Characterization and Quantification of Water in Smectites with Low-Field NMR

We show that simple low-field nuclear magnetic resonance (NMR) can provide quantitative measurements of water content in clays providing appropriate instrumentation and protocols. In particular, NMR is a good tool to study the swelling of smectites and the interlayer water content despite pore sizes of the order of or less than 1 nm and the presence of paramagnetic impurities yielding short T-1 and T-2 relaxation times. The NMR measurements performed at a magnetic field of 0.5 T were: (i) the standard CPMG sequence to deduce a distribution of transverse relaxation times T-2, (ii) the 2D T-1-T-2 sequence to analyze the mobility of protons, and (iii) a free induction decay (FID) sequence to detect very short relaxation time and evaluate the total amount of protons present in the system, including structural hydroxyls. We complemented the NMR data with small angle X-ray scattering (SAXS) to obtain the basal spacing d(001). On powder clays, the total interlayer water content is obtained from FID measurements after correction for hydroxyls (determined after heating the powders at 200 degrees C) and for external water (determined using freezing experiments). The interlayer water content obtained on a reference smectite is comparable to literature data On four hornoionic smectites (Na, Ca, Mg, and K) equilibrated at four different relative humidities (11, 33, 75, and 97%), we found a ranking of the water content in agreement with the ion radius. However, whatever the ion, the interlayer water content is linearly related to the basal spacing d(001). The transverse NMR relaxation time T-2 is not related to the spacing d(001).