Thermomechanical analysis of air-dried whole soil samples

Abstract

Macromolecular characteristics of organic matter in soil can greatly influence the sorption behavior of organic compounds. Previous investigations of the macromolecular nature of whole soil samples by differential scanning calorimetry (DSC) revealed step transitions in air-dried soil samples, which do not fully mimic typical glass transition behavior. This work further explores these step transitions through a second, independent method: thermomechanical analysis (TMA). TMA was validated with respect to detection, localization, and quantification of the step transitions in whole soil samples. TMA was found to sensitively detect the transitions, indicating sample softening near the step transition ( T g * ) , while localization of T g * by TMA appears to be accurate in pellets only. Quantification via difference between coefficients of initial expansion and final compression, respectively, is favorable for weak transitions, where the sensitivity of DSC is too low to obtain reliable results. Although the nonreversing nature of the step transition is atypical for glass transitions, the observed matrix softening suggests changes in matrix rigidity near T g * . This softening is hypothesized to be associated with water bridging individual soil structural units. The slow, nonreversing changes in soil rigidity are expected to affect the sorption behavior of organic contaminants, and the increase in compressibility further suggests relevance for soil mechanical properties.