BOOK OF ABSTRACTS 19 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Monday, 24 August 2026 / Hall B+C Analysis and Characterization SL3 From Dilution Simulations to 13C NMRDerived Endmembers – and Beyond: Insights into Humic Substances Mikhail Borisover1, Edgar Galicia-Andrés2,3, Marcos Lado4, Drazen Petrov2, Chris Oostenbrink2 1 Volcani Institute, Agricultural Research Organization, Institute of Soil, Water and Environmental Sciences, 68 HaMakabim Str. POB 15159 Rishon LeZion, 7505101, Israel, vwmichel@volcani.agri.gov.il 2 Institute of Molecular Modeling and Simulation, University of Natural Resources and Life Sciences (BOKU), Muthgasse, 18, 1190, Vienna, Austria 3 Institute of Soil Research, University of Natural Resources and Life Sciences (BOKU), Gregor-Mendel-Strasse 33, 1180, Vienna, Austria 4 Interdisciplinary Center for Chemistry and Biology (CICA), Faculty of Sciences, University of A Coruña, As Carballeiras, s/n, Campus Elviña, 15071 A Coruña, Spain Molecular modeling offers a way to probe the conformational and aggregation-related organization of humic substances (HS), enabling controlled “virtual experiments” that complement laboratory observations. One test to examine the molecular organization of HS is to explore the response of HS systems to dilution, a process directly linked to aggregate stability in aqueous environments. In this work, molecular dynamics simulations based on models generated with the Vienna Soil Organic Matter Modeller were used to examine the stability and evolution of Leonardite humic acid aggregates across a broad range of water contents, spanning conditions from water-rich HS phases to dilute HS solutions, by varying molecular sizes, ionization states, and counter-ion types (Na⁺, Ca²⁺) [1]. The simulations tracked the gradual decomposition of HS aggregates upon dilution, quantified through changes in intermolecular contacts, hydrogen bonding, cluster size distributions, and interaction energies. The results suggest that dilution induces a stepwise disaggregation process lacking a cooperative character, in which large assemblies progressively dissociate, highlighting the sensitivity of HS organization to the ionic environment. These findings provide a mechanistic framework for interpreting experimental observations of HS size distributions and dialysis behavior. These simulations also raise a fundamental question: what structural entities should be used as molecular inputs in such models? Earlier observations proposed that multiple HS can be represented as a mixture of a very limited number of independent components [2]. To address this on a quantitative basis and obtain uniquely
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