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SURFACE AREA AND LAYER CHARGE OF SMECTITE FROM CEC AND EGME/H₂O-RETENTION MEASUREMENTS

Jan rodo¹ and Douglas K. McCarty²

¹ Institute of Geological Sciences PAN, Senacka 1, 31002 Krakow, Poland
² Chevron ETC, 3901 Briarpark, Houston, TX 77042, USA

^* E-mail address of corresponding author: ndsrodon{at}cyf-kr.edu.pl

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), H₂O 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 H₂O from smectite on heating, and to introduce a correction for H₂O 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 H₂O coverage; (2) using mean EGME coverage; and (3) using a combination of H₂O 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 H₂O and EGME coverage on layer charge. The study demonstrates that Ca²⁺-smectite samples at 47% RH have H₂O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.

Key Words: CEC • Charge Density • EGME • Layer Charge • Smectite • Specific Surface Area • Water Sorption