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

<jats:title>Abstract</jats:title><jats:p>The total specific surface area (<jats:italic>TSSA</jats:italic>) and smectitic layer charge (<jats:italic>Q</jats:italic><jats:sub>s</jats:sub>) 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 <jats:italic>TSSA</jats:italic> and <jats:italic>Q</jats:italic><jats:sub>s</jats:sub> measurement techniques based on cation exchange capacity (<jats:italic>CEC</jats:italic>), H<jats:sub>2</jats:sub>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<jats:sub>2</jats:sub>O from smectite on heating, and to introduce a correction for H<jats:sub>2</jats:sub>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 <jats:italic>CEC</jats:italic> and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements.</jats:p><jats:p>A good agreement between <jats:italic>Q</jats:italic><jats:sub>s</jats:sub> from the structural formula and from <jats:italic>CEC</jats:italic> was obtained when this correction was applied. The <jats:italic>TSSA</jats:italic> of smectite was measured with similar accuracy (mean error of ±5–7%) by three techniques: (1) using mean H<jats:sub>2</jats:sub>O coverage; (2) using mean EGME coverage; and (3) using a combination of H<jats:sub>2</jats:sub>O coverage and <jats:italic>CEC</jats:italic>. 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<jats:sub>2</jats:sub>O and EGME coverage on layer charge. The study demonstrates that Ca<jats:sup>2+</jats:sup>-smectite samples at 47% RH have H<jats:sub>2</jats:sub>O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.</jats:p>