Effects of Hydrophobicity on Soil Water Vapor Sorption.

Soil water in arid and/or drought-stricken regions primarily exists as vapor phase, and is retained in soil through adsorption to particle surfaces. Despite predictions of greater drought frequencies and durations due to climate change, water vapor characteristics in soil have received relative little attention. Moreover, soil hydrophobicity can be a major problem in arid regions because it can prevent infiltration and recharge during periods of precipitation. Therefore, it is important to understand how hydrophobicity affects soil water vapor sorption and exchange. The objective of this study was to identify the effects of soil hydrophobicity on water vapor sorption (WVS) using two pure minerals (kaolinite and montmorillonite). We mixed each mineral with and without hydrophobic organic components (6% CTAC) to become hydrophobic (contact angle = 131.65°) and hydrophilic (contact angle = 10.02°), then measured their WVS isotherms and photographed the samples before and after adsorption-desorption cycle to observe the changes in surface morphology. The results showed that hydrophobic samples absorbed less water than hydrophilic samples, especially in the high range of water activity (a_w > 0.6) where the hydrophobic surfaces may have prevented water vapor condensation. This effect was observed in both minerals but was most obvious in montmorillonite samples. Vapor adsorption-desorption cycle created cracks in the montmorillonite samples but not the kaolinite ones. Our analysis showed that the cracks in the hydrophobic montmorillonite have smaller length/widths and areas than in the hydrophilic samples. These results raise the possibility that swelling soils may potentially undergo morphological development in response to cyclical vapor exchange, and hydrophobic surfaces may inhibit both the adsorption of capillary water and the formation of shrinkage cracks.