TY - JOUR

T1 - The effect of temperature and wetting–drying cycles on soil wettability

T2 - Dynamic molecular restructuring processes at the solid–water–air interface

AU - Bachmann, Joerg

AU - Söffker, Steffen

AU - Sepehrnia, Nasrollah

AU - Goebel, Marc-O.

AU - Woche, Susanne K.

N1 - Funding Information: Financial support provided by the German Research Foundation – DFG (BA 1359/17‐1 and CA 921/4‐1 for this project is greatly appreciated. We thank the students Lukas Blumenreuter, Ciarán Fitzgerald, Kevin Kaps and Kifah Olba for their excellent support in the laboratory. We further thank two anonymous reviewers for their helpful comments. Open Access funding enabled and organized by Projekt DEAL.

PY - 2021/9/1

Y1 - 2021/9/1

N2 - The impact of heat treatment and wetting–drying cycles on the wetting properties of sandy forest soils was explored. Topsoil and upper subsoil were sampled at three beech forest sites in northern Germany. The air-dried soils were treated at 20, 40 and 80°C for 24 h, with materials treated at 20°C serving as reference. After wetting, materials were air-dried or shock-frozen in liquid N2 and freeze-dried. Interfacial properties were monitored by sessile drop contact angles (CAs) and X-ray photoelectron spectroscopy (XPS), which provide physical and chemical information on the outermost particle interface layer. CAs of reference samples were around 90° and significantly increased after 80°C-treatment to >90°, whereas 40°C-treatment had in comparison to reference soils no distinct impact on CA. Depending on the initial temperature treatment, air-drying after wetting decreased CA to 60–80% and shock-freezing and freeze-drying decreased CA to 10–50% of the reference value. Results suggest that shock-freezing may preserve the organic matter molecular structure that prevails during contact with water at the solid–liquid interface, thus indicating the wettability of the wet surface. Generally, wetting–drying cycles had the least impact on 80°C-treated material. XPS analysis confirmed dynamic interfacial molecular restructuring processes by changes in O and C content and the content of non-polar C compounds. A second heat treatment after two wetting–drying cycles again proved the distinct and pronounced impact of 80°C-treatment on CA, especially with prior shock-freezing and freeze-drying. In conclusion, the findings of our study indicate a sensitive and partly reversible reorganization of the solid interfacial wetting properties. Results may conceptually be used to develop dynamic wettability models, which are needed to simulate the sensitive interplay between wettability and dynamic soil hydraulic functions at sites that are exposed to intensive and periodic moisture fluctuations. Highlights: T > 40°C increased contact angle; after wetting contact angle depended on how the water was removed Shock-freezing and freeze-drying preserves contact angle of the wetted state Contact angle changes confirm interface molecular restructuring due to heat treatment or wetting Contact angle change goes along with changes in element content within XPS analysis depth.

AB - The impact of heat treatment and wetting–drying cycles on the wetting properties of sandy forest soils was explored. Topsoil and upper subsoil were sampled at three beech forest sites in northern Germany. The air-dried soils were treated at 20, 40 and 80°C for 24 h, with materials treated at 20°C serving as reference. After wetting, materials were air-dried or shock-frozen in liquid N2 and freeze-dried. Interfacial properties were monitored by sessile drop contact angles (CAs) and X-ray photoelectron spectroscopy (XPS), which provide physical and chemical information on the outermost particle interface layer. CAs of reference samples were around 90° and significantly increased after 80°C-treatment to >90°, whereas 40°C-treatment had in comparison to reference soils no distinct impact on CA. Depending on the initial temperature treatment, air-drying after wetting decreased CA to 60–80% and shock-freezing and freeze-drying decreased CA to 10–50% of the reference value. Results suggest that shock-freezing may preserve the organic matter molecular structure that prevails during contact with water at the solid–liquid interface, thus indicating the wettability of the wet surface. Generally, wetting–drying cycles had the least impact on 80°C-treated material. XPS analysis confirmed dynamic interfacial molecular restructuring processes by changes in O and C content and the content of non-polar C compounds. A second heat treatment after two wetting–drying cycles again proved the distinct and pronounced impact of 80°C-treatment on CA, especially with prior shock-freezing and freeze-drying. In conclusion, the findings of our study indicate a sensitive and partly reversible reorganization of the solid interfacial wetting properties. Results may conceptually be used to develop dynamic wettability models, which are needed to simulate the sensitive interplay between wettability and dynamic soil hydraulic functions at sites that are exposed to intensive and periodic moisture fluctuations. Highlights: T > 40°C increased contact angle; after wetting contact angle depended on how the water was removed Shock-freezing and freeze-drying preserves contact angle of the wetted state Contact angle changes confirm interface molecular restructuring due to heat treatment or wetting Contact angle change goes along with changes in element content within XPS analysis depth.

KW - Contact angle

KW - Heat treatment

KW - Interfacial chemical composition

KW - Sandy forest soil

KW - Wetting–drying cycles

KW - X-ray photoelectron spectroscopy (XPS)

UR - http://www.scopus.com/inward/record.url?scp=85103026372&partnerID=8YFLogxK

U2 - 10.1111/ejss.13102

DO - 10.1111/ejss.13102

M3 - Article

AN - SCOPUS:85103026372

VL - 72

SP - 2180

EP - 2198

JO - European Journal of Soil Science

JF - European Journal of Soil Science

SN - 1351-0754

IS - 5

ER -