Potential of AFM–nanothermal analysis to study the microscale thermal characteristics in soils and natural organic matter (NOM)
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This exploratory study evaluates the potential of nanothermal analysis (nTA) coupled with atomic force microscopy (AFM) of soil samples for understanding physicochemical processes in soil and for linking the nanospatial and microspatial distribution of thermal characteristics with the macroscopic properties of soil samples.
Materials and methods
Soil and reference samples were investigated by differential scanning calorimetry and AFM-nTA. nTA was conducted on 16 points of each AFM image in two subsequent heating cycles (55–120°C and 55–300°C, respectively). Thermograms were subdivided into characteristic types and their spatial distribution was compared between sample replicates and materials.
Results and discussion
Thermogram types consisted of partly structured expansion and compression phases, suggesting material-specific thermal profiles. The distribution of thermogram types reflected sample-dependent nanoscale and microscale heterogeneity. Indications for water molecule bridge transitions were found by nTA in peat and soil. Organic materials generally revealed strong expansion and irreversible compression phases, latter probably due to the collapse of pore and aggregate structures. In contrast to charcoal and manure, peat shows strong expansion below 120°C and compression only above 120°C.
All investigated samples are heterogeneous on the nanoscale and microscale with respect to thermal behaviour. AFM-nTA allows distinguishing numerous different materials on nanometre and micrometre scales in soil samples. The material-dependent characteristics will help in understanding and learning more about the nanoscale distribution of different materials and properties. Related to the macroscopic thermal behaviour, this will allow studying links between the properties of biogeochemical interfaces and the processes governed by them.
KeywordsAtomic force microscopy Glass transition Nanothermal analysis Soil organic matter Thermal analysis Thermomechanical analysis
Atomic force microscopy
Atomic force microscopy coupled with nanothermal analysis
Apparent expansion coefficient
Differential scanning calorimetry
Energy-dispersive X-ray spectroscope (respective spectroscopy)
Environmental scanning electron microscope (respective microscopy)
Apparent compression coefficient
Localized thermomechanical analysis
Nuclear magnetic resonance
Region of interest
Soil organic matter
Glass transition temperature
WaMB transition temperature
Water molecule bridge
This study has been supported by the Deutsche Forschungsgemeinschaft, Project SCHA849/8 within the priority programme SPP 1315 Biogeochemical Interfaces in Soil. We also thank Dr. Jiri Kucerik for assistance in obtaining the ESEM pictures, Dr. Jette Schwarz for the DSC measurements and Ms. Priya Mary Abraham for assistance at AFM measurements and all of them for helpful discussions.
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