The ligand density critically affects the performance of ion-exchange resins in terms of adsorption capacity and transport characteristics. However, for tentacular and other polymer-modified exchangers, the mechanistic origins of the effect of ligand density on performance are not yet fully understood. This investigation focuses on understanding the ionic strength-dependent nanoscale changes in intraparticle architecture of tentacular cation exchangers using small-angle x-ray scattering (SAXS) and ultra-small-angle x-ray scattering (USAXS). Combined SAXS and USAXS intensity profiles for tentacular sorbents shows three different structural ranges in terms of the scattering vector q, which varies inversely with length scale. The low-q (0.0007-0.007 Å-1) region reflects changes in pore size; the intermediate-q region shows q-4 dependence, reflecting the mass fractal nature of the base matrix; and the high-q region shows a characteristic ionomer peak at 0.06 &lt; q &lt; 0.2 Å-1 that depends on the ligand density and ionic strength of the solution. Increasing the ionic strength leads to a decrease in the ionomer peak intensity and a shift in the maximum towards higher q values. These observations indicate a signature change in the polyelectrolyte layer from larger to smaller dimensions, reflecting the collapse of the grafted polyelectrolyte layer. An increase in the ionomer peak intensity with change in counterion (Na+ replaced by Cs+ or Rb+) confirms the structural origin of the ionomer peak as a polyelectrolyte layer of a tentacular sorbent. Different modeling approaches were used to determine the characteristic lengths scales of the grafted polyelectrolyte layer as well as the pore sizes for tentacular sorbents varying in ligand density.

Probing nanoscale effects of ligand density in tentacular ion exchangers using x-ray scattering

251st National Meeting (2016)

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