LAUSR

Abstract Soil Organic Matter (SOM) is composed of a complex and heterogeneous mixture of organic compounds. It has been imperative to understand the structure, water accessibility, conformation, interfaces and reactivity of SOM with its surrounding environment. Due to its complexity, the understanding of SOM can be improved via the use of standardized humic substances samples that permit scientist a systematic insight about the elemental composition and 13 C NMR carbon distribution, among others. This data was used as input parameters for the Vienna Soil Organic Matter Modeler 2 (VSOMM2; https://somm.boku.ac.at/ ), a program to produce representative, atomistic condensed-phase models of humic substances, that are chemically more diverse and more compact than previous models. Specifically in this work, we used the modeler to understand different properties of the standard Leonardite Humic Acid (LHA) at a nanoscopic level. These models aim to represent a complex set of molecules at the nanoscale. Together with simulation of the molecular dynamics, we tested potential energy, water dynamics and free energies of our system with different water content ranging in terms of heavy atoms fraction from 0.1 to 0.45. The water dynamic analyses show that an important fraction of water molecules are confined between organic compounds. In addition, preferential interactions analyses indicate the availability of humic substances to interact favorably with water. Moreover, absorption studies of water and methane molecules show that their values are affected by the presence of vacuum inside the system and the interconnection between water cavities. This work expands our understanding of how nanoscopic interactions affect the macroscopic properties of SOM.