Probing the Internal Structure of Layered Natural Organic Compounds On Mineral Surfaces by Neutron Reflectivity.

Organic carbon (OC) stabilization in soils plays a significant role in the global C cycle, therefore the understanding of the structure and function of the OC-soil mineral interface is of high importance.  To study the interface, films of simple OC compounds and natural organic matter (NOM) were deposited onto a soil mineral analogue (Al₂O₃) using spin coating and were exposed to humidity.  The thickness, density and structure of the films were studied using a depth-sensitive technique of neutron reflectivity. A single homogenous layer was observed when NOM and glucose (GL) were adsorbed onto Al₂O₃. However, when stearic acid (SA) was added to either NOM or GL, separate layers attributed to SA and either NOM or GL were detected. The formation of distinct, immiscible layers is due to insolubility of SA with NOM and GL in a humid environment. In contrast, humidity is readily solvated into the water-soluble compounds GL and NOM.  Our results suggest that the OC-mineral interface may form complex layering and that the sequence of the layering depends on the compounds and on the humidity of the environment.