Neutron Time-of-Flight Quantification of Water Desorption Isotherms of Montmorillonite

The multiple energy states of water held by surfaces of a clay mineral can be effectively probed with time-of-flight and fixed elastic window neutron scattering. We used these techniques to quantitatively differentiate water types, including rotational and translational diffusions, in Ca- and Na-montmorillonite (SAz-1) and charge-reduced preparations equilibrated at RH = 33% and 55%, whose gravimetric water contents are in proportion with their layer charge. Quasi-elastic neutron scattering results revealed significant differences within interlayer water populations and between interlayer and interparticle waters. Interlayer cationic and H-bonded waters have residence times ranging from a few nanoseconds to tenths of picoseconds, while interparticle water, obtained for the RH = 55% equilibrated samples, showed an average diffusivity faster than interlayer water, yet slower than bulk water. Our results enabled us to differentiate at least two water motions during dehydration of Ca- and Na-SAz-1 (initially equilibrated at RH = 55%) by using a "controlled water loss" time-of-flight procedure. This work confirms that (a) interlayer and cationic water in dioctahedral smectites are characterized by slower motions than interparticle water, (b) interlayer cations influenced the dynamics of water loss, probably through its affect on clay fabric, and (c) interparticle water behaves more like bulk water. At 55% RH the Ca montmorillonite held more interparticle water, but on dehydration under controlled conditions, it retained interlayer and cationic water more strongly than Na montmorillonite