2H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Abstract

Multiple quantum 2H NMR spectroscopy is used to study the structure and dynamics of D2O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and T2 relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the T2 values range from 20 to 180 ms, with the high hydration values having smaller couplings and longer T2 values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 D2O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel-channel model for Nafion hydration.