The activity coefficient of ions in solution was proposed by Bjerrum (1916) to depend on the cube root of concentration, because of a good fit with data for low and moderate salt concentration. However, Debye and H\"uckel (DH) later developed a theory that prevailed, and this theory has a square root dependence. The derivation of the DH equation is not easy or intuitive, and has various uncertain elements, and the fit to data is not very good for salts with non-unity valencies. We develop a model for the activity coefficient based on the Coulombic forces between an ion and the nearest ions of opposite charge, including the distribution of separations between these ions. For a symmetric salt, we only have to analyse the interactions between one anion and one cation, and we can derive an expression for the dilute limit that depends on the Bjerrum length and the cube root of salt concentration, the same as an expression put forward by Bjerrum. Our theory also analytically describes higher salt concentrations and a non-zero ion size, not only for 1:1 but also for 2:2 and 3:3 salts. For a 1:1 salt, the cube root scaling law describes data of the activity coefficient very well, both at low and intermediate salt concentrations, while the extended Bjerrum equation describes data even better and up to higher concentrations, and also includes the effect of ion size. For asymmetric salts (2:1 and 3:1 salts), a numerical procedure can be used which considers all possible orbits of two or three monovalent ions around a central multivalent ion of opposite charge, which also fits data accurately.