Abstract
The T 1 and T 2 proton n.m.r. relaxation times of water in aqueous sodium carboxymethylcellulose (NaCMC), with water contents below 2 g H 2 O g CMC , were measured between 340 and 120 K. The n.m.r. study revealed that (1) a distribution of correlation times and a correction of the Bloembergen, Purcell and Pound (BPP) theory for rigid lattice protons at low temperatures is necessary to simulate T 2 as a function of temperature, (2) non-freezing water is associated with two energetically distinct sorption sites, and (3) non-freezing water exhibits a temperature dependent motional anisotropy. Above 260 K, water at high-energy sites is responsible for an increase of T 2 with decreasing temperature. Below 260 K, T 2 shows a two-step decline that is caused by translational motion of the water molecules at the higher temperature and by rotational motion in the lower temperature region. The above mentioned motions are attributed to the motion of water molecules associated with lower-energy binding sites. At temperatures below 200 K, a Pake-type lineshape can be observed due to the increasing number of immobile water proton pairs. From a simulated lineshape analysis, the ratios of rotating to stationary water molecules were calculated as a function of temperature and water content.
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