Adsorption of cations onto positively charged surface mesopores

• The retention mechanism of nortryptiline on a new stationary phase are studied.
• This phase is made of silica bonded to ionic ligands.
• The theory is based on Donnan equilibrium between static and percolating moving phases.
• Retention depends on the electro-chemical potentials of analytes in the pores and the bulk.
• The retention of low concentration cations decreases with increasing surface charge density.

Uwe Neue developed a theoretical treatment to account for the adsorption of ions on mesopores of packing materials the walls of which are bonded to ionic ligands but left this work unfinished. We elaborated upon this treatment and refined it, based on the equivalence that he suggested between charged surface particles and a membrane that separates two ionic solutions but is impermeable to one specification. He had written that the electro-chemical potentials in both ionic solutions are equal (Donnan equilibrium). The equilibrium between the surface and the pore concentrations is accounted for by an homogeneous electrostatically modified Langmuir (EML) isotherm model. The theoretical results are presented for four different charge surface concentrations σ0 = 0, 0.001, 0.002, and 0.003 C/m2, using a phosphate buffer (WSpH=2.65) of ionic strength I = 10 mM. The average pore size, the specific surface area, and the specific pore volume of the stationary phase were Dp = 140 Å, Sp = 182 m2/g, and Vp = 0.70 cm3/g, respectively.The theoretical results provide the quantitative difference between the ionic strength, the pH, and the concentrations of all the ions in the pores and in the bulk eluent. The theory predicts (1) that the retention times of cations under linear conditions is lower and (2) that their band widths under overloaded conditions for a given retention factor shrinks when the surface charge density σ0 is increased. These theoretical results are in good agreement with experimental results published previously and explain them.