title: Spontaneous aggregation of humic acid observed with AFM at different pH.
abstract: Atomic force microscopy in contact (AFM-C) mode was used to investigate the molecular dynamics of leonardite humic acid (HA) aggregate formed at different pH values. HA nanoparticles dispersed at pH values ranging from 2 to 12 were observed on a mica surface under dry conditions. The most clearly resolved and well-resulted AFM images of single particle were obtained at pH 5, where HA appeared as supramolecular particles with a conic shape and a hole in the centre. Those observations suggested that HA formed under these conditions exhibited a pseudo-amphiphilic nature, with secluded hydrophobic domains and polar subunits in direct contact with hydrophilic mica surface. Based on molecular simulation methods, a lignin-carbohydrate complex (LCC) model was proposed to explain the HA ring-like morphology. The LCC model optimized the parameters of β-O-4 linkages between 14 units of 1-4 phenyl propanoid, and resulted in an optimized structure comprising 45-50 linear helical molecules looped spirally around a central cavity. Those results added new insights on the adsorption mechanism of HA on polar surfaces as a function of pH, which was relevant from the point of view of natural aggregation in soil environment.
authors: Dipartimento Agricoltura Ambiente Alimenti (DAAA), Università del Molise, Via De Sanctis 86100, Campobasso (CB), Italy. Electronic address: colombo@unimol.it.
authors: Claudio
authors: C
authors: Colombo
authors: Dipartimento Agricoltura Ambiente Alimenti (DAAA), Università del Molise, Via De Sanctis 86100, Campobasso (CB), Italy.
authors: Giuseppe
authors: G
authors: Palumbo
authors: Dipartimento Agricoltura Ambiente Alimenti (DAAA), Università del Molise, Via De Sanctis 86100, Campobasso (CB), Italy.
authors: Ruggero
authors: R
authors: Angelico
authors: Department of Geology and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea.
authors: Hyen Goo
authors: HG
authors: Cho
authors: Dipartimento di Scienze Agrarie (DipSA), Università di Bologna, V.le Fanin 40, 40127 Bologna, Italy.
authors: Ornella
authors: O
authors: Francioso
authors: Dipartimento di Agronomia Animali Alimenti Risorse Naturali e Ambiente - DAFNAE, Viale dell'Università 16, Agripolis, 5020 Legnaro, Italy.
authors: Andrea
authors: A
authors: Ertani
authors: Dipartimento di Agronomia Animali Alimenti Risorse Naturali e Ambiente - DAFNAE, Viale dell'Università 16, Agripolis, 5020 Legnaro, Italy.
authors: Serenella
authors: S
authors: Nardi
mesh_terms: D000327:Adsorption; D000538:Aluminum Silicates / Q000737:chemistry; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D018625:Microscopy, Atomic Force; D008903:Minerals / Q000032:analysis* / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D053758:Nanoparticles / Q000737:chemistry*; D012987:Soil / Q000737:chemistry
chemical_list: D000538:Aluminum Silicates; D006812:Humic Substances; D008903:Minerals; D012987:Soil; C568542:leonardite; C011934:mica
keywords: Atomic force microscopy (AFM); Humic acid; Leonardite; Lignin–carbohydrate complex (LCC) model; Molecular conformation; Particle size