Surface area and layer charge of smectite from CEC and EGME/H2O-retention measurements

The total specific surface area (TSSA) and smectitic layer charge (Q) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Q measurement techniques based on cation exchange capacity (CEC), HO retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of HO from smectite on heating, and to introduce a correction for HO remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements. A good agreement between Q from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5-7%) by three techniques: (1) using mean HO coverage; (2) using mean EGME coverage; and (3) using a combination of HO coverage and CEC. A reduction of the mean error from 5-7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of HO and EGME coverage on layer charge. The study demonstrates that Ca-smectite samples at 47% RH have HO contents corresponding to 88-107% of the theoretical mass of a monolayer and offers an explanation of this variation.