Magnetic resonance studies of the properties of water in cellulose acetate membranes

The properties of water in various cellulose acetate (CA) membranes have been investigated by magnetic resonance techniques. It has been observed that the longitudinal (T 1 ) relaxation time of 17 O in CA membranes containing H 2 17 O is considerably shorter (by a factor of 5 to 6) than the corresponding value in pure water. The T 1 values of 23 Na ions contained in these membranes were, however, shorter than the corresponding values observed in measurements of the aqueous mother solution by a factor smaller than 2. The EPR spectra of Mn 2+ and of Cr 3+ contained in CA membranes were practically identical to the spectra of the pure aqueous solution. The data suggest the existence, in the CA membrane, of two "types" of water; a small fraction of highly immobilized "bound" water and the remaining large fraction of "free" water displaying properties almost identical to those of pure bulk water. This model is supported by measurements of the longitudinal relaxation rates of H 2 17 O in CA membranes which were gradually dehydrated from their water.