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Modification of Soil Wetting Properties Always Involves the Interface Chemical Composition

Soil wetting properties, usually quantified by contact angle (CA), depend on the chemical composition of the soil particle interface layer that differs from that of the underlying mineral. X-ray photoelectron spectroscopy (XPS) with only shallow analysis depth identified the CA to respond on the surface O/C ratio. Both parameters were found to be negatively exponentially correlated and increasing CA along a soil chronosequence could be explained by a decreasing O/C ratio [1]. Here, CA was artificially modified. Sandy soils were treated at 80°C, subsequently wetted and the structure of the wetted state preserved by shock-freezing and freeze-drying. 80°C-treatment increased CA to >110°, while for the wetted state CA distincly decreased to <80°. Both modifications may be explained by conformational changes within the structure of organic compounds and specifically the orientation of non-polar functional groups with respect to pore space. XPS spectra revealed a decreased O/C ratio at 80°C-treatment and an increased O/C ratio at water treatment. C speciation indicated CA to be positively correlated with non-polar (C-C, C-H)-species [2]. Incubation of soil microaggregates (SMA) derived from silty soil induced subcritical water repellency in the originally wettable material that actually resulted in a slightly decreased O/C ratio. Further, fitted amounts of non-polar (C-C, C-H)-species supplemented the positive correlation found for the sandy soils (CA range 40-130°) in the small CA domain (CA range 9-25°). The results presented here not only confirm the negative correlation between CA and surface O/C ratio [1],