Ionic strength-dependent changes in tentacular ion exchangers with variable ligand density. I. Structural properties

• Published in: Journal of Chromatography A Vol. 1463; pp. 90 - 101
• Main Authors: Bhambure, Rahul, Gillespie, Christopher M., Phillips, Michael, Graalfs, Heiner, Lenhoff, Abraham M.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 09.09.2016
• Subjects: Adsorption; Cation Exchange Resins - chemistry; Cation-exchange chromatography (CEX); Chromatography, Gel; Inverse size-exclusion chromatography (ISEC); Ligand density; Ligands; Microscopy, Electron, Scanning; Osmolar Concentration; Porosity; Salt exclusion; Scanning electron microscopy (SEM); Scattering, Small Angle; Small-angle x-ray scattering (SAXS); X-Ray Diffraction; Small-angle x-ray scattering (SAXS); Inverse size-exclusion chromatography (ISEC); Salt exclusion; Cation-exchange chromatography (CEX); Scanning electron microscopy (SEM); Ligand density
• ISSN: 0021-9673; 1873-3778
• DOI: 10.1016/j.chroma.2016.08.010

•Ligand density can appreciably affect the structure and properties of ion exchangers.•Effects of ligand density on structure of tentacular resins were investigated.•Increasing ligand density was found to result in pore constriction.•SAXS measurements indicate the presence of ionomers (ion clusters). The ligand density critically affects the performance of ion-exchange resins in such measures as the adsorption capacity and transport characteristics. However, for tentacular and other polymer-modified exchangers, the mechanistic basis of the effect of ligand density on performance is not yet fully understood. In this study we map the ionic strength-dependent structural changes in tentacular cation exchangers with variable ligand densities as the basis for subsequent investigation of effects on functional properties. Inverse size-exclusion chromatography (ISEC), scanning electron microscopy (SEM) and small-angle x-ray scattering (SAXS) were used to assess the effect of ionic strength on the pore size and intraparticle architecture of resin variants with different ligand densities. Comparison of ISEC and cryo-SEM results shows a considerable reduction in average pore size with increasing ligand density; these methods also confirm an increase of average pore size at higher ionic strengths. SAXS analysis of ionic strength-dependent conformational changes in the grafted polyelectrolyte layer shows a characteristic ionomer peak at values of the scattering vector q (0.1–0.2Å−1) that depend on the ligand density and the ionic strength of the solution. This peak attribution reflects nanoscale changes in the structure of the grafted polyelectrolyte chains that can in turn be responsible for observed pore-size changes in the resins. Finally, salt breakthrough experiments confirm a stronger Donnan exclusion effect on pore accessibility for small ions in the high ligand density variant.