Characterization of hydrophobic soils: a novel approach using mid-infrared photoacoustic spectroscopy

Previous research has produced conclusive evidence to show that soil hydrophobicity is affected by soil organic matter (SOM) and soil water content (WC). Soil hydrophobicity that responds to WC is a unique form of hydrophobicity called reversible hydrophobicity. The mechanistic processes by which water and SOM interact continue to be a subject of investigation. This article presents a novel application of photoacoustic Fourier transform infrared spectroscopy (FT-IR) for the investigation of reversibly hydrophobic soils. Photoacoustic FT-IR data show that, in response to hydration, surface molecules on wetted soil particles interacted differently with mid-infrared radiation than surface molecules present on air-dried soil. This can be interpreted as an effect of the reorienting of amphiphilic molecules in response to hydration-driven entropic processes. These results suggest that the photoacoustic FT-IR method can be used to elucidate how SOM and water interact at the molecular scale to drive soil hydrophobicity.