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As soils become dry, water primarily exists as vapor that adsorbs to and desorbs from particle surfaces. The drying process also often enhances soil water repellency, yet the effects of repellency on soil water vapor sorption and exchange are not well understood. The objective of this study was to quantify the water vapor sorption dynamics of two minerals (Ca2+ saturated kaolinite and montmorillonite), in which water repellency was induced using two organic agents: cetyl trimethylammonium bromide (similar to 4% by mass) and stearic acid (0.5%, 5%, 10%, 20%, and 35% by mass). Samples were then analyzed for water vapor sorption isotherms, solid-water contact angles, organic carbon content, mineral surface morphology, hydrophobic functional groups (using Fourier transform infrared spectroscopy), zeta potential, and particle size distributions. The results showed that the water repellent treatments altered particle surface potentials and decreased surface areas relative to the controls. As a result, those samples adsorbed significantly less water and had less hysteresis between adsorption and desorption isotherms than the nontreated controls (p < 0.05). Differences in water adsorption were most pronounced for water activities > 0.6, where hydrophobic compounds may have inhibited water vapor condensation, even at low concentrations. In contrast, solid-water contact angles were small in montmorillonite treatments with < 10% stearic acid, suggesting that low levels of hydrophobic compounds may have greater effect on vapor sorption compared to liquid water imbibition. Altogether, these results imply that repellency may reduce water retention in dry soils and enhance water vapor losses.

Plain Language Summary Many soils become water repellent as they dry, such that water drops cannot penetrate the soil surface, yet specific interactions between water vapor and water-repellent soil particles remain poorly understood. In this study we applied two different chemicals to clay minerals to induce varying degrees of water repellency and then measured the ability of treated particles to adsorb and rerelease water vapor. The water repellent minerals adsorbed less water vapor than nontreated minerals, because the applied chemicals (1) reduced the ability of water vapor to access wettable sites on mineral surfaces, (2) caused particles to aggregate together, further limiting the surface area available to water vapor, and (3) altered the ability of minerals to attract water vapor onto their exposed surfaces. These findings suggest that soil water repellency may enhance water vapor losses from drying soils, yet more study is needed to verify that these results translate to natural field soils.