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The statistical thermodynamic (ST) model was used to study nonlinear binary protein adsorption equilibria on an anion exchanger. Single-component and binary protein adsorption isotherms of bovine hemoglobin (Hb) and bovine serum albumin (BSA) on DEAE Spherodex M were determined by batch adsorption experiments in 10 mM Tris-HCl buffer containing a specific NaCl concentration (0.05, 0.10, and 0.15 M) at pH 7.40. The ST model was found to depict the effect of ionic strength on the single-component equilibria well, with model parameters depending on ionic strength. Moreover, the ST model gave acceptable fitting to the binary adsorption data with the fitted single-component model parameters, leading to the estimation of the binary ST model parameter. The effects of ionic strength on the model parameters are reasonably interpreted by the electrostatic and thermodynamic theories. The effective charge of protein in adsorption phase can be separately calculated from the two categories of the model parameters, and the values obtained from the two methods are consistent. The results demonstrate the utility of the ST model for describing nonlinear binary protein adsorption equilibria.

The statistical thermodynamic (ST) model was used to study nonlinear binary protein adsorption equilibria on an anion exchanger. Single-component and binary protein adsorption isotherms of bovine hemoglobin (Hb) and bovine serum albumin (BSA) on DEAE Spherodex M were determined by batch adsorption experiments in 10 mM Tris-HCl buffer containing a specific NaCl concentration (0.05, 0.10, and 0.15 M) at pH 7.40. The ST model was found to depict the effect of ionic strength on the single-component equilibria well, with model parameters depending on ionic strength. Moreover, the ST model gave acceptable fitting to the binary adsorption data with the fitted single-component model parameters, leading to the estimation of the binary ST model parameter. The effects of ionic strength on the model parameters are reasonably interpreted by the electrostatic and thermodynamic theories. The effective charge of protein in adsorption phase can be separately calculated from the two categories of the model parameters, and the values obtained from the two methods are consistent. The results demonstrate the utility of the ST model for describing nonlinear binary protein adsorption equilibria.

knowledge of mass transfer kinetics is required to optimize the chromatographic purification process, as the process performance is mass-transfer-limited in many applications. Liquid chromatography models on the basis of continuity equations are useless unless they are fed with appropriate adsorption isotherm equations, describing the basic thermodynamic equilibria between biomolecules in the mobile phase and the stationary phase. Therefore the study of adsorption equilibrium is of major importance for liquid chromatography [1][2][3].Currently, many mathematical models have been proposed for describing the adsorption equilibrium of biomolecules. Among these models, the Langmuir-type isotherms are the most famous models and are widely used to describe the nonlinear adsorption behavior in preparative ion-exchange chromatography [4]. Expressed in the simple and explicit form, Langmuir isotherms can represent the singlecomponent adsorption equilibrium data very well. However, the Langmuir isotherms are recognized to be thermodynamically inconsistent for multi-component ion-exchange systems, and the effect of ionic strength on protein adsorption is not explicitly expressed in these models [5,6]. Based on the mass action law, the stoichiometric displacement (SD) model was proposed to study protein adsorption in ion exchange systems [1][2][3][4][5][6][7]. However, it is a non-mechanistic model on the basis of oversimplified hypotheses which restrict it to rather ideal cases. Brooks and Cramer [8] combined the SD model and the concept of macromolecule steric shielding effect together to bring forward the steric mass-action (SMA) model for ion exchange adsorption. This model is considered to be one of the best models so far for protein ion exchange adsorption equilibrium and has found many applications in multi-component ion exchange chromatography [9,10]. The SMA model has also been extended to describe the dyeligand affinity adsorption equilibrium of proteins [11]. Recently, Shen and Frey [12] further investigated the SMA model by incorporating the effects of charge regulation on the ion-exchange adsorption of proteins. Abstract A study of nonlinear competitive adsorption equilibria of proteins is of fundamental importance in understanding the behavior of preparative chromatographic separation. This work describes the nonlinear binary protein adsorption equilibria on ion exchangers by the statistical thermodynamic (ST) model. The single-component and binary protein adsorption isotherms of bovine hemoglobin (Hb) and bovine serum albumin (BSA) on SP Sepharose FF were determined by batch adsorption experiments in 0.05 mol/L sodium acetate buffer at three pH values (4.5, 5.0 and 5.5) and three NaCl concentrations (0.05, 0.10 and 0.15 mol/L) at pH 5.0. The ST model was found to depict the effects of pH and ionic strength on the single-component equilibria well, with model parameters depending on the pH and ionic strength. Moreover, the ST model gave acceptable fitting to the binary adsorption data with the fitted singlecompo...

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