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

Abstract

Abstract The total specific surface area ( TSSA ) and smectitic layer charge ( Q s ) 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 s measurement techniques based on cation exchange capacity ( CEC ), H 2 O 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 H 2 O from smectite on heating, and to introduce a correction for H 2 O 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 s 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 H 2 O coverage; (2) using mean EGME coverage; and (3) using a combination of H 2 O 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 H 2 O and EGME coverage on layer charge. The study demonstrates that Ca 2+ -smectite samples at 47% RH have H 2 O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.