| Abstract
| - The diffusion of ions in polyelectrolyte gels has been investigated both theoretically andexperimentally. A simple phenomenological model was first developed, building upon the geometricobstruction theory developed originally for the diffusion of neutral and charged species in neutral gels.The electrostatic obstruction is accounted for by (a) adopting an effective chain diameter obtained fromthe Dobrynin scaling theory for polyelectrolytes and (b) adding an exclusion layer of thickness δ(cs) aroundthe charged chains to describe the electrostatic interaction between the ions and the chains. Then thediffusion of water and organic ions in an organically cross-linked polyacrylamide gel was studiedexperimentally using nuclear magnetic resonance (NMR). The relative diffusion coefficients Dr = Dg/D0(Dg = diffusion in gel, D0 = diffusion in solvent) of the different species were measured for various polymer,cross-linker, and salt concentrations. The calculated predictions and the measured data were found to bein good agreement. For water molecules, we obtain Dr = 0.92 ± 0.01 independently of salt content, whichis consistent with a pure geometric obstruction. For monovalent anions and cations (butyrate, Bu-, andtetramethylamonium, TMA+) Dr increases from about 0.5 to 0.75 when the ion concentration increasesfrom 0 to 150 mol/m3. For the divalent cation (putrescine, Pu2+) the Dr are generally higher and increasefrom 0.6 to about 0.92, indicating an almost complete elimination of the electrostatic obstruction, consistentwith a more efficient screening of the charges in the chains.
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