14N NMR spectroscopy of NH4+ as a probe of ion‐exchange membranes

Abstract 14 N NMR Spectroscopy of NH 4 + is a convenient and sensitive probe of structure and mobility in Nafion, Dow and Neosepta cation‐exchange membranes. NH 4 + ions bound to these commercially produced membranes give rise to small 14 N quadrupolar splittings ranging from 10 to 160 Hz. Moderate stretching of Nafion and Dow membranes induces much larger splittings of up to 2 kHz. The splitting has also been found to be dependent on the orientation of the membranes in the magnetic field B 0 . In Nafion it reaches a minimum at an angle of 45–55° between the direction of stretch and B 0 . The quadrupolar splittings also increase with increase in temperature. 14 N spin–lattice relaxation times in all three membranes at 300 K are about 20 times shorter than in aqueous solution, consistent with strongly bound NH 4 + ions. The apparent activation energies for 14 N relaxation, derived from the temperature dependence of the T 1 , data, show that reorientation of the bound NH 4 + ions requires ca. 20 kJ mol −1 in Nafion and Dow membranes and ca. 30 kJ mol −1 in Neosepta, compared with 6.5 kJ mol −1 for free NH 4 + in aqueous solution.