Hierarchical self-assembling properties of natural organic matter's components

Natural organic matter (NOM) has been described as a hierarchical self-assembling system consisting of three components; non-amphiphilic, amphiphilic and lipid fractions. Serving as the first hierarchical level, L-0 is formed by self-assembly of amphiphilic components (HA(2)) with lipid components (L-1) while the second hierarchical level is the result of the association of L-0 with non-amphiphilic components (HA(1)). As an initial step toward determining the mechanisms that govern NOM self-assembly processes, a comparative study of self-assembly of humic acid and humin was designed to identify the primary factors affecting this process. The two hierarchical levels for humic acid and humin were treated as binary mixtures and were characterized by the sign and magnitude of their excess heat capacity (c(p)(E)). The data for the first hierarchical level showed that the c(p)(E) values were positive for humic acid and negative for humin, and the distribution of components HA(2) and L-1 was different too. The data for the second hierarchical level showed similar c(p)(E) values for both humic acid and humin as well as similar distributions of HA(1) and L-0. The similarities and differences in the self-assembly of humic acid and humin are discussed in terms of the concentration and the nature of components and a hypothetical mechanism is proposed. The findings of this study performed on operationally defined NOM fractions isolated using two different extractants from two geomaterials with different chemical characteristics and geochemical histories, suggests that the hierarchical associative mechanism is one that is characteristic of NOM. (c) 2013 Elsevier Ltd. All rights reserved.