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

The dependence of the relationship between hygroscopic water content (θ_h) and clay content on water activity is not well understood. Here, we determined the correlation coefficients between θ_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 θ_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 θ_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 θ_h and clay content from.82 to.96. Compared with the correlation (r =.96) between θ_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 θ_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.