Retention of ions on nonporous charged stationary phases

The interactions between ion-exchange resins and counterions consist of several mechanisms, such as ion-pair formation between active sites and counterions, specific adsorption, solvation changes, and double-layer accumulation. The double-layer accumulation of ions, which is a typical nonstoichiometric mechanism, is an important factor governing overall ion-exchange chromatographic retention when a major part of the stationary-phase surface is in contact with eluent flows. Nonporous stationary phases, where solutes are accessible to the surfaces by convection as well as by diffusion, possibly highlight this nonstoichiometric contribution through the coupling of a flow profile with an electrostatic potential function. The retention of ions on nonporous stationary phases has been interpreted by a model derived on the basis of the Poisson-Boltzmann equation including solvation change terms. Unusual retention behaviors have been confirmed only for anions, and can be explained by the model including the assumption that anions undergo solvation changes in a thin layer (approximately 5 nm thickness) at the vicinity of the stationary phase; the thickness should be a function of eluent flow rates. This strongly suggests that there is a difference in solvation nature between cations and anions. It can be inferred that water molecules interacting with polymer domains of the stationary phase behave like single molecules and cannot form a stable hydration shell around an anion as usually seen in bulk solution.