Surface Interaction Strength Research Articles

Oil Film Formation and Delamination Process on Nanostructured Surfaces in Boundary Lubrication: A Coarse-Grained Molecular Dynamics Study

Abstract A series of coarse-grained molecular dynamics simulations were conducted to investigate the temporal evolution of frictional behaviors of lubricants between sliding nanostructured iron surfaces. Grain boundary atoms were given a stronger interaction with the lubricant molecules. We varied the surface distance and interaction strength between grain boundary atoms and lubricants. It was found that, below the critical compressive stress, the oil film detached from the surface at first and then attached to it after several nanoseconds due to the localized molecular rearrangement within lubricants and the enhancement of interaction between lubricants and iron surfaces. The transition times required for oil film formation and de-lamination both increased as it approached the critical compressive stress. Larger interaction strength increased the delamination time but barely affected the formation time.

Nonmonotonic polymer translocation kinetics through nanopores under changing surface-polymer interactions.

Understanding the dynamics of polymers in confined environments is pivotal for diverse applications ranging from polymer upcycling to bioseparations. In this study, we develop an entropic barrier model using self-consistent field theory that considers the effect of attractive surface interactions, solvation, and confinement on polymer kinetics. In this model, we consider the translocation of a polymer from one cavity into a second cavity through a single-segment-width nanopore. We find that, for a polymer in a good solvent (i.e., excluded volume, u0 > 0), there is a nonmonotonic dependence of mean translocation time (τ) on surface interaction strength, ɛ. At low ɛ, excluded volume interactions lead to an energetic penalty and longer translocation times. As ɛ increases, the surface interactions counteract the energetic penalty imposed by excluded volume and the polymer translocates faster through the nanopore. However, as ɛ continues to increase, an adsorption transition occurs, which leads to significantly slower kinetics due to the penalty of desorption from the first cavity. The ɛ at which this adsorption transition occurs is a function of the excluded volume, with higher u0 leading to an adsorption transition at higher ɛ. Finally, we consider the effect of translocation across different size cavities. We find that the kinetics for translocation into a smaller cavity speeds up while translocation to a larger cavity slows down with increasing ɛ due to higher surface contact under stronger confinement.

MULTISCALE SIMULATION OF HYDRODYNAMIC STEP BEARING WITH ULTRA-LOW CLEARANCE INVOLVING SURFACE ROUGHNESS

The numerical calculation was carried out for the multiscale hydrodynamic step bearing with ultra-low clearance and nanoscale surface roughness based on the newly developed multiscale approach. The moving surface was assumed as perfectly smooth and the stationary surface was imposed with the sinusoidal surface roughness. Due to the surface roughness, locally the physically adsorbed boundary layer may be only present, while in other areas, the sandwich films consisting of both the adsorbed boundary layers and the intermediate continuum fluid film may be present. In this bearing, the fluid film may be locally severely squeezed so that the local film pressure is very high. In this circumstance, the influences of the film pressure on both the film viscosity and density were considered. The results show that with the increase of the surface roughness, the hydrodynamic pressure and carried load of the bearing are very significantly increased especially for a stronger fluid-bearing surface interaction; the fluid piezo-viscous effect becomes stronger with the increases of both the surface roughness and the fluid-bearing surface interaction strength.

Evolution of phase diagrams of polymer adsorption over attractive surfaces as a function of flexibility and solvent quality

ABSTRACT We describe the equilibrium conformations of a polymer near an attractive surface for different conditions in solvent quality and polymer stiffness. Using molecular dynamics techniques in a canonical ensemble, we show the evolution of the phase diagrams as function of temperature and surface interaction strength. Phase diagrams show an ampler variety of possible conformations due to changes in solvent quality compared to stiffness for which variations in conformations appear mainly in the degree of adsorption. We show that the adsorption transition is independent of solvent quality and stiffness. When both parameters change simultaneously, solvent effects are stronger than the stiffness secluding the stiff conformations to regions of low temperature and high surface attraction. A remarkable observation is that the radius of gyration perpendicular component to the surface, , signals the adsorption transition at coincidental points for each system independently of solvent and stiffness variations in all simulations, which may be of aid in locating this transition in other systems.

Friction hysteretic behavior of supported atomically thin nanofilms

Hysteretic friction behavior has been observed on varied 2D nanofilms. However, no unanimous conclusion has yet been drawn on to the exact mechanism or relative contribution of each mechanism to the observed behavior. Here we report on hysteretic friction behavior of supported atomically thin nanofilms studied using atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. Load dependent friction measurements were conducted on unheated and heated samples of graphene, h-BN, and MoS2 supported by silica substrates. Two diverging friction trends are reported: the unheated samples showed higher friction during unloading than during loading, and the heated samples showed a reversed hysteresis. Further, the friction force increased sub-linearly with normal force for heated samples, compared with unheated samples. Tapping mode AFM suggested that the interaction strength of the substrate was increased with heating. Roughened substrates in the MD simulations that mimicked strong/weak interaction forces reproduced the experimental observations and revealed that the evolution of real contact area in different interface interaction situation caused the diverging behaviors. Surface roughness and interaction strength were found to be the key parameters for controlling the out-of-plane deformation of atomically thin nanofilms.

The effect of nanoparticle softness on the interfacial dynamics of a model polymer nanocomposite.

The introduction of soft organic nanoparticles (NPs) into polymer melts has recently expanded the material design space for polymer nanocomposites, compared to traditional nanocomposites that utilize rigid NPs, such as silica, metallic NPs, and other inorganic NPs. Despite advances in the fabrication and characterization of this new class of materials, the effect of NP stiffness on the polymer structure and dynamics has not been systematically investigated. Here, we use molecular dynamics to investigate the segmental dynamics of the polymer interfacial region of isolated NPs of variable stiffness in a polymer matrix. When the NP-polymer interactions are stronger than the polymer-polymer interactions, we find that the slowing of segmental dynamics in the interfacial region is more pronounced for stiff NPs. In contrast, when the NP-polymer interaction strength is smaller than the matrix interaction, the NP stiffness has relatively little impact on the changes in the polymer interfacial dynamics. We also find that the segmental relaxation time τα of segments in the NP interfacial region changes from values lower than to higher than the bulk material when the NP-polymer interaction strength is increased beyond a "critical" strength, reminiscent of a binding-unbinding transition. Both the NP stiffness and the polymer-surface interaction strength can thus greatly influence the relative segmental relaxation and interfacial mobility in comparison to the bulk material.

Nanoconfined Fluid Critical Properties Variation over Surface Wettability

The characterization of nanoconfined fluid critical properties is critical for nanoconfined fluid phase behavior and flow capacity evaluation. To date, fluid–surface interaction strength is considered as the original cause for the shift of fluid critical properties, which is closely related to pore size and surface wettability. However, the majority of research is devoted to investigating the impact of pore size, while investigations about surface wettability on nanoconfined fluid critical properties are lacking. In order to fill the knowledge gap, in-depth investigations have been carried out to shed light on the wettability effect on nanoconfined fluid critical properties. First, focusing on the relative strength between fluid–fluid interactions and fluid–surface interactions, a robust correlation is proposed to relate surface contact angle, a macroscopic form of wettability effect, to critical property shift. Then, a linear correlation between the wettability effect and the adsorption thickness is established, and as a result, effective pore size can be described as a function of wettability as well. Finally, a prediction model for nanoconfined fluid critical properties is developed, and its reliability is well-clarified against 86 cases, collected from previous molecular simulation results or experiments. The results show the following: (a) The magnitude of the shift of fluid critical property can reach as high as 60%, dominating the variation characteristics of nanoconfined fluid phase behavior. (b) Both the pore size shrinkage and strong fluid-affinity surface are underlying mechanisms for suppressed critical properties at nanoscale. (c) The confined effect arisen from the cylindrical geometry is 2.24 times greater than that for slit nanopores, essentially stemming from the fluid force condition exerted by the pore surface.

Cellular avidity as a novel biomarker for candidate selection of cell-based immunotherapies

Abstract Cellular immunotherapies are increasingly complex in design. Affinity readouts, i.e., the measurements of the strength of single receptor-ligand interactions, are frequently used to assess the binding of chimeric antigen receptors (CARs) or T cell receptors (TCRs) to their ligand. However, affinity is a poor predictor of effector function of the engineered downstream product. Therefore, new tools are needed to evaluate the binding strength of CAR and TCR-transgenic cells to their cognate targets in a more biologically relevant context. Recent studies demonstrated the utility of measuring cellular avidity as a novel biomarker for identifying and developing potent and safe immunotherapies. Unlike affinity, cellular avidity is driven by the overall strength of dynamic surface interactions between effector cells and their targets by integrating receptor density, the sum of individual affinities, and engagement of the multitude of co-receptors within the immunological synapse. Here, we show that increased specific avidity, i.e., TCRs with the strongest antigen binding and the lowest background, correlated with improved effector function both in vitro and in vivo. For CAR-T, higher cellular avidity was significantly correlated with improved tumor control in in vivo murine models, but also associated with toxicities in patients, suggesting fine-tuning of cell therapies to the desired avidity for ideal function is needed.

RNA Secondary Structures Regulate Adsorption of Fragments onto Flat Substrates.

RNA is a functionally rich molecule with multilevel, hierarchical structures whose role in the adsorption to molecular substrates is only beginning to be elucidated. Here, we introduce a multiscale simulation approach that combines a tractable coarse-grained RNA structural model with an interaction potential of a structureless flat adsorbing substrate. Within this approach, we study the specific role of stem-hairpin and multibranch RNA secondary structure motifs on its adsorption phenomenology. Our findings identify a dual regime of adsorption for short RNA fragments with and without the secondary structure and underline the adsorption efficiency in both cases as a function of the surface interaction strength. The observed behavior results from an interplay between the number of contacts formed at the surface and the conformational entropy of the RNA molecule. The adsorption phenomenology of RNA seems to persist also for much longer RNAs as qualitatively observed by comparing the trends of our simulations with a theoretical approach based on an ideal semiflexible polymer chain.

Correlating the strength of reducing agent adsorption with Ag/Al2O3 catalyst performances in selective catalytic reduction (SCR) of NOx

NMR relaxation has been used to characterise the strength of surface interaction of reducing agent species in Ag/Al2O3 catalysts used for the selective catalytic reduction (SCR) of NOx. Several classes of compounds, including aliphatic and aromatic hydrocarbons, as well as alcohols, were investigated. The results show that the strength of interaction of the reducing agent with the catalyst surface, characterised by the NMR relaxation time ratio T1/T2, plays an important role in determining the catalyst performances, with reducing agents able to compete with water for adsorption over the surface giving better catalytic activities. In particular, a clear trend was observed when comparing the T1/T2 ratio of different reducing agents with the initial activity of the catalyst, in particular with the activation temperature, with species such as alcohols, which showed a higher affinity with the catalyst, showing an easier initial activation. Whilst SCR is a complex reaction, whereby a series of phenomena are responsible for the overall catalytic activity, the strength of surface interaction of the reducing agent is a useful indicator for the initial catalytic activation in such processes.

Mimicking Biosintering: The Identification of Highly Condensed Surfaces in Bioinspired Silica Materials.

Interfacial interactions between inorganic surfaces and organic additives are vital to develop new complex nanomaterials. Learning from biosilica materials, composite nanostructures have been developed, which exploit the strength and directionality of specific polyamine additive-silica surface interactions. Previous interpretations of these interactions are almost universally based on interfacial charge matching and/or hydrogen bonding. In this study, we analyzed the surface chemistry of bioinspired silica (BIS) materials using solid-state nuclear magnetic resonance (NMR) spectroscopy as a function of the organic additive concentration. We found significant additional association between the additives and fully condensed (Q4) silicon species compared to industrial silica materials, leading to more overall Q4 concentration and higher hydrothermal stability, despite BIS having a shorter synthesis time. We posit that the polyfunctionality and catalytic activity of additives in the BIS synthesis lead to both of these surface phenomena, contrasting previous studies on monofunctional surfactants used in most other artificial templated silica syntheses. From this, we propose that additive polyfunctionality can be used to generate tailored artificial surfaces in situ and provide insights into the process of biosintering in biosilica systems, highlighting the need for more in-depth simulations on interfacial interactions at silica surfaces.

GUAIFENESIN MODIFIES AIRWAY MUCUS, REDUCES MUCUS-CELL SURFACE INTERACTIONS, AND INCREASES MUCOCILIARY TRANSPORT IN AN ORGANOTYPIC IN VITRO MODEL OF MUCUS HYPERSECRETION

SESSION TITLE: Phenotyping and Management of COPD SESSION TYPE: Original Investigations PRESENTED ON: October 18-21, 2020 PURPOSE: Airway mucus hypersecretion is a hallmark of chronic bronchitis (CB) and is strongly associated with cough discomfort, reduced pulmonary function, recurring respiratory infections, and the development of obstructive lung disease. Guaifenesin (GGE) is currently the only FDA-approved oral expectorant and the most common active ingredient in prescription and over-the-counter medicines for treatment of acute respiratory congestion and stable CB. Studies in patients with CB demonstrate that GGE reduces airway secretion viscoelasticity and mucus adhesion, inhibits cough reflex sensitivity, and increases mucociliary clearance (MCC). However, despite its broad use, the mechanism of action for GGE remains unclear. Here, we investigate whether GGE acts directly on the respiratory epithelium to modify epithelial cell secretions, reduce mucus/mucosa interactions, and enhance mucociliary transport (MCT) using an organotypic in vitro model of mucus hypersecretion. METHODS: Primary human bronchial epithelial cells (hBECs) were collected from healthy (HC), heavy smoker (HS; 48.3±18.9 pack year), and COPD (51.3±10.3 pack year) donor lungs (N=3 per group), and differentiated at an air-liquid interface to a mucociliary phenotype. At culture day 35, 20µg/ml GGE was supplemented to the basolateral medium and cultures were maintained in the presence of drug for an additional 15 days. Apical secretions were allowed to accumulate for the duration of the 15-day treatment. Following treatment, mucus concentration, mucus/mucosa interaction strength, in vitro MCT, ciliary beat frequency (CBF), and mucin protein expression were compared. RESULTS: Apical secretions collected from HS and COPD cultures had a higher mucus concentration (12.3±3.0 and 11.1±2.2 % solids, respectively) than HC cultures (7.6±1.1). Moreover, increased mucus concentration was associated with greater mucus/mucosa interaction strength and lower baseline MCT as compared to HC. In all donor groups, 15-day GGE treatment reduced mucus concentration (? % solids: 2.2±1.2 HC, 1.9±0.6 HS, and 2.4±1.8 COPD) and mucus/mucosa interaction strength (? mN/cm: 1.8±0.4 HC, 3.6±1.9 HS, and 1.6±0.4 COPD). GGE-mediated reductions in mucus concentration and mucus/mucosa interaction strength translated to increased in vitro MCT (? µm/sec: 35.1±21.9 HC, 19.6±30.1 HS, and 4.7±0.8 COPD). GGE treatment did not significantly alter MUC5AC or MUC5B secreted protein levels, or CBF. CONCLUSIONS: Our findings indicate that GGE significantly reduces airway mucus concentration, reduces mucus-cell surface interaction strength, and increases MCT in vitro through direct interactions with airways epithelial cells in all donor groups (HC, HS, and COPD). CLINICAL IMPLICATIONS: These data suggest that GGE may act directly at the level of the respiratory epithelium to modify airway mucus and improve MCC in conditions characterized by mucus hypersecretions. DISCLOSURES: Global Medical Lead Respiratory relationship with Reckitt Benckiser Please note: $1001 - $5000 Added 04/10/2020 by Graça Maria Alves da Cunha Coutinho, source=Web Response, value=Salary No relevant relationships by Brian Button, source=Web Response Receive research support relationship with Reckitt Benckiser Please note: >$100000 Added 04/09/2020 by Phillip Clapp, source=Web Response, value=Grant/Research Support No relevant relationships by Miriam Figueira, source=Web Response no disclosure on file for Ly Nguyen; Employee relationship with Rekitt Benckiser Please note: >$100000 Added 06/11/2020 by Karin Nicholson, source=Web Response, value=Salary No relevant relationships by Timother Shea, source=Web Response No relevant relationships by Holly Thacker, source=Web Response

Development of empirical relationships for surface accommodation coefficients through investigation of nano-poiseuille flows using molecular dynamics method

In this work, we have studied the effects of macroscopic (temperature of gas and wall atoms; velocity of gas near the wall) and microscopic properties (energy well-depth parameter, $$\epsilon$$ , used in Lennard–Jones potential to describe gas–surface interaction strength) on both energy accommodation coefficients (EAC) and tangential momentum accommodation coefficient (TMAC) using molecular dynamics approach. The effect of aforementioned properties is studied through classical force-driven nano-Poiseuille flow using argon gas and platinum walls. Parameters like temperature of wall, external force acting on gas atoms, and energy well depth ( $$\epsilon$$ ) between gas and surface are varied systematically to study the effects of these parameters on accommodation coefficients. Empirical relationship between the accommodation coefficients (TMAC and EAC) and the above-mentioned properties is obtained by performing non-linear regression analysis.

Impact of Surfactants and Stabilizers on Palladium Nanoparticle–Hydrogen Interaction Kinetics: Implications for Hydrogen Sensors

Surfactants and stabilizers are always present on the surfaces of colloidal nanocrystals due to their critical function in promoting selective facet growth and since they are essential to prevent aggregate formation in solution. After synthesis, however, the presence of these molecules on the surface of a nanocrystal is problematic because they potentially significantly alter the nature of the interaction with the environment, which is critical for sensor or catalysis applications. Here, we quantitatively scrutinize this effect experimentally for the four most common stabilizers in Pd nanoparticle synthesis: cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide (TOAB), cetyltrimethylammonium chloride (CTAC), and poly(vinylpyrrolidone) (PVP). We use the surface-catalyzed hydrogen sorption and hydride formation reaction in Pd as a model system, due to its high relevance for hydrogen sensors. Specifically, we map in detail the (de)hydrogenation kinetics of arrays of nanofabricated Pd nanodisks in the presence of the surfactants and benchmark it with an uncoated Pd reference. As the key results, we find that the cationic surfactants significantly decelerate the (de)hydrogenation surface reaction, with the amplitude of deceleration mediated by the interplay between the halide-ion–Pd surface interaction strength and surfactant surface density. In contrast, a polymeric PVP coating is found to significantly accelerate hydrogen sorption. For the Pd-based hydrogen sensor application, our findings thus provide important insights for the appropriate choice of a surfactant to minimize the negative impact on hydrogen sorption kinetics and thus hydrogen detection response/recovery times. In a wider perspective, our results dramatically show how nanoparticles can attain different properties depending on what types of surfactants and stabilizers are present on their surface and how critical the quantitative understanding of their impact is for a specific application.

Nanolamellar Tantalum Interfaces in the Osteoblast Adhesion.

The design of topographically patterned surfaces is considered to be a preferable approach for influencing cellular behavior in a controllable manner, in particular to improve the osteogenic ability of bone regeneration. In this study, we fabricated nanolamellar tantalum (Ta) surfaces with lamellar wall thicknesses of 40 and 70 nm. The cells attached to nanolamellar Ta surfaces exhibited higher protein adsorption and expression of β1 integrin, as compared to the nonstructured bulk Ta, which facilitated the initial cell attachment and spreading. We thus, as expected, observed significantly enhanced osteoblast adhesion, growth, and alkaline phosphatase activity on nanolamellar Ta surfaces. However, the beneficial effects of nanolamellar structures on osteogenesis became weaker as the lamellar wall thickness increased. The interaction between cells and Ta surfaces was examined through adhesion forces using atomic force microscopy. Our findings indicated that the Ta surface with a lamellar wall thickness of 40 nm exhibited the strongest stimulatory effect. The observed strongest adhesion force between the cell-attached tip and the Ta surface with a 40 nm thick lamellar wall encouraged the much stronger binding of cells with the surface and thus well-attached, -stretched, and -grown cells. We attributed this to the increase in the available contact area of cells with the thinner nanolamellar Ta surface. The increased contact area allowed the enhancement of the cell surface interaction strength and, thus, improved osteoblast adhesion. This study suggests that the thin nanolamellar topography shows immense potential in improving the clinical performance of dental and orthopedic implants.

Roles of Surface Energy and Temperature in Heterogeneous Ice Nucleation

Heterogeneous ice nucleation (HIN) is strongly related to the dynamics of hydrogen bonds in water at an interface. In this work, we investigate the microscopic kinetics of HIN through molecular dynamics simulations. The dynamics of hydrogen bond network (HBN) at interfaces is studied under the coupled effects of thermal fluctuation and water–surface molecular interactions. It is revealed that the lasting time of the HBN at the interface is critical to HIN. Under comparable thermal and surface effects, which result in a proper lasting time of the HBN, HIN is promoted. However, if the thermal effect or the surface effect dominates over the other, the lasting time of the HBN at the interface would be either too long or too short, leading to the failure of HIN. By varying the water–surface interaction strength, i.e., binding energy, and temperature, a diagram of HIN events is presented, which shows that HIN is only favored in certain temperature and surface energy ranges.

Adsorption of dihalogen molecules on pristine graphene surface: Monte Carlo and molecular dynamics simulation studies.

Weak interactions of graphene surface with reactive molecular impurities are the subject of many studies since noncovalent functionalization of surface via molecular doping is a powerful tool for tuning the electronic properties of graphene layers. In this work, the adsorption of diatomic halogen gas molecules, F2, Cl2, Br2, I2 onto bilayer and multilayer pristine graphene surfaces were studied comparatively by Monte Carlo (MC) and molecular dynamics (MD) simulation techniques in canonical ensemble. The adsorption sites, adsorption capacity, coverage factors, adsorption isotherms, and adsorption kinetics were investigated and the adsorption energies were calculated for all adsorbates. Graphene was modeled as a two-dimensional layer of 200 carbon atoms in a honeycomb arrangement. The COMPASS force field was used in the simulations. The adsorption isotherms were obtained and fitted to Langmuir model. The kinetics of adsorption was studied and found to be first order. Both the monolayer and the multilayer adsorption of halogen molecules showed that van der Waals volumes of halogen molecules and also their polarizabilities display a competitive role in the saturation capacity and the strength of surface interactions.

Catalytic aspects of metallophthalocyanines adsorbed on gold-electrode. Theoretical exploration of the binding nature role.

The need of deeper insights regarding the inner working of catalysts represents a current challenge in the search of new ways to tune their activities towards new chemical transformations. Within this field, metallophthalocyanines-based (MPc) electrocatalysis has gained tremendous attention due to their versatility, low cost, great stability and excellent turn-over properties. In this concern, here we present a quantum chemical study of the formation of supramolecular complexes based on the adsorption of MPcs on gold substrates, and the effect of the substrate on their electrocatalytic properties. For this purpose, we used iron- (FePc), cobalt- (CoPc) and copper-phthalocyanines (CuPc). To model the gold surface we used two gold clusters of different sizes, given by Au26 and Au58 accounting for gold electrode Au(111) surface. Thus, both electronic and binding strength features of the adsorption process between the complexes were analyzed in detail in order to gain a deeper description of the nature of the MPc-Au(111) formation, by using Density Functional Theory (DFT) calculations, at the PBE and TPSS levels including the dispersive contribution according to the Grimme approach (D3). Our results show that dispersion forces rule the MPc-gold interaction, with binding strengths ranging between 61 and 153 kcal mol-1, in agreement to the reported experimental data. To provide a detailed picture of our findings we used the non-covalent interactions index (NCIs) analysis, which offers additional chemical insights regarding the forces that control their interaction strength. Finally, our calculations revealed that among the three MPcs, CuPc required less energy for its oxidation. However, the removal of the electron involves a tremendous decrease of the MPc-gold surface interaction strength thus suggesting its desorption, which would prevent the required reversibility of the redox reaction, explaining its low performance observed experimentally.

Impact of surface interactions on the phase behavior of Y-shaped molecules

In recent years the statistical mechanics of non-spherical molecules, such as polypeptide chains and protein molecules, has garnered considerable attention as the phase behavior of these systems has important scientific and health implications. More recently, it has been realized that surface binding may have a considerable impact on this behavior. With this in mind, we examine here the role of surface interactions on the phase behavior of Y-shaped molecules (a simplified model of immunoglobulin) using grand-canonical Monte Carlo simulation. In particular, we investigate the critical behavior of a system of such molecules on a hexagonal lattice using histogram reweighting, multicanonical sampling, and finite-size scaling. After obtaining the critical properties of the three-dimensional bulk system, we investigate the impact of a patterned solid surface on these properties as a function of patterning geometry and surface interaction strength. Our results suggest avenues for tailoring phase behavior by selectively controlling surface characteristics.

ChemInform Abstract: Ferroelectric‐Based Catalysis: Switchable Surface Chemistry

AbstractReview: 138 refs.