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

✍ Scribed by Haryadi; M. Rankothge; J. Hook; L. van Gorkom; G. Moran

• Publisher: John Wiley and Sons
• Year: 1994
• Tongue: English
• Weight: 494 KB
• Volume: 32
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260320803

✦ Synopsis

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.