The present study compares the water-vapor adsorption capacity of bentonites (natural cation population) with the Enslin-Neff method. Water-vapor adsorption at 50% r.h. (relative humidity) or 70% r.h. is known to depend heavily on the amount of permanent charge and on the type of exchangeable cation. At ~80% r.h. Na+- and Ca2+/Mg2+-dominated bentonites take up equal amounts of water. Comparing the water-uptake capacity at 80% r.h. with the cation exchange capacity (CEC) revealed a close correlation between these two variables. Appreciable scatter apparent from this plot, however, suggests that additional factors influence the water-uptake capacity. Water adsorption at external surfaces was considered to be one of these factors and was, in fact, implicated by N2-adsorption data. The ratio of external/internal water ranged from 0 to 1, which suggests that water-adsorption values cannot be applied in the calculation of the internal surface area without correction for external water.

The Enslin-Neff water-uptake capacity, on the other hand, is unaffected by microstructural features (e.g. specific surface area and porosity). The amount of exchangeable Na+ is the most important factor. However, the relationship between the Na+ content and the Enslin value is not linear but may be explained by percolation theory.

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