Variability in water sorption and in particle thickness of standard smectites: Fid-Bau Portal

Variability in water sorption and in particle thickness of standard smectites

Stepkowska, E.T / Pérez-Rodrı́guez, J.L / Maqueda, C / Starnawska, E

AbstractVariability 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 °C and variable room temperature) and on sorption time (ts=1, 12 and 32 days). The particle thickness, δ was estimated, assuming a given number of molecular layers of sorbed water at the given RH. δ 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, δ=22 and 20 unit layers [u.l.], respectively), the thinnest ones-in Ca-smectite (SAz-1) and in nontronite (SWa-1, δ=5–6 u.l.) and the intermediate both in Na-smectite (SWy-1) and in Ca-smectite (STx-1), δ=12 and 8 u.l., respectively. On prolonged storage at RH=1.0, ts=32 days, the smectite particles disintegrated gradually into separate crystallites, δ=3–5 u.l., thus close to the thickness measured by XRD. An exception was hectorite, SHCa-1, with δ=6–8 u.l. possibly due to “gluing” by calcium carbonate present. The decrease in δ (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.