Adsorption of Water by Homoionic Exchange Forms of Wyoming Montmorillonite (SWy-1)

Peter L. Hall and Douglas M. Astill1
Schlumberger Cambridge Research, P.O. Box 153, Cambridge CB3 0HG, United Kingdom
1 Present address: Astromed Ltd., Cambridge Science Park, Milton Rd., Cambridge CB4 4GS, United Kingdom.

Abstract: Adsorption isotherms for water on homoionic (Ca2+, Li+, Na+, and K+) exchanged forms of Crook County, Wyoming, montmorillonite (CMS Source Clay SWy-1) were measured between 25° and 70°C using a vacuum microbalance having automated control of water vapor pressure. From these adsorption data, integral enthalpies and entropies of adsorption were calculated. Both quantities were negative, but decreased in magnitude with increasing amounts of adsorbed water. For all four cationic forms of the clay, the amount of initial water adsorption at 25°C and at low relative humidities was sensitive to the sample temperature during prior evacuation of water, less water being adsorbed by samples evacuated at 100°C compared with samples evacuated at 25°. For Ca- and Na-montmorillonite, these changes were reversible after several subsequent desorption/adsorption cycles, but recovery was not observed for the Li-SWy-1 clay, probably because of migration of Li+ into the aluminosilicate structure.

Key Words: Adsorption • Enthalpy • Entropy • Montmorillonite • Relative humidity • Water

Clays and Clay Minerals; August 1989 v. 37; no. 4; p. 355-363; DOI: 10.1346/CCMN.1989.0370409
© 1989, The Clay Minerals Society
Clay Minerals Society (www.clays.org)