Variability in water sorption and in particle thickness of standard smectites

Variability in sorbed water content was studied on several standard smectites, as dependent on the relative humidity (RH = 0.5, 0.95 and 1.0), on temperature (constant 30 degreesC and variable room temperature) and on sorption time (t(s) = 1, 12 and 32 days). The particle thickness, 6 was estimated, assuming a given number of molecular layers of sorbed water at the given RH. 6 decreased with the increase in the factors mentioned above. The thickest particles in dry state were found in hectorite and in synthetic mica/smectite (SHCa-1 and Syn-1, delta = 22 and 20 unit layers [u.l.], respectively), the thinnest ones-in Ca-smectite (SAz-1) and in nontronite (SWa-1, delta = 5-6 u. l.) and the intermediate both in Na-smectite (SWy-1) and in Ca-smectite (STx-1), delta = 12 and 8 u.l., respectively. On prolonged storage at RH = 1.0, ts = 32 days, the smectite particles disintegrated gradually into separate crystallites, delta = 3 5 u.l., thus close to the thickness measured by XRD. An exception was hectorite, SHCa-1, with delta = 6-8 u.l. possibly due to gluing by calcium carbonate present. The decrease in delta (delamination) observed on increasing RH was partly reversible. The hysteresis between water sorption on its increasing (WS) and water retention on its lowering to RH = 0.5 (WR) was especially pronounced in SHCa-1, SWy-1 and Syn-1 of initially thick particles. This was due either to a different number of the sorbed water layers, higher in WR, or to incomplete collapse in WR. (C) 2004 Elsevier B.V. All rights reserved.