Molecular-scale Understanding Research Articles

Molecular fingerprint-aided prediction of organic solute rejection in reverse osmosis and nanofiltration

Reverse osmosis and nanofiltration are used to purify feedwaters that contain a range of harmful organic solutes. The rejection of many of these solutes is poorly understood due to our limited ability to experimentally measure removal of any given compound. In this work, we present a machine learning approach that predicts organic solute rejection using molecular fingerprints that encode chemical structure features, such as functional groups and rings, into simple binary vectors. We trained machine learning models on a database of 1906 membrane rejection measurements including 228 organic compounds and 39 types of reverse osmosis and nanofiltration membranes. Three types of molecular fingerprint models (structural key, circular, and path based) were compared, and we observed that the Molecular Access System (MACCS) structural key had high performance (coefficient of determination of 0.87 with the testing set), fast calculation time due to its short bit-length, and easy interpretability. In addition to evaluating prediction performance, Shapley Additive Explanations (SHAP) analysis was implemented to gain a better molecular-scale understanding of membrane rejection, identifying molecular substructures that are important in determining their rejection. Overall, this work presents a method to predict the rejection of compounds that uses readily available molecular structure information and improves our ability to understand rejection mechanisms.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012).

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

Electrolyte Mixtures and Organic Cathode Materials for Rechargeable Aluminum Batteries

Rechargeable aluminum (Al) metal batteries are an emerging energy storage technology ideal for use at a global scale: Al metal is energy dense, low cost, inherently safe, earth abundant, and highly recyclable. Despite such great promise, their technological progress has been hindered by the few electrolytes able to reversibly electrodeposit Al metal at room temperature, coupled with the limited number of positive electrode materials that are compatible with them. In this context, recent progress will be discussed in the development of electrolyte mixtures and organic cathode materials for rechargeable aluminum batteries. Chloroaluminate ionic liquid electrolytes and ionic liquid analogues prepared using mixtures of organic cations and/or neutral solvent species will be presented, which exhibit improved electrochemical properties for aluminum electrodeposition and in aluminum-graphite batteries, over temperatures ranging from ambient conditions down to -60 °C. Different organic structures will also be presented as organic cathode materials for aluminum batteries, where the effects of molecular structure on bulk energy storage properties will be discussed. Molecular-scale understanding of their ionic and electronic charge storage mechanisms will be elucidated through a combination of solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations. Lastly, by using different electrolytes in combination with an organic cathode, it will be shown that electrolyte speciation can affect the local environments of charge-compensating ions in cycled organic electrodes. Overall, the results and analyses are aimed at developing next-generation rechargeable aluminum metal batteries for diverse energy storage applications.

CO2 absorption enhancement of fluorinated ionic liquids on nonaqueous biphasic absorbents: Experimental and theoretical study

Nonaqueous biphasic absorbent decreasing the regeneration heat consumption by phase separation (lean-phase and rich-phase) and water evaporation elimination is a promising CO2 capture absorbent, but the low CO2 loading capacities are of great challenges. In this study, ionic liquids (IL) were proposed as catalysts for enhancing the CO2 loading in monoethanolamine (MEA)-1-Butanol nonaqueous biphasic absorbents. Three types of fluorinated IL including 1‑butyl‑3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1‑butyl‑3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), and 1‑butyl‑3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bmim][TF2N]) under different concentrations (0–10wt%) were detailly analyzed to explore the catalyzation of IL on phase separation behavior and absorption performance. Phase separation study showed that the MEA-1-Butanol-IL system exhibited the wonderful phase separation behavior with a volume ratio (rich-phase) between 30 and 40 %. Three phases were observed in MEA-1-Butanol-[Bmim][BF4] and MEA-1-Butanol-[Bmim][PF6] systems after CO2 absorption and the components of each phase were identified by 13C nuclear magnetic resonance (NMR) spectroscopy. In absorption study, the absorption enhancement of IL was in this order: [Bmim][TF2N] > [Bmim][BF4] > [Bmim][PF6]. MEA-1-Butanol-10wt% [Bmim][TF2N] system could improve 35.9 % CO2 loading and 24.2 % absorption rate compared to basic MEA-1-Butanol. A molecular scale understanding of the three-steps reaction mechanism of MEA-1-Butanol-IL absorbing CO2 was revealed, and the different catalyzation of three types of IL can be attributed to the different affinity for CO2. This study provides new insights into designing nonaqueous biphasic solvents and a promising MEA-1-Butanol-IL system with high CO2 capture performance.

Direct Experimental Observations of Ion Distributions during Overcharging at the Muscovite-Water Interface by Adsorption of Rb+ and Halides (Cl- , Br- , I- ) at High Salinity.

Classical electric double layer (EDL) models have been widely used to describe ion distributions at charged solid-water interfaces in dilute electrolytes. However, the chemistry of EDLs remains poorly constrained at high ionic strength where ion-ion correlations control non-classical behavior such as overcharging, i. e., the accumulation of counter-ions in amounts exceeding the substrate's surface charge. Here, we provide direct experimental observations of correlated cation and anion distributions adsorbed at the muscovite (001)-aqueous electrolyte interface as a function of dissolved RbBr concentration ([RbBr]=0.01-5.8 M) using resonant anomalous X-ray reflectivity. Our results show alternating cation-anion layers in the EDL when [RbBr]≳100 mM, whose spatial extension (i. e., ~20 Å from the surface) far exceeds the dimension of the classical Stern layer. Comparison to RbCl and RbI electrolytes indicates that these behaviors are sensitive to the choice of co-ion. This new in-depth molecular-scale understanding of the EDL structure during transition from classical to non-classical regimes supports the development of realistic EDL models for technologies operating at high salinity such as water purification applications or modern electrochemical storage.

Probing Molecular Structures at Electrified Interfaces Using in-Situ Surface-Enhanced Raman Spectroscopy

The molecular scale understanding of electrode-electrolyte interfaces is significant, as interfacial molecular structures play a critical role in determining various properties such as capacitance and catalytic activity. However, the limited understanding exists on the interfaces, especially at a molecular level, due to difficulties to selectively probe the interfaces without an interference from the bulk. In this talk, I will present our recent observations on molecular configurations at electrified interfaces using in-situ surface-enhanced Raman spectroscopy (SERS). To be specific, we study interfaces of ionic liquids and aqueous electrolytes, considering they are promising candidates for various electrochemical applications. Our SERS spectra display a strong enough interface-sensitivity providing comprehensive information on a potential-dependence of ion configurations such as molecular position and orientation. We expect the molecular scale understandings provided by our studies are beneficial to a wide range of electrochemical applications including supercapacitors, batteries, and electrocatalysts.

Interfaces in reinforced epoxy resins: from molecular scale understanding towards mechanical properties

ContextWe report on atomic level of detail analyses of polymer composite models featuring epoxy resin interfaces to silica, iron oxide, and cellulose layers. Using “reactive” molecular dynamics simulations to explore epoxy network formation, resin hardening is investigated in an unprejudiced manner. This allows the detailed characterization of salt-bridges and hydrogen bonds at the interfaces. Moreover, our sandwich-type composite systems are subjected to tensile testing along the interface normal. To elucidate the role of relaxation processes, we contrast (i) direct dissociation of the epoxy-metal oxide/cellulose contact layer, (ii) constant strain-rate molecular dynamics studies featuring (visco-)elastic deformation and bond rupture of the epoxy resin, and (iii) extrapolated relaxation dynamics mimicking quasi-static conditions. While the fracture mechanism is clearly identified as interface dissociation of the composite constituents, we still find damaging of the nearby polymer phase. The observed plastic deformation and local cavitation are rationalized from the comparably large stress required for the dissociation of salt-bridges, hydrogen bonds, and van der Waals contacts. Indeed, the delamination of the contact layers of epoxy resins with slabs of silica, magnetite, and cellulose call for a maximum stress of 33, 26, and 21 MPa, respectively, as compared to 84 MPa required for bulk epoxy yielding.MethodsMolecular dynamics simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code were augmented by a Monte Carlo–type procedure to probe epoxy bond formation (Macromolecules 53(22): 9698–9705). The underlying interaction models are split into conventional Generalized Amber Force Fields (GAFF) for non-reacting moieties and a recently developed reactive molecular mechanics potential enabling epoxy bond formation and cleavage (ACS Polymers Au 1(3): 165–174).

Rheological Properties of Small-Molecular Liquids at High Shear Strain Rates.

Molecular-scale understanding of rheological properties of small-molecular liquids and polymers is critical to optimizing their performance in practical applications such as lubrication and hydraulic fracking. We combine nonequilibrium molecular dynamics simulations with two unsupervised machine learning methods: principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE), to extract the correlation between the rheological properties and molecular structure of squalane sheared at high strain rates (106-1010s-1) for which substantial shear thinning is observed under pressures P∈0.1-955 MPa at 293 K. Intramolecular atom pair orientation tensors of 435×6 dimensions and the intermolecular atom pair orientation tensors of 61×6 dimensions are reduced and visualized using PCA and t-SNE to assess the changes in the orientation order during the shear thinning of squalane. Dimension reduction of intramolecular orientation tensors at low pressures P=0.1,100 MPa reveals a strong correlation between changes in strain rate and the orientation of the side-backbone atom pairs, end-backbone atom pairs, short backbone-backbone atom pairs, and long backbone-backbone atom pairs associated with a squalane molecule. At high pressures P≥400 MPa, the orientation tensors are better classified by these different pair types rather than strain rate, signaling an overall limited evolution of intramolecular orientation with changes in strain rate. Dimension reduction also finds no clear evidence of the link between shear thinning at high pressures and changes in the intermolecular orientation. The alignment of squalane molecules is found to be saturated over the entire range of rates during which squalane exhibits substantial shear thinning at high pressures.

Site-specific isotope fractionation during Zn adsorption onto birnessite: Insights from X-ray absorption spectroscopy, density functional theory and surface complexation modeling

Birnessite minerals help control the fate of Zn in surface environments and readily fractionate Zn isotopes through adsorption reactions, yet little is known about the role played by various reactive sites in stable isotopic fractionation. Here we present the Zn isotope fractionation data cause by adsorption on birnessite under different reaction times, pH values, and Zn concentrations. We observe that isotopic equilibrium of Zn is attained after ∼120 h of reaction time at pH 6. At pH 3–5 and Zn concentrations of 0.05–0.3 mM, the isotopic fractionation (Δ66Znadsorbed-aqueous) is around −0.46 ± 0.04‰, and gradually increases to −0.09 ± 0.05‰ at pH 6–8 and Zn concentrations of 0.2 mM. The change in Zn isotopic compositions as a function of pH and Zn concentration is well described using the surface complexation model, where two binding sites are involved: external edge sites and interlayer vacancies. According to this model, two different isotopic fractionation factors of Zn are calculated: Δ66Znadsorbed-aqueous = −0.46 ± 0.04‰ for adsorption on vacancy sites and Δ66Znadsorbed-aqueous = 0.52 ± 0.04‰ for binding to edge sites. Extended X-ray absorption fine structure spectroscopy (EXAFS) demonstrates that Zn forms triple-corner-sharing (TCS) octahedral complex on birnessite vacancies at pH 3 and Zn concentrations of 0.05–0.2 mM, where Zn is coordinated on one side to three oxygen atoms of the Mn vacancy (∼2.03 Å) and to three water molecules on the other side (∼2.15 Å), suggesting the formation of distorted ZnO octahedra (average bond length: ∼2.09 Å). At pH 6 and 8, double-corner-sharing (DCS) complexes on layer edges formed in addition to the TCS octahedral complex on vacancies. Density functional theory (DFT) optimisations suggest that DCS Zn complex exist in tetrahedral coordination. Based on EXAFS spectroscopy, DFT optimisations and surface complexation modeling, the distinct isotopic fractionation of Zn is related to the differences in Zn local structure at different reactive sites of birnessite. Our results provide a molecular-scale understanding of Zn isotopic fractionation in natural birnessite-containing settings, as well as new insights into predicting the links between adsorption and fractionation of other similar metals.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials.

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Chemo-mechano-structural interplay in fully hydrated ion clusters on hexagonal boron nitride nanosheet surfaces

The theoretical understanding of the coupling relationship between ionic thermodynamics and electron kinetics in the vicinity of fluid–solid interfaces is highly important. In this study, the chemo–mechano–structural interplay of the hydrated ion cluster that governs the equilibrium state in adsorption is elucidated using all-atom model computations. Specifically, the physicochemical properties of four (sodium, chloride, hydronium, and ammonium) fully hydrated ions in bulk solution and on the surface of a hexagonal boron nitride nanosheet (hBNNS) substrate were comparatively investigated. The results suggest that the intrinsic structure and charged state of these ions are key in determining the internal structure—symmetry, water ligand alignment, and hydrogen-bonded network structure—of the hydrated ion clusters. The shape of the hydrated ion cluster has a significant impact on the mechanical behavior during inner-sphere adsorption of the ion. Furthermore, the electron kinetics of the hydrated ion–hBNNS interface are strongly dependent on the arrangement of the water ligands and water network inside the ion cluster. This study is the first to elucidate the coupled mechanism of multiple hydrated ion clusters, providing new insights into the molecular scale understanding of electrical double-layer electrode, desalination separator, and proton transport membrane systems in practical engineering.

Molecular scale assessment of defluoridation of coal-mining wastewater by calcined Mg/Al layered double hydroxide using 19F solid-state NMR, XPS, and HRTEM.

Calcination is an effective way to improve the F- adsorption capacity of layered double hydroxide (LDH) materials, however, a molecular scale understanding of the enhanced defluoridation capability of calcined LDHs (CLDH) is lacking. This study investigated the mechanisms of F- adsorption by CLDH using 19F solid-state NMR, X-ray photoelectron spectroscopy (XPS), and high-resolution TEM. Under calcination process, LDH underwent three periods: surface dehydration below 200°C, structural dehydroxylation at 200-400°C, and release of interlayer carbonate groups above 400°C. Additionally, XPS and XRD characterization showed that CLDH could not recover to the original structural symmetry even after rehydration and reconstitution. The F- affinity was greatly enhanced for the calcined LDH, especially at high pH. At pH 10, the adsorption capacity could reach 22.0mgF-/g for CLDH (500°C calcined), about 6 times larger than that of LDH. The XRD analyses revealed that the F-adsorbed CLDH had a poorer crystalline degree as the calcination temperature increased, consistent with the TEM observation of abundant defects and Mg/Al oxides on the CLDH sheets. 19F solid-state NMR spectra of the CLDH after F- adsorption showed that the formation of surface Al-F is the predominant F- adsorption mode at pH 7, whereas the Mg-F local coordination mode is the pronounced F- adsorption mechanism under alkaline conditions (pH 10). The present study provided a comprehensive understanding of CLDH in F- adsorption and suggested that calcination is a promising treatment for promoting the efficacy of polluted anion scavenging.

Beyond Aquaporins: Recent Developments in Artificial Water Channels.

A molecular scale understanding of the fast and selective water transport in biological water channels, aquaporins (AQPs), has inspired attempts to mimic its performance in synthetic structures. These synthetic structures, referred to as artificial water channels (AWCs), present several advantages over AQPs in applications. After over a decade of efforts, the unique transport properties of AQPs have been reproduced in AWCs. Further, recent developments have shown that the performance of benchmark AQP channels can be exceeded by new AWC designs using novel features not seen in biology. In this Perspective, we provide a brief overview of recent AWC developments, and share our perspective on forward-looking AWC research.

Droplet wettability and repellency on fluorinated lubricant-infused surfaces: A molecular dynamics study

Fluorinated lubricant-infused surfaces are gaining remarkable attention as a promising candidate of self-cleaning and antifouling materials and liquid-resistant surface coatings. Despite all the lubricant-infused surfaces are related to a surface lubrication stabilized in a porous or nanotextured solid, they may present varying droplet morphologies regarding to the state of cloaking, lubricant ridges, and the wetting state. In this work, molecular dynamics simulations were performed to investigate the droplet wettability and repellency on fluorinated lubricant-infused surfaces. A phase diagram of possible droplet configurations at static equilibration has been developed and compared with theoretical predictions by examining the effects of interfacial tensions and lubricant thickness comprehensively. The static and dynamic droplet repellencies were further quantified in terms of the static, advancing, and receding contact angles and the shedding velocity. In particular, the distinctive “slippery Cassie” surface could effectively conquer the droplet pinning instability by energetically favorably “slippery Wenzel-to-Cassie” transition as well as self-replenishing of lubricant, as compared to the superhydrophobic surface that is susceptible to the irreversible transition to Wenzel state. The findings in this work are believed to provide molecular scale understanding and intuitive guidelines for inspiring applications of fluorinated lubricant-infused surfaces by designing surface wettability and lubricant thickness level rationally.

Investigating Nanoscale Electron Transfer Processes at the Cell-Mineral Interface in Cobalt-Doped Ferrihydrite Using Geobacter sulfurreducens: A Multi-Technique Approach

Cobalt is an essential element for life and plays a crucial role in supporting the drive to clean energy, due to its importance in rechargeable batteries. Co is often associated with Fe in the environment, but the fate of Co in Fe-rich biogeochemically-active environments is poorly understood. To address this, synchrotron-based scanning X-ray microscopy (SXM) was used investigate the behaviour of cobalt at the nanoscale in Co-Fe(III)-oxyhydroxides undergoing microbial reduction. SXM can assess spatial changes in metal speciation and organic compounds helping to elucidate the electron transfer processes occurring at the cell-mineral interface and inform on the fate of cobalt in redox horizons. G. sulfurreducens was used to reduce synthetic Co-ferrihydrite as an analogue of natural cobalt-iron-oxides. Magnetite [Fe(II)/Fe(III)3O4] production was confirmed by powder X-ray diffraction (XRD), SXM and X-ray magnetic circular dichroism (XMCD) data, where best fits of the latter suggested Co-bearing magnetite. Macro-scale XAS techniques suggested Co(III) reduction occurred and complementary SXM at the nanoscale, coupled with imaging, found localised biogenic Co(III) reduction at the cell-mineral interface via direct contact with outer membrane cytochromes. No discernible localised changes in Fe speciation were detected in the reordered cobalt-iron-oxides that were formed and at the end point of the experiment only 11% Co and 1.5% Fe had been solubilised. The solid phase retention, alongside the highly localised and preferential cobalt bioreduction observed at the nanoscale is consistent with retention of Co in redox zones. This work improves our fundamental molecular-scale understanding of the fate of Co in complex environmental systems and supports the development of biogenic Co-doped magnetite for industrial applications from drug delivery systems to magnetic recording media.

Genesis of MoS2 from model-Mo-oxide precursors supported on γ-alumina

The molecular-scale understanding of the genesis of the MoS2 phase from Mo-oxide precursors supported on alumina is highly challenging with a strong impact on the activation process of heterogeneous industrial catalysts. By means of density functional theory (DFT), we quantify the activation free energies of the elementary steps involved in the sulfo-reduction mechanisms of Mo-trioxide oligomers and the stability of the corresponding Mo-oxysulfide intermediates supported on the γ-alumina (100) surface. The Gibbs free energy profiles highlight the characteristic chemical reactivity of various oxygen sites involved in the O/S exchange mechanism and reveal that interfacial oxygen atoms (Mo-O-Al) are the most challenging sites to be exchanged with S. We quantitatively compare the two main paths proposed experimentally: the one involving Mo-oxysulfide and Mo-trisulfide intermediates and the second one involving only Mo-oxysulfide. While O/S exchange requires moderate activation energies, the rate-determining steps correspond to S- and O-removal on small MonO3n-xSx or MonS3n (n ≤ 3) oligomeric intermediates. To overcome these high energy steps, the small Mo-trisulfide (MonS3n) oligomers are proposed to be fast diffusing surface species and to promote the growth process towards the targeted MoS2 phase. A reconstruction from a chain to a triangular Mo3S9 conformer also facilitates this phase transformation.

Manipulation of Precise Molecular Arrangements and Their Photochemical Properties on Inorganic Surfaces via Multiple Electrostatic Interactions

Abstract Multiple (or multivalent) interaction is the key in many biological systems. One of the most important (photo-)chemical reactions, photosynthesis, is driven by regularly aligned molecules by multiple interactions between proteins and molecules. A grand challenge of modern chemistry therefore includes the construction of supramolecular assemblies and control of their functions for mimicking nature and beyond. While most synthetic systems depend on covalent, coordination and hydrogen bonds between molecules, my approach focuses on multiple electrostatic interactions with two-dimensional clay mineral nanosheets. I here summarize my recent work on manipulation of precise molecular arrangements and photochemical properties via multiple electrostatic interactions. This Account mainly consists of the three parts; 1: manipulation of photochemical properties of molecules and new emission enhancement phenomenon (chapters 2–7), 2: efficient photochemical reactions and artificial photosynthesis model (chapters 8–14), and 3: molecular-scale understanding by means of electron microscopy (chapters 15–17).

Tip Charge Dependence of Three-Dimensional AFM Mapping of Concentrated Ionic Solutions.

A molecular scale understanding of the organization and structure of a liquid near a solid surface is currently a major challenge in surface science. It has implications across different fields from electrochemistry and energy storage to molecular biology. Three-dimensional AFM generates atomically resolved maps of solid-liquid interfaces. The imaging mechanism behind those maps is under debate, in particular, for concentrated ionic solutions. Theory predicts that the observed contrast should depend on the tip's charged state. Here, by using neutrally, negatively, and positively charged tips, we demonstrate that the 3D maps depend on the tip's polarization. A neutral tip will explore the total particle density distribution (water and ions) while a charged tip will reveal the charge density distribution. The experimental data reproduce the key findings of the theory.

Advances in molecular dynamics simulation on heterogeneous nucleation of gas hydrate

Gas hydrate is a nonstoichiometric crystalline material. It is attracting more and more attention not only because natural gas hydrate is a potential energy resource, but also because hydrate-based technologies can be potentially applied to the industry of energy and environment such as energy storage, carbon sequestration, etc. The nucleation mechanism of gas hydrate is a key issue in studying reservoir formation of natural gas hydrate and the kinetic properties of gas hydrate as an energy-storage media. Molecular dynamics simulation can provide microscopic insights into molecular-scale understanding of hydrate nucleation. In this review, the advances on the study of heterogeneous hydrate nucleation by molecular dynamics simulation are summarized, with a specific focus on the factors affecting nucleation processes. The interfacial energy barrier for nucleation will be lowered by molecular structure of interfacial water between hydrate and solid substrate. The interface and surface properties of solid substrate, including hydrophilicity and hydrophobicity, surface roughness, layer-accumulated adsorption potential, crystallinity and surface layer-charge are also recognized as being associated with the process of hydrate nucleation by influencing the dynamics of gas and water molecules. In case of hydrate nucleation in confined pore space, confinement effect, pore size and elasticity are identified as the controlling factors.

Molecular-Level Understanding of the Influence of Ions and Water on HMGB1 Adsorption Induced by Surface Hydroxylation of Titanium Implants.

Due to its excellent chemical and mechanical properties, titanium has become the material of choice for orthopedic and dental implants to promote rehabilitation via bone anchorage and osseointegration. Titanium osseointegration is partially related to its capability to form a TiO2 surface layer and its ability to interact with key endogenous proteins immediately upon implantation, establishing the first bone-biomaterial interface. Surgical trauma caused by implantation results in the release of high-mobility group box 1 (HMGB1) protein, which is a prototypic DAMP (damage-associated molecular pattern) with multiple roles in inflammation and tissue healing. To develop different surface strategies that improve the clinical outcome of titanium-based implants by controlling their biological activity, a molecular-scale understanding of HMGB1-surface interactions is desired. Here, we use molecular dynamics (MD) computer simulations to provide direct insight into the HMGB1 interactions and the possible molecular arrangements of HMGB1 on fully hydroxylated and nonhydroxylated rutile (110) TiO2 surfaces. The results establish that HMGB1 is most likely to be adsorbed directly onto the surface regardless of surface hydroxylation, which is undesirable because it could affect its biological activity by causing structural changes to the protein. The hydroxylated TiO2 surface shows a greater affinity for HMGB1 than the nonhydroxylated surface. The water layer on the nonhydroxylated TiO2 surface prevents ions and the protein from directly contacting the surface. However, it was observed that if the ionic strength increases, the total number of ions adsorbed on the two surfaces increases and the protein's direct adsorption ability decreases. These findings will help to understand the HMGB1-TiO2 interactions upon implantation as well as the development of different surface strategies by introducing ions or ionic materials to the titanium implant surface to modulate its interactions with HMGB1 to preserve biological function.