Surface Area And Layer Charge Of Smectite From CEC And EGME/H2O-Retention Measurements

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

JAN S´ RODON´ 1 AND DOUGLAS K. MCCARTY2
1 Institute of Geological Sciences PAN, Senacka 1, 31002 Krakow, Poland 2 Chevron ETC, 3901 Briarpark, Houston, TX 77042, USA

Abstract—The total specific surface area (TSSA) and smectitic layer charge (Qs) 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 Qs 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 to110ºC, because the
sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption
measurements. A temperature of 200ºC was found to be the optimum reference for such measurements.
A good agreement between Qs 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 Ca 2+-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.