Macromolecular Characteristics of Natural Organic Matter. 1. Insights from Glass Transition and Enthalpic Relaxation Behavior

This is the first of a series of papers focusing on an experimental investigation of mechanisms contributing to the sequestration of hydrophobic organic compounds (HOCs) by macromolecular natural organic matter (NOM). It presents the results of a detailed study by differential scanning calorimetry (DSC) of NOM-related glass transition and enthalpic relaxation phenomena. Parallel measurements for model synthetic organic macromolecules of well characterized structure provide a basis for interpretation of the observed behaviors of the structurally heterogeneous and relatively ill-characterized natural organic materials investigated. The effects of varied DSC heating rates reveal transition temperature and enthalpic relaxation responses consistent with the Gibbs-Dimarzio glass transition theory, thus providing definitive evidence to support our earlier reports of the occurrence of glassy/rubbery state transitions in NOM macromolecules. Quantification of maximum changes in heat capacity for enthalpic relaxation phenomena provides insights into the types of physical and chemical bonds that limit glassy-state macromolecular mobility. Marked reductions by sorbed water of the effective temperature at which glass transition phenomena occur for hydrophilic NOMs are observed, suggesting that the thermodynamic states of NOM macromolecules are influenced in natural systems by the presence of large concentrations of sorbing molecules. The thermodynamic states of the more hydrophobic components or regions of NOM macromolecules, for example, are likely influenced by large amounts of sorbed HOCs in a manner similar to that affected by sorbed water for hydrophilic macromolecules. Finally, ramifications of the findings with respect to alternative remediation end points are discussed.