The Environmental Chemical Sciences (ECS) program of the Division of Chemistry will support the research project of Prof. James Rice of South Dakota State University. Prof. Rice and his students will investigate the role of nanoparticles in the formation of self assembled Natural Organic Matter (NOM). They will apply scanning electron microscopy, atomic force microscopy, differential scanning calorimetry and isotopically-labeled lipids and amphiphiles as probes in combination with solid-state NMR spectroscopy, small-angle neutron scattering, and neutron spin-echo spectrometry to provide molecular level understanding of the self assembly process leading to achieve this goal.
The project has the potential to increase our molecular level understanding of mechanisms that contribute to the persistence of organic carbon in soils and thus the global carbon cycle. As a practical outcome, the ability to control NOM self-assembly could lead to the development of sequestration mechanisms and strategies to increase its residence time in the soil. The project will provide excellent training opportunities to students and postdoctoral fellows in a highly interdisciplinary research area of great environmental importance.
Natural organic matter (NOM) is one of the predominant forms of organic carbon the earth’s surface. Our recent work has shown that NOM is a self-assembled material formed via a multi-step process that involves the interaction of amphiphilic and lipid components: humic acid, HA0 is the result of the interaction of an aromatic humic acid-like but nonamphiphilic fraction (HA1) and ta lipid fraction (L0) and the L0 fraction is first formed by the interaction of another lipid fraction (L1), and an amphiphilic material (HA2). The structural organization of these components is the result of a complex, multi-step process that is difficult to study because of a lack of specific knowledge about and experimental control over the molecular interactions that occur. The ability to "disassemble" and "reassemble" NOM could provide an opportunity for experimental control over the molecular interactions that occur and creates the possibility of applying a "soft materials" chemistry approach to understand not only the processes that create NOM, but the chemical properties of the material that results by incorporating probes of the self-assembly process into the process itself, allowing direct observation of the mechanism and the properties of the resulting material. The overall goal of this project was to identify the conditions under which NOM assembles into supramolecular structures and explore the nature chemical characteristics of the resulting material. As a result of this project it is now possible to reassemble both HA0 and L0 under laboratory conditions. This is accomplished via the formation of an aqueous emulsion under acidic conditions and HA1/L0 or HA2/L1 ratio of 75:25 (w:w). The resulting materials have chemical characteristics (heat capacity, 13C NMR spectra) similar to the authentic samples. Pulsed-filed gradient NMR studies of the interactions occurring during self-assembly indicate that important mechanisms in each level of the assembly mechanism involve interactions between aliphatic and aromatic components with π-π, Van der Waal and electrostatic interactions contributing. The diffusion coefficients of the assembled materials were smaller than those of their components indicating that the assembled particles are more "compact" than the molecules that they form from. Studies with scanning electron microscopy, atomic force microscopy and electromotive force microscopy indicate that different polarity (hydrophobic and hydrophilic) domains exist in the assembled materials. 2H NMR studies of isotopically labeled probe molecules indicate that different mobility domains exist in the "reassembled" materials similar to the different domains that exist in the authentic materials.
