Pore Structure Compartmentalization for Advanced Characterization of Metal-Organic Framework Materials.

Metal-organic frameworks (MOFs) are nanoporous crystals which are widely used as selective adsorbents, separation membranes, catalysts, gas and energy storage media, and drug delivery vehicles. The unique adsorption and transport properties of MOFs are determined by their complex three-dimensional (3D) networks of pores, cages, and channels that differ in size, shape, and chemical composition. While the morphological structure of MOF crystals is known, practical MOF materials are rarely ideal crystals. They contain secondary phases, binders, residual chemicals, and various types of defects. It is of paramount importance to evaluate the degree of crystallinity and accessibility of different pore compartments to adsorb guest molecules. To this end, we recently suggested the method of fingerprint isotherms based on the comparison of the experimentally measured adsorption isotherms and theoretical isotherms on ideal MOF crystals produced by Monte Carlo (MC) simulations and decomposed with respect to different pore compartments [Parashar, S. ACS Appl. Nano Mater. 2021, 4, 5531-5540 and Dantas, S.; Neimark, A. V. ACS Appl. Mater. Interfaces 2020, 12, 15595-15605]. In this work, we develop an automated algorithm for pore network compartmentalization that is a prerequisite for calculations of the fingerprint isotherms. The proposed algorithm partitions the unit cell into realistically shaped compartments based on the geometric pore size distribution. The proposed method is demonstrated on several characteristic systems, including Cu-BTC, IRMOF-1, UiO-66, PCN-224, ZIF-412, and 56 structures from the CoRE MOF database.

Surface-Constrained Metropolis Monte Carlo: Simulation of Reactions on Triply Periodic Minimal Surfaces.

Triply periodic minimal surfaces (TPMS) inspired by nature serve as a foundation for developing novel nanomaterials, such as templated silicas, graphene sponges, and schwarzites, with customizable optical, poroelastic, adsorptive, catalytic, and other properties. Computer simulations of reactions on TPMS using reactive intermolecular potentials hold great promise for constructing and screening potential TPMS with the desired properties. Here, we developed an off-lattice, surface-constrained Metropolis Monte Carlo (SC-MMC) algorithm that utilized a temperature quench process. The presented SC-MMC algorithm was used to investigate the process of graphitization reactions on the Schwarz primitive, Schwarz diamond, and Schoen gyroid TPMS, all with a cubic lattice parameter of 8 nm. We show that the optimized carbon TPMS exhibits a low energy, approximately -7.1 eV/atom, comparable to that of graphite and diamond crystals, along with a variety of topological defects. Furthermore, these structures showcase extensive and smooth surfaces characterized by a negative discrete Gaussian curvature, a distinctive feature indicative of an interconnected morphology. They possess specific surface areas of ∼2700 m2/g, comparable to graphene, and exhibit a significant porosity of around 90%. The theoretical X-ray correlation functions and nitrogen adsorption isotherms confirm that the constructed TPMS exhibit remarkably similar surface properties, although the pore space topology varies significantly.

Extra adsorption and adsorbate superlattice formation in metal-organic frameworks.

Metal-organic frameworks (MOFs) have a high internal surface area and widely tunable composition, which make them useful for applications involving adsorption, such as hydrogen, methane or carbon dioxide storage. The selectivity and uptake capacity of the adsorption process are determined by interactions involving the adsorbates and their porous host materials. But, although the interactions of adsorbate molecules with the internal MOF surface and also amongst themselves within individual pores have been extensively studied, adsorbate-adsorbate interactions across pore walls have not been explored. Here we show that local strain in the MOF, induced by pore filling, can give rise to collective and long-range adsorbate-adsorbate interactions and the formation of adsorbate superlattices that extend beyond an original MOF unit cell. Specifically, we use in situ small-angle X-ray scattering to track and map the distribution and ordering of adsorbate molecules in five members of the mesoporous MOF-74 series along entire adsorption-desorption isotherms. We find in all cases that the capillary condensation that fills the pores gives rise to the formation of 'extra adsorption domains'-that is, domains spanning several neighbouring pores, which have a higher adsorbate density than non-domain pores. In the case of one MOF, IRMOF-74-V-hex, these domains form a superlattice structure that is difficult to reconcile with the prevailing view of pore-filling as a stochastic process. The visualization of the adsorption process provided by our data, with clear evidence for initial adsorbate aggregation in distinct domains and ordering before an even distribution is finally reached, should help to improve our understanding of this process and may thereby improve our ability to exploit it practically.

Modeling Gas-Liquid Interfaces by Dissipative Particle Dynamics: Adsorption and Surface Tension of Cetyl Trimethyl Ammonium Bromide at the Air-Water Interface.

Adsorption of surfactants at gas-liquid interfaces that causes reduction in the surface tension is a classical problem in colloid and interface science with multiple practical applications in oil and gas recovery, separations, cosmetics, personal care, and biomedicine. Here, we develop an original coarse-grained model of the liquid-gas interface within the conventional dissipative particle dynamics (DPD) framework with the goal of quantitatively predicting the surface tension in the presence of surfactants. As a practical case-study example, we explore the adsorption of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) on the air-water interface. The gas phase is modeled as a DPD fluid composed of fictitious hard-core "gas" beads with exponentially decaying repulsive potentials to prevent penetration of the liquid phase components. A rigorous parametrization scheme is proposed based on matching the bulk and interfacial properties of water and octane taken as the reference compounds. Quantitative agreement between the simulated and experimental surface tension of CTAB solutions is found for a wide range of bulk surfactant concentrations (∼10-3 to ∼1 mmol/L) with the reduction of the surface tension from ∼72 mN/m (pure water) to the limiting value of ∼37.5 mN/m at the critical micelle concentration. The gas phase DPD model with the proposed parametrization scheme can be extended and applied to modeling various gas-liquid interfaces with surfactant and lipid monolayers, such as bubble suspensions, foams, froths, etc.

Deciphering the Relations between Pore Structure and Adsorption Behavior in Metal-Organic Frameworks: Unexpected Lessons from Argon Adsorption on Copper-Benzene-1,3,5-tricarboxylate.

Consistent adsorption characterization of metal-organic frameworks (MOFs) is imperative for their wider adoption in industry and practical applications. Current approaches are based on the conventional intuitive representation of MOF pore space as a regular network of pore compartments (cages and channels), adsorption in which occurs independently according to their geometric dimensions. Here, we demonstrate that this conventional approach is unable to describe even qualitatively the shape of Ar adsorption isotherms on hydrated and dehydrated Cu-BTC structures, one of the most well-known MOF materials. A combination of geometric characterization of MOF crystallographic structure, molecular simulation, and virtual visualization of the adsorption process reveals that the filling of the adjacent pore compartments proceeds in parallel in a complex cooperative fashion. The proposed synergistic approach helps us to understand the relations between pore structure geometric and chemical features and adsorption behavior, laying down a foundation for improved methods for MOF characterization.

Phase Behavior and Capillary Condensation Hysteresis of Carbon Dioxide in Mesopores.

Carbon dioxide adsorption on micro- and mesoporous carbonaceous materials in a wide range of temperatures and pressures is of great importance for the problems of gas separations, greenhouse gas capture and sequestration, enhanced hydrocarbon recovery from shales and coals, as well as for the characterization of nanoporous materials using CO2 as a molecular probe. We investigate the influence of temperature on CO2 adsorption focusing on the capillary condensation and hysteresis phenomena. We present experimental data on the adsorption of CO2 on CMK-3, ordered carbon with mesopores of ∼5-6 nm, at various temperatures (185-273 K) and pressures (up to 35 bars). Using Monte Carlo (MC) simulations in the grand canonical and mesocanonical ensembles, we attempt to predict the transition from reversible capillary condensation to hysteretic adsorption-desorption cycles that is experimentally observed with the decrease of temperature. We show that although the desorption at all temperatures occurs at the conditions of pore vapor-liquid equilibrium, the capillary condensation is a nucleation-driven process associated with an effective energy barrier of ∼43 kT, specific to the sample used in this work. This barrier can be overcome at the equilibrium conditions in the region of reversible condensation at temperatures higher than 240 K. At lower temperatures, the regime of developing hysteresis is observed with progressively widening hysteresis loops. The position of capillary condensation transition is estimated using the pressure dependence of the energy barrier calculated by the thermodynamic integration of the van der Waals-type continuous canonical isotherm simulated with the gauge cell MC method. These findings lay the foundation for developing kernels of CO2 adsorption and desorption isotherm for calculating the pore size distribution in the entire range of micropore and mesopore sizes from one high-pressure experimental isotherm.

Mechanical Characterization of Hierarchical Structured Porous Silica by in Situ Dilatometry Measurements during Gas Adsorption.

Mechanical properties of hierarchically structured nanoporous materials are determined by the solid phase stiffness and the pore network morphology. We analyze the mechanical stiffness of hierarchically structured silica monoliths synthesized via a sol-gel process, which possess a macroporous scaffold built of interconnected struts with hexagonally ordered cylindrical mesopores. We consider samples with and without microporosity within the mesopore walls and analyze them on the macroscopic level as well as on the microscopic level of the mesopores. Untreated as-prepared samples still containing some organic components and the respective calcined and sintered counterparts of varying microporosity are investigated. To determine Young's moduli on the level of the macroscopic monoliths, we apply ultrasonic run time measurements, while Young's moduli of the mesopore walls are obtained by analysis of the in situ strain isotherms during N2 adsorption at 77 K. For the latter, we extended our previously reported theoretical approach for this type of materials by incorporating the micropore effects, which are clearly not negligible in the calcined and most of the sintered samples. The comparison of the macro- and microscopic Young's moduli reveals that both properties follow essentially the same trends, that is, calcination and sintering increase the mechanical stiffness on both levels. Consequently, stiffening of the monolithic samples can be primarily attributed to stiffening of the backbone material which is consistent with the fact that the morphology on the mesopore level is mainly preserved with the post-treatments applied.

In Situ Small-Angle Neutron Scattering Investigation of Adsorption-Induced Deformation in Silica with Hierarchical Porosity.

Adsorption-induced deformation of a series of silica samples with hierarchical porosity has been studied by in situ small-angle neutron scattering (SANS) and in situ dilatometry. Monolithic samples consisted of a disordered macroporous network of struts formed by a 2D lattice of hexagonally ordered cylindrical mesopores and disordered micropores within the mesopore walls. Strain isotherms were obtained at the mesopore level by analyzing the shift of the Bragg reflections from the ordered mesopore lattice in SANS data. Thus, SANS essentially measured the radial strain of the cylindrical mesopores including the volume changes of the mesopore walls due to micropore deformation. A H2O/D2O adsorbate with net zero coherent neutron scattering length density was employed in order to avoid apparent strain effects due to intensity changes during pore filling. In contrast to SANS, the strain isotherms obtained from in situ dilatometry result from a combination of axial and radial mesopore deformation together with micropore deformation. Strain data were quantitatively analyzed with a theoretical model for micro-/mesopore deformation by combining information from nitrogen and water adsorption isotherms to estimate the water-silica interaction. It was shown that in situ SANS provides complementary information to dilatometry and allows for a quantitative estimate of the elastic properties of the mesopore walls from water adsorption.

Adhesion and Separation of Nanoparticles on Polymer-Grafted Porous Substrates.

This work explores interactions of functionalized nanoparticles (NP) with polymer brushes (PB) in a binary mixture of good and poor solvents. NP-PB systems are used in multiple applications, and we are particularly interested in the problem of chromatographic separation of NPs on polymer-grafted porous columns. This process involves NP flow through the pore channels with walls covered by PBs. NP-PB adhesion is governed by adsorption of polymer chains to NP surface and entropic repulsion caused by the polymer chain confinement between NP and the channel wall. Both factors depend on the solvent composition, variation of which causes contraction or expansion of PB. Using dissipative particle dynamics simulations in conjunction with the ghost tweezers free energy calculation technique, we examine the free energy landscapes of functionalized NPs within PB-grafted channels depending on the solvent composition at different PB grafting densities and polymer-solvent affinities. The free energy landscape determines the probability of NP location at a given distance to the surface, positions of equilibrium adhesion states, and the Henry constant that characterizes adsorption equilibrium and NP partitioning between the stationary phase of PB and mobile phase of flowing solvent. We analyze NP transport through a polymer-grafted channel and calculate the mean velocity and retention time of NP depending on the NP size and solvent composition. We find that, with the increase of the bad (poor) solvent fraction and respective PB contraction, NP separation exhibits a transition from the hydrodynamic size exclusion regime with larger NPs having shorter retention time to the adsorption regime with smaller NPs having shorter retention time. The observed reversal of the sequence of elution is reminiscent of the critical condition in polymer chromatography at which the retention time is molecular weight independent. This finding suggests the possibility of the existence of an analogous special regime in nanoparticle chromatography at which NPs with like surface properties elute together regardless of their size. The latter has important practical implications: NPs can be separated by surface chemistry rather than by their size employing the gradient mode of elution with controlled variation of solvent composition.

Phenol Molecular Sheets Woven by Water Cavities in Hydrophobic Slit Nanospaces.

Despite extensive research over the last several decades, the microscopic characterization of topological phases of adsorbed phenol from aqueous solutions in carbon micropores (pore size < 2.0 nm), which are believed to exhibit a solid and quasi-solid character, has not been reported. Here, we present a combined experimental and molecular level study of phenol adsorption from neutral water solutions in graphitic carbon micropores. Theoretical and experimental results show high adsorption of phenol and negligible coadsorption of water in hydrophobic graphitic micropores (super-sieving effect). Graphic processing unit-accelerated molecular dynamics simulation of phenol adsorption from water solutions in a realistic model of carbon micropores reveal the formation of two-dimensional phenol crystals with a peculiar pattern of hydrophilic-hydrophobic stripes in 0.8 nm supermicropores. In wider micropores, disordered phenol assemblies with water clusters, linear chains, and cavities of various sizes are found. The highest surface density of phenol is computed in 1.8 nm supermicropores. The percolating water cluster spanning the entire pore space is found in 2.0 nm supermicropores. Our findings open the door for the design of better materials for purification of aqueous solutions from nonelectrolyte micropollution.

Coarse-grained model of nanoscale segregation, water diffusion, and proton transport in Nafion membranes.

We present a coarse-grained model of the acid form of Nafion membrane that explicitly includes proton transport. This model is based on a soft-core bead representation of the polymer implemented into the dissipative particle dynamics (DPD) simulation framework. The proton is introduced as a separate charged bead that forms dissociable Morse bonds with water beads. Morse bond formation and breakup artificially mimics the Grotthuss hopping mechanism of proton transport. The proposed DPD model is parameterized to account for the specifics of the conformations and flexibility of the Nafion backbone and sidechains; it treats electrostatic interactions in the smeared charge approximation. The simulation results qualitatively, and in many respects quantitatively, predict the specifics of nanoscale segregation in the hydrated Nafion membrane into hydrophobic and hydrophilic subphases, water diffusion, and proton mobility. As the hydration level increases, the hydrophilic subphase exhibits a percolation transition from a collection of isolated water clusters to a 3D network of pores filled with water embedded in the hydrophobic matrix. The segregated morphology is characterized in terms of the pore size distribution with the average size growing with hydration from ∼1 to ∼4 nm. Comparison of the predicted water diffusivity with the experimental data taken from different sources shows good agreement at high and moderate hydration and substantial deviation at low hydration, around and below the percolation threshold. This discrepancy is attributed to the dynamic percolation effects of formation and rupture of merging bridges between the water clusters, which become progressively important at low hydration, when the coarse-grained model is unable to mimic the fine structure of water network that includes singe molecule bridges. Selected simulations of water diffusion are performed for the alkali metal substituted membrane which demonstrate the effects of the counter-ions on membrane self-assembly and transport. The hydration dependence of the proton diffusivity reproduces semi-qualitatively the trend of the diverse experimental data, showing a sharp decrease around the percolation threshold. Overall, the proposed model opens up an opportunity to study self-assembly and water and proton transport in polyelectrolytes using computationally efficient DPD simulations, and, with further refinement, it may become a practical tool for theory informed design and optimization of perm-selective and ion-conducting membranes with improved properties.

Adhesion of Phospholipid Bilayers to Hydroxylated Silica: Existence of Nanometer-Thick Water Interlayers.

Lipid bilayers attached to solid surfaces play an important role in bioinspired materials and devices and serve as model systems for studies of interactions of cell membranes with particles and biomolecules. Despite active experimental and theoretical studies, the interactions of lipid membranes with solid substrates are still poorly understood. In this work, we explore, using atomistic molecular dynamics simulations, the equilibrium and stability of a phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine membrane supported on hydroxylated amorphous silica. We reveal two distinct types of thermodynamically stable states, characterized by different widths of the water layer between the membrane and the substrate. In α-states, the membrane is closely attached with the lipid head groups interacting directly with surface hydroxyls; however, because of the molecular level roughness of the amorphous silica surface, there exists an inhomogeneous water layer trapped between the substrate and the membrane. In ß-states, the membrane is separated from the silica surface by a water film of ∼2.5 nm in thickness. The thermodynamic equilibrium is quantified in terms of the disjoining pressure isotherm as a function of membrane-substrate separation, which has a double sigmoidal shape with two minima and one maximum, which correspond to the limits of stability of α- and ß-states. The thermodynamic properties and bilayer structure are compared with experimental findings and simulation results for relevant systems.

Determination of Isosteric Heat of Adsorption by Quenched Solid Density Functional Theory.

The heat of adsorption is one of the most important parameters characterizing energetic heterogeneity of the adsorbent surface. Heats of adsorption are either determined directly by calorimetry or calculated from adsorption isotherms measured at different temperatures using the thermodynamic Clausius-Clapeyron equation. Here, we present a method for calculating the isosteric heat of adsorption that requires as input only a single adsorption isotherm measured at one temperature. The proposed method is implemented with either nonlocal (NLDFT) or quenched solid (QSDFT) density functional theory models of adsorption that are currently widely used for calculating pore size distributions in various micro- and mesoporous solids. The pore size distribution determined from the same experimental isotherm is used for predicting the isosteric heat. The QSDFT method has advantages of taking into account two factors contributing to the structural heterogeneity of adsorbents: the molecular level roughness of the surface and the pore size distribution. The method is illustrated with examples of low temperature nitrogen and argon adsorption on selected samples of carbons of different degree of graphitization and MCM-41 mesoporous silicas of different pore size. The isosteric heat predictions from the NLDFT and QSDFT methods are compared against relevant experiments and the results of Monte Carlo (MC) simulations, with good agreement found in the cases where the surface model adequately reflects the pore surface roughness. Analyses with the QSDFT method show that the isosteric heat of adsorption significantly depends of the molecular level roughness of the adsorbent surface, which is ignored in NLDFT and MC models. The proposed QSDFT method with further verification can be used for calculating the isosteric heat as an additional parameter characterizing the adsorbent surface in parallel with routine calculations of the pore size distribution from a single adsorption isotherm.

Adsorption-Induced Deformation of Hierarchically Structured Mesoporous Silica-Effect of Pore-Level Anisotropy.

The goal of this work is to understand adsorption-induced deformation of hierarchically structured porous silica exhibiting well-defined cylindrical mesopores. For this purpose, we performed an in situ dilatometry measurement on a calcined and sintered monolithic silica sample during the adsorption of N2 at 77 K. To analyze the experimental data, we extended the adsorption stress model to account for the anisotropy of cylindrical mesopores, i.e., we explicitly derived the adsorption stress tensor components in the axial and radial direction of the pore. For quantitative predictions of stresses and strains, we applied the theoretical framework of Derjaguin, Broekhoff, and de Boer for adsorption in mesopores and two mechanical models of silica rods with axially aligned pore channels: an idealized cylindrical tube model, which can be described analytically, and an ordered hexagonal array of cylindrical mesopores, whose mechanical response to adsorption stress was evaluated by 3D finite element calculations. The adsorption-induced strains predicted by both mechanical models are in good quantitative agreement making the cylindrical tube the preferable model for adsorption-induced strains due to its simple analytical nature. The theoretical results are compared with the in situ dilatometry data on a hierarchically structured silica monolith composed by a network of mesoporous struts of MCM-41 type morphology. Analyzing the experimental adsorption and strain data with the proposed theoretical framework, we find the adsorption-induced deformation of the monolithic sample being reasonably described by a superposition of axial and radial strains calculated on the mesopore level. The structural and mechanical parameters obtained from the model are in good agreement with expectations from independent measurements and literature, respectively.

Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths.

Fabrication of nanographene shows a promising route for production of designed porous carbons, which is indispensable for highly efficient molecular separation and energy storage applications. This process requires a better understanding of the mechanical properties of nanographene in their aggregated structure. We studied the structural and mechanical properties of nanographene monoliths compressed at 43 MPa over different times from 3 to 25 h. While in monoliths compressed over shorter time adsorption isotherms of Ar at 87 K or N2 at 77 K exhibited a prominent hysteresis due to presence of predominant mesopores, compression for long time induces a low pressure hysteresis. On the other hand, compression for 25 h increases the microporosity evaluated by Ar adsorption, not by N2 adsorption, indicating that 25 h compression rearranges the nanographene stacking structure to produce ultramicropores that can be accessible only for Ar. TEM, X-ray diffraction, and Raman spectroscopic studies indicated that the compression for 25 h unfolds double-bent-like structures, relaxing the unstable nanographene stacked structure formed on the initial compression without nanographene sheets collapse. This behavior stems from the highly elastic nature of the nanographenes.

Deformation of Microporous Carbons during N2, Ar, and CO2 Adsorption: Insight from the Density Functional Theory.

Using the nonlocal density functional theory, we investigate adsorption of N2 (77 K), Ar (77 K), and CO2 (273 K) and respective adsorption-induced deformation of microporous carbons. We show that the smallest micropores comparable in size and even smaller than the nominal molecular diameter of the adsorbate contribute significantly to the development of the adsorption stress. While pores of approximately the nominal adsorbate diameter exhibit no adsorption stress regardless of their filling level, the smaller pores cause expansive adsorption stresses up to almost 4 GPa. Accounting for this effect, we determined the pore-size distribution of a synthetic microporous carbon by simultaneously fitting its experimental CO2 adsorption isotherm (273 K) and corresponding adsorption-induced strain measured by in situ dilatometry. Based on the pore-size distribution and the elastic modulus fitted from CO2 data, we predicted the sample's strain isotherms during N2 and Ar adsorption (77 K), which were found to be in reasonable agreement with respective experimental data. The comparison of calculations and experimental results suggests that adsorption-induced deformation caused by micropores is not limited to the low relative pressures typically associated with the micropore filling, but is effective over the whole relative pressure range up to saturation pressure.

Coarse-grained model of water diffusion and proton conductivity in hydrated polyelectrolyte membrane.

Using dissipative particle dynamics (DPD), we simulate nanoscale segregation, water diffusion, and proton conductivity in hydrated sulfonated polystyrene (sPS). We employ a novel model [Lee et al. J. Chem. Theory Comput. 11(9), 4395-4403 (2015)] that incorporates protonation/deprotonation equilibria into DPD simulations. The polymer and water are modeled by coarse-grained beads interacting via short-range soft repulsion and smeared charge electrostatic potentials. The proton is introduced as a separate charged bead that forms dissociable Morse bonds with the base beads representing water and sulfonate anions. Morse bond formation and breakup artificially mimics the Grotthuss mechanism of proton hopping between the bases. The DPD model is parameterized by matching the proton mobility in bulk water, dissociation constant of benzenesulfonic acid, and liquid-liquid equilibrium of water-ethylbenzene solutions. The DPD simulations semi-quantitatively predict nanoscale segregation in the hydrated sPS into hydrophobic and hydrophilic subphases, water self-diffusion, and proton mobility. As the hydration level increases, the hydrophilic subphase exhibits a percolation transition from isolated water clusters to a 3D network. The analysis of hydrophilic subphase connectivity and water diffusion demonstrates the importance of the dynamic percolation effect of formation and breakup of temporary junctions between water clusters. The proposed DPD model qualitatively predicts the ratio of proton to water self-diffusion and its dependence on the hydration level that is in reasonable agreement with experiments.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes.

This study describes a novel approach for the in situ synthesis of metal oxide-polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high-resolution TEM, SEM/EDX, UV-vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10-20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.

Deformation of Microporous Carbon during Adsorption of Nitrogen, Argon, Carbon Dioxide, and Water Studied by in Situ Dilatometry.

Adsorption-induced deformation of a monolithic, synthetic carbon of clearly distinguishable micro- and mesoporosity was analyzed by in situ dilatometry with N2 (77 K), Ar (77 K), CO2 (273 K), and H2O (298 K). A characteristic nonmonotonic shape of the strain isotherm showing contraction of the sample at initial micropore adsorption followed by expansion toward completion of micropore filling was found for all adsorbates. However, the extent of contraction and expansion varied significantly with the adsorbate type. The deformation differences observed were compared with the density ratio of the adsorbates within the micropores and the respective unconfined fluids. In particular, CO2 caused the least contraction of the sample, while in parallel adsorbed CO2 molecules were predicted to be considerably compacted inside carbon micropores compared to bulk liquid CO2. On the contrary, the packing of H2O molecules within carbon micropores is less dense than in the bulk liquid and adsorption of H2O produced the most pronounced contraction. N2 and Ar, both exhibiting essentially the same densities in adsorbed and bulk liquid phase, induced very similar deformation of the sample. These findings support theoretical predictions, which correlate adsorption-induced deformation and packing of molecules adsorbed in micropores. Additionally for the first time, we demonstrated with the N2 strain isotherm the existence of two nonmonotonic stages of subsequent contraction and expansion in the regions of micropore and mesopore filling. This characteristic behavior is expected for any micro- and mesoporous material.

Adhesion of nanoparticles to polymer brushes studied with the ghost tweezers method.

Mechanisms of interactions between nanoparticles (NPs) and polymer brushes (PBs) are explored using dissipative particle dynamics simulations and an original "ghost tweezers" method that emulates lab experiments performed with optical or magnetic tweezers. The ghost tweezers method is employed to calculate the free energy of adhesion. Ghost tweezers represents a virtual harmonic potential, which tethers NP with a spring to a given anchor point. The average spring force represents the effective force of NP-PB interaction as a function of the NP coordinate. The free energy landscape of NP-PB interactions is calculated as the mechanical work needed to transfer NP from the solvent bulk to a particular distance from the substrate surface. With this technique, we explore the adhesion of bare and ligand-functionalized spherical NPs to polyisoprene natural rubber brush in acetone-benzene binary solvent. We examine two basic mechanisms of NP-PB interactions, NP adhesion at PB exterior and NP immersion into PB, which are governed by interplay between entropic repulsive forces and enthalpic attractive forces caused by polymer adsorption at the NP surface and ligand adsorption at the substrate. The relative free energies of the equilibrium adhesion states and the potential barriers separating these states are calculated at varying grafting density, NP size, and solvent composition.