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Abstract
The ability of metal surfaces to dissociate hydrogen molecules is key to many ways that metals react to their environment. Often, the barrier to dissociation is linked to the formation of surface hydrogen adatoms. Here, we show that hydrogen can be more strongly bound to water-covered surfaces in the form of hydronium ions than as adatoms. Density functional theory reveals that the hydronium binding is proportional to the surface electronic work function. For the case of Pt(111), a particularly high work function surface, the proton affinity of adsorbed water films can be 0.4 eV larger than that of the bare metal surface. This binding is large enough to make the water films susceptible to the formation of hydroxyl and hydronium ion pairs. We present evidence from scanning tunneling microscopy for the existence of hydronium ions in water films on Pt(111). This new insight into the stability of hydronium-containing water layers provides a basis for more realistic models of the chemical reactivity of water films on metals.

7 pages, 4 figures.-- PACS nrs.: 73.20.-r. ; Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of ~1.8–2.0 eV with respect to the vacuum for the unoccupied resonance of the ns valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the ns electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces. ; This work was supported by the U.S. Department of Energy Grant No. DE-FG02-03ER15434. Some calculations were performed in the Environmental Molecular Sciences Laboratory, a user facility sponsored by the DOE Office of Biological and Environmental Research. N.P. thanks the Alexander von Humboldt Foundation for support. H.P. thanks Ikerbasque for the support of his stay at DIPC. The San Sebastián group acknowledges support by the Spanish MEC grant, the European FP6-NoE grant "Nanoquanta", the UPV/EHU and the projects "NANOMATERIALES" and "NANOTRON" of the Basque Government and the Diputación de Guipúzcoa. ; Peer reviewed

An optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2–10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO2 coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO2/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO2 based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10−16 cm2 s−1 at a temperature of 100 °C. Temperature‐dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of Al2O3, which is known as a poor oxygen ion conductor within the analyzed temperature regime. ; We thank Herbert Over from the Institute of Physical Chemistry at the JLU Giessen for sharing his knowledge and experience and helping improve the manuscript. Financial support was provided by the DFG via the GrK (Research training group) 2204 "Substitute Materials for sustainable Energy Technologies." M.C. thanks Ramón y Cajal program RYC‐2013‐12448. The authors also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and MAT2017‐83169‐R AEI/FEDER,UE. This article is based upon work from COST Action MP1402 "Hooking together European research in atomic layer deposition (HERALD)," supported by COST (European Cooperation in Science and Technology). S.M.S. acknowledges funding from "Programa Internacional de Becas 'la Caixa"'‐Severo Ochoa." ICN2 members acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017‐85087‐C3. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, grant no. SEV‐2013‐0295) and is funded by the CERCA Programme /Generalitat de Catalunya. Part of this work was performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. This received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 654360 NFFA‐Europe. ; Peer reviewed

It is widely known that surfaces play an important role in numerous biological processes and technological applications. Thus, being able to modify surface properties provides an opportunity to control many phenomena occurring at interfaces. One way of controlling surface properties is to adsorb a polymer film onto the surface, for example through layer-by-layer (LbL) deposition of polyelectrolytes. This simple but versatile technique enables various polymers, proteins, colloidal particles etc. to be incorporated into the film, resulting in a multifunctional coating. Due to recent legislations and a consumer demand for more environmentally friendly products, we have chosen to use natural polymers (biopolymers) from renewable resources. The focus of this thesis has been on the adsorption of biopolymers and their layer-by-layer formation at solid-liquid interfaces; these processes have been studied by a wide range of techniques. The main method was the quartz crystal microbalance with dissipation monitoring (QCM-D), which measures the adsorbed mass, including trapped solvent and the viscoelastic properties of an adsorbed film. This technique was often complemented with an optical method, such as ellipsometry or dual polarization interferometry (DPI), which provided information about the "dry" polymer or protein adsorbed mass. From this combination, the solvent content and density of the layers was evaluated. In addition, the surface force apparatus (SFA), X-ray photoelectron spectroscopy (XPS), total internal reflection fluorescence (TIRF), and fluorescence resonance energy transfer (FRET) were utilized, providing further information about the film structure, chemical composition, and polymer inter-layer diffusion. Adsorption studies of the glycoprotein mucin, which has a key role in the mucousal function, showed that despite the net negative charge of mucin, it adsorbed on negatively charged substrates. The adsorbed layer was highly hydrated and the segment density on the substrate was low. We showed the ...

Tuning the thermodynamic state of a material has a tremendous impact on its performance. In the case of polymers placed in proximity of a solid wall, this is possible by annealing above the glass transition temperature, Tg, which induces the formation of an adsorbed layer. Whether heating to higher temperatures would result in desorption, thereby reverting the thermodynamic state of the interface, has so far remained elusive, due to the interference of degradation. Here, we employ fast scanning calorimetry, allowing to investigate the thermodynamics of the interface while heating at 104 K s−1. We show that applying such rate to adsorbed polymer layers permits avoiding degradation and, therefore, we provide clear-cut evidence of desorption of a polymer melt. We found that the enthalpy and temperature of desorption are independent of the annealing temperature, which, in analogy to crystallization/melting, indicates that adsorption/desorption is a first order thermodynamic transition. ; D.C. acknowledges financial support from the project PGC2018-094548-B-I00 (MICINN-Spain and FEDER–UE) and the project IT-1175-19 (Basque Government). S.N. acknowledges financial supports from the Action Concertée Recherche-ULB under project SADI and the Fonds de la Recherche Scientifique FNRS under Grant EXOTICAGE. ; Peer reviewed

The intermolecular interaction among phthalocyanine molecules deposited on Ag(111) has been investigated at submonolayer coverages. By means of infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction, unambiguous evidence for an attractive interaction between neighboring titanyl-phthalocyanine (TiOPc) molecules is found for dilute layers, which is in contrast to the intermolecular repulsion reported for the copper-phthalocyanine (CuPc)/Ag(111) system. Accordingly, the formation of dense two-dimensional islands of TiOPc molecules with a well-defined long-range order (commensurate phase) is observed upon cooling to temperatures below 75 K, while a disordered arrangement is retained for adsorbed CuPc. Using density functional theory (DFT) calculations as a guide, these differences have been traced to the competition of attractive van der Waals and repulsive electrostatic interactions. Specifically, the vertically oriented molecular dipoles are substantially smaller for adsorbed TiOPc as compared to CuPc due to a compensation of the oppositely oriented dipole moments arising from the Pauli push-back effect on the one hand and the internal molecular dipole associated with the axial Ti–O group on the other. A detailed account and comparison of the molecule–substrate interaction, molecular binding geometries, and intermolecular interactions of TiOPc and CuPc on the Ag(111) surface are provided in a theoretical analysis based on DFT. ; We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project ID 223848855-SFB 1083 "Structure and Dynamics of Internal Interfaces". L.F. acknowledges financial support from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement MagicFACE no. 797109. D.S.-P. and A.X.B.-R. acknowledge the Spanish Agencia Estatal de Investigación (grant nos. MAT2016-78293-C6-4-R and PID2019-107338RB-C66) and Dep. Educación of the Basque Government and UPV/EHU (grant no. IT1246-19) for support. ; Peer reviewed

[EN] The hydrogenation of CO2 into hydrocarbons promoted by the action of sunlight has been studied on Co nanoparticles covered by thin carbon layers. In particular, nearly 100% selectivity to hydrocarbons is obtained with increased selectivities towards C2 + hydrocarbons and alcohols (mainly ethanol) when using nanostructured materials comprising metallic cobalt nanoparticles, carbon layers, and sodium as promoter (NaCo@C). In the contrary, larger amount of CH4 and lower selectivity to C2 + hydrocarbons and alcohols were obtained in the conventional thermal catalytic process. When using Co@C nanoparticles in the absence of Na or bare Co3O4 as catalyst, methane is essentially the main product (selectivity > 96%). Control experiments in the presence of methanol as a hole scavenger suggest the role of light in generating charges by photon absorption as promoting factor. The reaction mechanism for photoassisted CO2 hydrogenation on the Co-based catalysts was investigated by near ambient-pressure X-ray photoelectron (AP-XPS) and in situ Raman spectroscopies, which provided information on the role of light and Na promoter in the modulation of product distribution for CO2 hydrogenation. Spectroscopic studies suggested that surface CO2 dissociation to CO, the stabilization of CO adsorbed on the surface of Na-Co@C catalyst and the easy desorption of reaction products is a key step for photothermal CO2 hydrogenation to ethanol and C2 + hydrocarbons. ; L.L. thanks ITQ for providing a contract. A.V.P. thanks the Spanish Government (Agencia Estatal de Investigacion) and the European Union (European Regional Development Fund) for a grant for young researchers (CTQ2015-74138-JIN, AEI/FEDER/UE). J.C. thanks the Spanish Government-MINECO for a "Severo Ochoa" grant (BES-2015-075748). The AP-XPS experiments were performed at NAPP endstation of CIRCE beamline at ALBA Synchrotron with the collaboration of ALBA staff. The authors also thank the Microscopy Service of UPV for kind help on FESEM, TEM and STEM measurements. Financial ...

The dramatic consequences that the orientation adopted by the molecular dipoles, in diverse arrays of chloroaluminum phthalocyanine (ClAlPc) on Au(111), have on the ulterior adsorption and growth of C60 are explored by means of an all scanning probe microscopy approach. The unidirectional downwards organization of the molecular dipoles at the first layer reduces charge transfer from the metal to C60. Imbalance between attractive and repulsive interactions of the fullerenes are crucial for their ordered supramolecular aggregation. The effect at the basis of such self-assembling seems to be released by the all upwards dipole orientation adopted on the ClAlPc second layer. The low electronic corrugation of the bilayer repercutes in a higher mobility of the fullerenes which for similar coverages diffuse large distances to reach uncoverd first layer regions. Density functional theory calculations corroborate the experimental observations indicating the relevance of charge transfer, potential energy surface corrugation, C60 on-surface diffusion barriers and screening. The structure of the co-adsorbed C60 and ClAlPc layers strongly depends on the deposition sequence. Phase-separation, where each molecule adopts the single-component assembly, occurs if C60 is deposited first. The present results contribute to understanding the influence of the dipolar nature of molecular layers on the electronic and structure of donor/acceptor heterojunctions, which is crucial for device design via engineering the energy levels algingment at the organic-organic and organic-metal interfaces. ; This work has been supported by the Spanish Government under the project PID2019-110907GB-I00, MAT2016-77852-C2- 1-R and MAT2017-85089-C2-1-R (AEI/FEDER, UE) and the ''Severo Ochoa'' Program for Centres of Excellence in R&D (CEX2019-000917-S). We also acknowledge the Generalitat de Catalunya grant 2017 SGR668. J.I.M. acknowledges Comunidad de Madrid via "Programa de Investigación Tecnologías 2018" (FOTOART-CM S2018/NMT-4367), and the EU Innovation Program under grant agreement 881603 (GrapheneCore3- Graphene-based disruptive technologies). ; Peer reviewed

The final publication is available via https://doi.org/10.1021/acsanm.8b01876 . ; Adsorption of molecules on high-surface-area materials is a fundamental process critical to many fields of basic and applied chemical research; for instance, it is among the simplest and most efficient principles for separating and remediating polluted water. However, established experimental approaches for investigating this fundamental process preclude in situ monitoring and thus obtaining real-time information about the ongoing processes. In this work, mid-infrared attenuated total reflection (ATR) spectroscopy is introduced as a powerful technique for quantitative in situ monitoring of adsorption processes and thus enrichment of traces of organic pollutants from aqueous solution in ordered mesoporous silica films. The synthesis, functionalization, and characterization of two silica films with 3D hexagonal and cubic pore structure on silicon ATR crystals are presented. Benzonitrile and valeronitrile as model compounds for aromatic and aliphatic water pollutants are enriched in hydrophobic films, while the matrix, water, is excluded from the volume probed by the evanescent field. Enrichment times of 200 and >100, respectively, yielding detection limits in the low ppm range. Furthermore, fast and complete desorption of the analyte, ensuring reliable regeneration of the sensor, was verified. Lastly, we derive and experimentally validate equations for ATR spectroscopy with thin film adsorption layers to quantify the absolute mass of adsorbed pollutant in the film. The excellent agreement between recorded absorptions at target wavenumbers of the target analytes and corresponding simulations corroborates the validity of the chosen approach. ; European Union's Horizon 2020

Carbide signatures are ubiquitous in the surface analyses of industrially sputter-deposited transition metal nitride thin films grown with carbon-less source materials in typical high-vacuum systems. We use high-energy-resolution photoelectron spectroscopy to reveal details of carbon temporal chemical state evolution, from carbide formed during film growth to adventitious carbon adsorbed upon contact with air. Using in-situ grown Al capping layers that protect the as-deposited transition metal nitride surfaces from oxidation, it is shown that the carbide forms during film growth rather than as a result of post deposition atmosphere exposure. The XPS signature of carbides is masked by the presence of adventitious carbon contamination, appearing as soon as samples are exposed to atmosphere, and eventually disappears after one week-long storage in lab atmosphere. The concentration of carbon assigned to carbide species varies from 0.28 at% for ZrN sample, to 0.25 and 0.11 at% for TiN and HfN, respectively. These findings are relevant for numerous applications, as unintentionally formed impurity phases may dramatically alter catalytic activity, charge transport and mechanical properties by offsetting the onset of thermally induced phase transitions. Therefore, the chemical state of C impurities in PVD-grown films should be carefully investigated. (C) 2016 Elsevier B.V. All rights reserved. ; Funding Agencies|German Research Foundation (DFG) [SFB-TR 87]; VINN Excellence Center Functional Nanoscale Materials (FunMat) [2005-02666]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant) [SFO-Mat-LiU 2009-00971]; Knut and Alice Wallenberg Foundation [2011.0143]

The nontrivial effect of molecular dipoles on the surface work function of metals is explored for the unidirectional ordered arrays forming the first and second layers of chloroaluminum phthalocyanine on Au(111). This phthalocyanine is a nonplanar molecule with permanent electric dipole perpendicular to its molecular π-plane that can adopt two distinct configurations (Cl-up and Cl-down) when adsorbed on surfaces. The ordered array forming the first layer is known to consist of all Cl-up molecules, whereas the less-studied second layer is formed by molecules in the Cl-down configuration. The inverted orientation of the molecules in these two layers constitutes our benchmark system to investigate the influence of the dipole array orientation on the surface work function. The present study includes an experimental and theoretical approach that combines diverse imaging and spectroscopic scanning probe microscopies, in ultrahigh vacuum, with first-principles density functional theory-based atomistic simulations. Experiment and theory show a chiral organization transferred from the first layer to the growing film that is reflected in the electronic structure. We demonstrate that the obtained surface work function changes are smaller in magnitude than expected from a dipolar approximation because of charge rearrangement at and beyond the monolayer. We provide understanding of the crucial interplay between the interlayer and organic/metal interactions and quantify their effect on the electron density distribution and on the work function changes. ; This work has been supported by the Spanish Government under projects MAT2013-47869-C4-1-P, MAT2015-72848- EXP /AEI and MAT2016-77852-C2-1-R (AEI/FEDER, UE) and the Generalitat de Catalunya 2017 SGR 668. R. P-R. thanks financial support through BES-2014-067942. We acknowledge the specific agreement between ICMAB-CSIC and the Synchrotron Light Facility ALBA and the Spanish MINECO through the project MAT2015-68994-REDC and the "Severo Ochoa" Program for Centers of Excellence in R&D (SEV-2015-0496). J.I.M. acknowledges the financial support by the Spanish MINECO via grant no. MAT2017-85089-C2-1- R and the "Ramon y Cajal ́ " Program (grant no. RYC-2015- 17730), and the EU through the Innovation Program (grant 696656: Graphene Core1. Graphene-based disruptive technologies), as well as the use of computing resources from CTICSIC. ; Peer reviewed

Open Access. ; We investigate the role of structure and chemical composition on the uptake of poly(ethylene oxide) by a series of graphite oxides (GOs) and thermally reduced GOs, leading to the formation of polymer-intercalated GO and polymer-adsorbed graphene nanostructures. To this end, a series of poly(ethylene oxide) (PEO) - GO hybrid materials exhibiting a variable degree of GO oxidation and exfoliation has been investigated in detail using a combination of techniques including X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetry, scanning-electron microscopy, and nitrogen adsorption. Intercalation of the polymer phase into well-defined GO galleries is found to correlate well with both the degree of GO oxidation and with the presence of hydroxyl groups. The latter feature is an essential prerequisite to optimize polymer uptake owing to the predominance of hydrogen-bonding interactions between intercalant and host. Unlike the bulk polymer, these intercalation compounds show neither crystallisation nor glass-transition associated with the polymer phase. Exfoliation and reduction of GO result in high-surface-area graphene layers exhibiting the highest polymer uptake in these GO-based materials. In this case, PEO undergoes surface adsorption, where we observe the recovery of glass and melting transitions associated with the polymer phase albeit at significantly lower temperatures than the bulk. © 2012 Elsevier Ltd. All rights reserved. ; The authors gratefully acknowledge the support of the Spanish Ministry of Education (MAT2007-63681), the European Union (502235-2 SOFTCOMP), and the Basque Government (IT-436-07). FBB acknowledges financial support from BERC-MPC. FFA acknowledges financial support from the UK Science and Technology Facilities Council. ; Peer Reviewed

[EN] Calcium is an element widely used in the development of biomaterials for bone tissue engineering as it plays important roles in bone metabolism and blood coagulation. The Ca ions can condition the microenvironment at the tissue-material interface, affecting the protein deposition process and cell responses. The aim of this study was to analyze the changes in the patterns of protein adsorption on the silica hybrid biomaterials supplemented with different amounts of CaCl2, which can function as release vehicles. This characterization was carried out by incubating the Ca-biomaterials with human serum. LC-MS/MS analysis was used to characterize the adsorbed protein layers and compile a list of proteins whose affinity for the surfaces might depend on the CaCl2 content. The attachment of pro- and anti-clotting proteins, such as THRB, ANT3, and PROC, increased significantly on the Ca-materials. Similarly, VTNC and APOE, proteins directly involved on osteogenic processes, attached preferentially to these surfaces. To assess correlations with the proteomic data, these formulations were tested in vitro regarding their osteogenic and inflammatory potential, employing MC3T3-E1 and RAW 264.7 cell lines, respectively. The results confirmed a Ca dose-dependent osteogenic and inflammatory behavior of the materials employed, in accordance with the protein attachment patterns. ; This work was supported by MINECO [MAT2017-86043-R]; Universitat Jaume I [Grant numbers Predoc/2014/25, UJI-B2017-37]; Basque Government [Grant numbers IT611-13, Predoc/2016/1/0141]; University of the Basque Country [Grant number UFI11/56]. CIC bioGUNE is supported by Basque Department of Industry, Tourism and Trade (Etortek and Elkartek programs), the Innovation Technology Department of the Bizkaia County; The ProteoRed-ISCIII (Grant PRB3 IPT17/0019); CIBERehd Network, and Severo Ochoa Grant (SEV-2016-0644). Authors would like to thank Antonio Coso and Jaime Franco (GMI-Ilerimplant) for their inestimable contribution to this study, and Raquel ...

26 páginas, 8 figuras, 2 tablas ; Marine sediments are home to one of the richest species pools on Earth, but logistics and a dearth of taxonomic work-force hinders the knowledge of their biodiversity. We characterized α- and β-diversity of deep-sea assemblages from submarine canyons in the western Mediterranean using an environmental DNA metabarcoding. We used a new primer set targeting a short eukaryotic 18S sequence (ca. 110 bp). We applied a protocol designed to obtain extractions enriched in extracellular DNA from replicated sediment corers. With this strategy we captured information from DNA (local or deposited from the water column) that persists adsorbed to inorganic particles and buffered short-term spatial and temporal heterogeneity. We analysed replicated samples from 20 localities including 2 deep-sea canyons, 1 shallower canal, and two open slopes (depth range 100–2,250 m). We identified 1,629 MOTUs, among which the dominant groups were Metazoa (with representatives of 19 phyla), Alveolata, Stramenopiles, and Rhizaria. There was a marked small-scale heterogeneity as shown by differences in replicates within corers and within localities. The spatial variability between canyons was significant, as was the depth component in one of the canyons where it was tested. Likewise, the composition of the first layer (1 cm) of sediment was significantly different from deeper layers. We found that qualitative (presence-absence) and quantitative (relative number of reads) data showed consistent trends of differentiation between samples and geographic areas. The subset of exclusively benthic MOTUs showed similar patterns of β-diversity and community structure as the whole dataset. Separate analyses of the main metazoan phyla (in number of MOTUs) showed some differences in distribution attributable to different lifestyles. Our results highlight the differentiation that can be found even between geographically close assemblages, and sets the ground for future monitoring and conservation efforts on these bottoms of ecological and economic importance. ; The sampling has been done in the framework of the INDEMARES project (LIFE+ NAT/E/ 000732) of the European Union and the DOSMARES project (CTM2010-21810) of the Spanish Government. A European Science Foundation ConGenOmics Exchange Grant (n 3919) funded MG stay at the LECA lab. This research has been funded by projects BENTHOMICS CTM2010-22218 and CHALLENGEN CTM2013-48163 of the Spanish ; Peer reviewed

Transport and reactivity of carbon in the critical zone are highly controlled by reactions of dissolved organic matter (DOM) with subsurface soils, including adsorption, transformation and exchange. These reactions are dependent on frequent wet–dry cycles common to the unsaturated zone, particularly in semi-arid regions. To test for an effect of wet–dry cycles on DOM interaction and stabilization in subsoils, samples were collected from subsurface (Bw) horizons of an Entisol and an Alfisol from the Catalina-Jemez Critical Zone Observatory and sequentially reacted (four batch steps) with DOM extracted from the corresponding soil litter layers. Between each reaction step, soils either were allowed to air dry (wet–dry treatment) before introduction of the following DOM solution or were maintained under constant wetness (continually wet treatment). Microbial degradation was the dominant mechanism of DOM loss from solution for the Entisol subsoil, which had higher initial organic C content, whereas sorptive retention predominated in the lower C Alfisol subsoil. For a given soil, bulk dissolved organic C losses from solution were similar across treatments. However, a combination of Fourier transform infrared (FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic analyses revealed that wet–dry treatments enhanced the interactions between carboxyl functional groups and soil particle surfaces. Scanning transmission X-ray microscopy (STXM) data suggested that cation bridging by Ca2+ was the primary mechanism for carboxyl association with soil surfaces. STXM data also showed that spatial fractionation of adsorbed OM on soil organo-mineral surfaces was diminished relative to what might be inferred from previously published observations pertaining to DOM fractionation on reaction with specimen mineral phases. This study provides direct evidence of the role of wet–dry cycles in affecting sorption reactions of DOM to a complex soil matrix. In the soil environment, where wet–dry cycles occur at different frequencies from site to site and along the soil profile, different interactions between DOM and soil surfaces are expected and need to be considered for the overall assessment of carbon dynamics. ; Binational Agricultural Research and Development (BARD) program [FI-534-2015]; National Science Foundation [EAR 13-31408]; Canadian Foundation for Innovation; Natural Sciences and Engineering Research Council of Canada; University of Saskatchewan; Government of Saskatchewan; Western Economic Diversification Canada; National Research Council Canada; Canadian Institutes of Health Research ; Open access journal. ; This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.