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| - Dynamics, Site Binding, and Distribution of Counterions in Polyelectrolyte SolutionsStudied by Electron Paramagnetic Resonance Spectroscopy
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| - A microscopic picture of counterion condensation in liquid and glassy frozen solutions of the cationicpolyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) is derived from data obtained by acombination of continuous-wave and pulse electron paramagnetic resonance techniques. The condensation ofsuch divalent anions to the polyelectrolyte chain in the presence of a large excess of polyelectrolyte andmonovalent counterions can be described by a dynamic equilibrium between specifically site-bound andnonspecifically territorially bound counterions with exchange between the two states proceeding on timescales significantly shorter than 1 ns. No free divalent counterions are detected. The dynamic electrostaticattachment is manifest in an axially symmetric rotational diffusion tensor, with the unique axis of fast rotationcorresponding to the electrostatic bond between one sulfonate group of Fremy's salt and the quaternaryammonium group of the PDADMAC repeat unit. A distance of 0.43 nm between the electron spin and the14N nucleus of the ammonium group is found by electron spin−echo envelope modulation spectroscopy,suggesting that the site-bound state corresponds to contact ion pairs. The same experiment provides an estimateof 20% site-bound and 80% territorially bound divalent counterions in glycerol/water glassy frozen solution.Pulse electron−electron double resonance measurements show that on nanometer length scales the counterionsare virtually homogeneously distributed in three dimensions for high polyelectrolyte concentration but linearlydistributed along stretched chains at low polyelectrolyte concentrations where no overlap of chains is expected.
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