Effects of increasing water activity on the relationship between water vapor sorption and clay content

The dependence of the relationship between hygroscopic water content (theta(h)) and clay content on water activity is not well understood. Here, we determined the correlation coefficients between theta(h) (or the slope of soil water vapor adsorption isotherms, S-vai) and clay content at nine water activity levels (i.e., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9) and investigated the evolution of correlation coefficients with increasing water activity for 65 soils with mixed and illitic clay mineralogy. The correlation coefficient between theta(h0.1) (water content at a water activity of 0.1) and clay content was the lowest (.82) due to the differences in soil-specific hydration processes. At the water activity range from 0.1 to 0.6, the S-vai showed stronger correlations (r = .91-.93) with clay content than theta(h0.1) because the multilayer sorption controlled by van der Waals force is related to clay content. When water activity increased from 0.1 to 0.6, the observed strong correlation between S-vai and clay content resulted in an increase of correlation coefficient between theta(h) and clay content from .82 to .96. Compared with the correlation (r = .96) between theta(h) and clay content at the beginning of condensation process, a weaker correlation (r = .87-.92) between S-vai and clay content was observed during condensation process, which led to a slight strengthening of the correlation between theta(h) and clay content at high water activities (>0.6). This was explained by the different clay particle properties and water activity thresholds for initiation of condensation.