Relating saturation capacity to charge density in strong cation exchangers

- General correlation between surface charge density and binding capacity was found.
- Steric effects ultimately limit binding capacity when charge density is increased.
- Equilibrium and theoretical maximum binding capacity of resins can be predicted.

In this work the relation between physical and chemical resin characteristics and the total amount of adsorbed protein (saturation capacity) for ion-exchange resins is discussed. Eleven different packing materials with a sulfo-functionalization and one multimodal resin were analyzed in terms of their porosity, pore size distribution, ligand density and binding capacity. By specifying the ligand density and binding capacity by the total and accessible surface area, two different groups of resins were identified: Below a ligand density of approx. 2.5 μmol/m2 area the ligand density controls the saturation capacity, while above this limit the accessible surface area becomes the limiting factor. This results in a maximum protein uptake of around 2.5 mg/m2 of accessible surface area. The obtained results allow estimating the saturation capacity from independent resin characteristics like the saturation capacity mainly depends on “library data” such as the accessible and total surface area and the charge density. Hence these results give an insight into the fundamentals of protein adsorption and help to find suitable resins, thus limiting the experimental effort in early process development stages.