Experimental study and modeling of the influence of mixed electrolytes on adsorption of macromolecules on a hydrophobic resin

• Mixed electrolytes are used in hydrophobic interaction chromatography.
• The ions and not the salts drive the adsorption.
• Synergetic effects are found using mixed electrolytes.
• A mathematical model describes the dependencies.

The influence of mixed electrolytes on the adsorption of macromolecules on a hydrophobic resin is explored. The macromolecules are native lysozyme, di-PEG-lysozyme-5kDa and pure PEG-6kDa, the adsorbent is Toyopearl PPG-600M, a mildly hydrophobic resin. The solvent is a 25 mM aqueous sodium phosphate buffer at pH 7.0 containing additional salts with an overall ionic strength of 3000 mM. The studied salts are sodium chloride, ammonium sulfate, sodium sulfate and ammonium chloride. Both pure salts as well as binary and ternary mixtures of these salts with varying ratios of the amounts of the salts are studied. For all solvents adsorption equilibrium isotherms are measured at 25 °C. Mixing electrolytes can lead to synergetic effects, i.e. the adsorption in the mixed electrolyte system is sometimes distinctly larger than expected from the results of the single electrolyte systems. The single ions, not the salts, drive the adsorption. A mathematical model is developed, which describes the influence of the different ions and their cross-interactions on the studied adsorption process.