Adsorption Of Organic Molecules Research Articles

Interplay between Supramolecularity and Substrate Symmetry in the Dehydrogenation of d-Alaninol on Cu(100) and Cu(110) Surfaces

The adsorption of organic chiral molecules on metallic substrates is widely studied as a tool to obtain chiral surfaces. The chirality transfer from the molecules to the surface is strongly driven by the availability of hydrogen atoms, which guides a specific chiral self-assembled structure. In this paper we report, by combination of photoelectron spectroscopy, low-energy electron diffraction, and density functional theory calculations, on the adsorption of the d-enantiomer of alaninol on Cu(100) and on Cu(110) with the aim of revealing dehydrogenation in the formation of the molecular chiral superstructure. We show that, on both surfaces, at low coverage alaninol is dehydrogenated at the hydroxyl group, whereas at saturated coverage the substrate symmetry, in combination with intermolecular interactions, induces partial amino group dehydrogenation on Cu(100) or inhibits hydroxyl group dehydrogenation on Cu(110).

Influence of the solvent on the stability of bis(terpyridine) structures on graphite.

The effect of solvation on the adsorption of organic molecules on graphite at room temperature has been addressed with force-field molecular dynamics simulations. As a model system, the solvation of a bis(terpyridine) isomer in water and 1,2,4-trichlorobenzene was studied with an explicit solvation model. The inclusion of solvation has a noticeable effect on adsorption energies. Although the results of the various considered force fields differ quite significantly, they all agree that the adsorption of BTP from the TCB solvent is almost thermoneutral. The substrate simply acts as a template to allow a planar arrangement of the network, which is stabilized by the intermolecular interaction. Using an atomic thermodynamics approach, the order of the stability of various network structures as a function of the chemical potential is derived yielding a sequence in agreement with the experiment.

Force Field for Tricalcium Silicate and Insight into Nanoscale Properties: Cleavage, Initial Hydration, and Adsorption of Organic Molecules

Improvements in the sustainability and durability of building materials depend on understanding interfacial properties of various mineral phases at the nanometer scale. Tricalcium silicate (C3S) is the major constituent of cement clinker and we present and validate a force field for atomistic simulations that provides excellent agreement with available experimental data, including X-ray structures, cleavage energies, elastic moduli, and IR spectra. Using this model and available measurements, we quantify key surface and interface properties of the dry and superficially hydrated mineral. An extensive set of possible cleavage planes shows cleavage energies in a range of 1300 to 1600 mJ/m2 that are consistent with the observation of faceted crystallites with an aspect ratio near one. Using pure and hydroxylated surface models that represent the first step in the hydration reaction, we examined the adsorption mechanism of several organic amines and alcohols at different temperatures. Strong attraction between −20 and −50 kcal/mol is found as a result of complexation of superficial calcium ions, electrostatic interactions, and hydrogen bonds on the ionic surface. Agglomeration of cleaved C3S surfaces in the absence of organic molecules was found to recover less than half the original cleavage energy (∼450 mJ/m2) associated with reduced Coulomb interactions between reconstructed surfaces. Additional adsorption of organic compounds below monolayer coverage reduced the attraction between even surfaces to less than 5% of the original cleavage energy (∼50 mJ/m2) related to their action as spacers between cleaved surfaces and mitigation of local electric fields. Computed agglomeration energies for a series of adsorbed organic compounds correlate with the reduction in surface forces in the form of measured grinding efficiencies. The force field is extensible to other cement phases and compatible with many platforms for molecular simulations (PCFF, COMPASS, CHARMM, AMBER, OPLS-AA, CVFF).

Adsorption of Small Organic Molecules on Graphene

We present a combined experimental and theoretical quantification of the adsorption enthalpies of seven organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate, hexane, and toluene) on graphene. Adsorption enthalpies were measured by inverse gas chromatography and ranged from -5.9 kcal/mol for dichloromethane to -13.5 kcal/mol for toluene. The strength of interaction between graphene and the organic molecules was estimated by density functional theory (PBE, B97D, M06-2X, and optB88-vdW), wave function theory (MP2, SCS(MI)-MP2, MP2.5, MP2.X, and CCSD(T)), and empirical calculations (OPLS-AA) using two graphene models: coronene and infinite graphene. Symmetry-adapted perturbation theory calculations indicated that the interactions were governed by London dispersive forces (amounting to ∼60% of attractive interactions), even for the polar molecules. The results also showed that the adsorption enthalpies were largely controlled by the interaction energy. Adsorption enthalpies obtained from ab initio molecular dynamics employing non-local optB88-vdW functional were in excellent agreement with the experimental data, indicating that the functional can cover physical phenomena behind adsorption of organic molecules on graphene sufficiently well.

Two Organic Phase Suspension Polymerization for Novel Hypercrosslinked Resin Bead by Polycondensation of CMB

The suspension polymerization with two organic phases was adopted to prepare spherical hypercrosslinked resin by self-polycondensation of 4,4′-bis-(chloromethyl)-1,1′-biphenyl (CMB). The chemical structure, morphology and pore characteristics of the novel spherical resin were characterized with Fourier transform infrared spectroscopy (FTIR), micrograph and Brunauer-Emmett-Teller (BET). It is found that the suspension system and stirring speed impose a great influence upon the regularity and size distribution of hypercrosslinked beads. To prepare CMB resin beads with diameter of about 300 μm, the optimal condition is as follows: stirring speed 300 r·min−1, and the volume ratio of the two organic phases (nitrobenzene/dimethyl silicon oil) 1:5. After the self-polycondensation and succedent post-crosslinking of CMB monomer, the spherical adsorbent presents high special surface area (1190 m2·g−1) and abundant pore volume (0.714 cm3·g−1), and could be potentially applied in the adsorption of various organic molecules and synthesis of porous ion exchanger.

Molecular chirality in surface science

The adsorption of ever-larger organic molecules onto well-defined solid surfaces has been pursued during the last decade of surface science. One important consequence of adsorbing large molecules is a mismatch of molecular size and surface lattice, which leads frequently to mirror symmetry breaking and establishes handedness to the adsorbate complex, even if the molecule itself is not chiral. In addition, adsorption of chiral molecules, i.e., those lacking mirror symmetry before adsorption, has been performed increasingly. Such studies are not limited to the liquid–solid interface, but also cover the UHV domain. The principles of mirror symmetry breaking due to molecular adsorption are reviewed and ramifications of chiral bias in molecular layers are discussed.

Interstratification Patterns from the pH-Dependent Intercalation of a Tetracycline Antibiotic within Montmorillonite Layers

Little is known about the distribution of the intercalated molecules within the interstratified layers resulting from the pH-dependent interlayer adsorption of ionizable organic molecules, including antibiotics, within smectite-type clay minerals. Here we employed experimental and simulated X-ray diffraction (XRD) to characterize interstratification (or mixed layering) from the intercalation of oxytetracycline (OTC), a commonly used tetracycline antibiotic, within Na-montmorillonite layers at pHs 4, 5, 6, and 8. Our XRD data reveal that OTC is distributed nonrandomly in the interlayers such that Na- and OTC-saturated interlayers coexist. The profile of the full width at half-maximum intensity (fwhm), monitored as a function of increasing layer-to-layer distance (d001), resulting from an increasing amount of intercalated OTC, reflects such mixed-layer crystals under the acidic pH conditions. A minimum in fwhm occurs at a d spacing of about 1.8 nm, which is to be the optimal d001 for OTC-saturated layers, in agreement with molecular modeling results. Using the coordinates of the thermodynamically favorable configuration of the adsorptives in a model montmorillonite interlayer, we simulated XRD profiles to unravel the different patterns of interstratification from the experimental data. At both pHs 4 and 5, Na- and OTC-interlayers are randomly interstratified, whereas at pH 6, these layers are clustered, as characterized by a segregated interstratification pattern. The theoretical layer stacking sequences of the simulated XRD illustrate, as pH increases, the clustering of similar layer types with the exclusion of OTC intercalation from clay populations enriched in Na. At pH 8, both fwhm and d001 indicate OTC adsorption primarily on external surface sites, not within interlayers. Our findings imply that, in addition to chemical speciation, a pH-dependent formation of montmorillonite crystallites with unexfoliated layers may be responsible both for the decreased OTC intercalation and for the increased binding on external sites, thus resulting in the different patterns of interstratification as a function of increasing pH.

Tuning the Surface Structure of Nitrogen-Doped TiO2Nanofibres-An Effective Method to Enhance Photocatalytic Activities of Visible-Light-Driven Green Synthesis and Degradation

Nitrogen-doped TiO2 nanofibres of anatase and TiO2(B) phases were synthesised by a reaction between titanate nanofibres of a layered structure and gaseous NH3 at 400-700 °C, following a different mechanism than that for the direct nitrogen doping from TiO2. The surface of the N-doped TiO2 nanofibres can be tuned by facial calcination in air to remove the surface-bonded N species, whereas the core remains N doped. N-Doped TiO2 nanofibres, only after calcination in air, became effective photocatalysts for the decomposition of sulforhodamine B under visible-light irradiation. The surface-oxidised surface layer was proven to be very effective for organic molecule adsorption, and the activation of oxygen molecules, whereas the remaining N-doped interior of the fibres strongly absorbed visible light, resulting in the generation of electrons and holes. The N-doped nanofibres were also used as supports of gold nanoparticle (Au NP) photocatalysts for visible-light-driven hydroamination of phenylacetylene with aniline. Phenylacetylene was activated on the N-doped surface of the nanofibres and aniline on the Au NPs. The Au NPs adsorbed on N-doped TiO2(B) nanofibres exhibited much better conversion (80 % of phenylacetylene) than when adsorbed on undoped fibres (46 %) at 40 °C and 95 % of the product is the desired imine. The surface N species can prevent the adsorption of O2 that is unfavourable for the hydroamination reaction, and thus, improve the photocatalytic activity. Removal of the surface N species resulted in a sharp decrease of the photocatalytic activity. These photocatalysts are feasible for practical applications, because they can be easily dispersed into solution and separated from a liquid by filtration, sedimentation or centrifugation due to their fibril morphology.

Atomic‐Scale Probing the Priority of Oxidation Sites of an Organic Molecule Adsorbed at the CuO/Cu(1 1 0) Interface

AbstractCatalytic reactions occurring on oxide‐supported Cu and Cu‐supported oxide catalysts are of fundamental interest in both industrial and scientific fields. Though the interface between Cu and oxide species has been regarded as the reaction site, the exact role of the interface in the catalytic reactions is not fully understood yet. In this work, by using a model system including CuO chains supported on Cu(1 1 0) and an organic molecule with alkynyl and aldehyde groups, we attempted to obtain a fundamental understanding of the role of the interface in the catalytic reactions. A combination of high‐resolution scanning tunneling microscopy imaging and density functional theory calculations provided direct evidence at the atomic scale that the interface between the CuO chains and the Cu(1 1 0) surface is the most favorable site for the adsorption and activation of organic molecules, and the oxygen of CuO has an activity to oxidize both the alkynyl and aldehyde groups of the molecules adsorbed at the interface.

Deposition Kinetics of Quantum Dots and Polystyrene Latex Nanoparticles onto Alumina: Role of Water Chemistry and Particle Coating

A clear understanding of the factors controlling the deposition behavior of engineered nanoparticles (ENPs), such as quantum dots (QDs), is necessary for predicting their transport and fate in natural subsurface environments and in water filtration processes. A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to study the effect of particle surface coatings and water chemistry on the deposition of commercial QDs onto Al2O3. Two carboxylated QDs (CdSe and CdTe) with different surface coatings were compared with two model nanoparticles: sulfate-functionalized (sPL) and carboxyl-modified (cPL) polystyrene latex. Deposition rates were assessed over a range of ionic strengths (IS) in simple electrolyte (KCl) and in electrolyte supplemented with two organic molecules found in natural waters; namely, humic acid and rhamnolipid. The Al2O3 collector used here is selected to be representative of oxide patches found on the surface of aquifer or filter grains. Deposition studies showed that ENP deposition rates on bare Al2O3 generally decreased with increasing salt concentration, with the exception of the polyacrylic-acid (PAA) coated CdTe QD which exhibited unique deposition behavior due to changes in the conformation of the PAA coating. QD deposition rates on bare Al2O3 were approximately 1 order of magnitude lower than those of the polystyrene latex nanoparticles, likely as a result of steric stabilization imparted by the QD surface coatings. Adsorption of humic acid or rhamnolipid on the Al2O3 surface resulted in charge reversal of the collector and subsequent reduction in the deposition rates of all ENPs. Moreover, the ratio of the two QCM-D output parameters, frequency and dissipation, revealed key structural information of the ENP-collector interface; namely, on bare Al2O3, the latex particles were rigidly attached as compared to the more loosely attached QDs. This study emphasizes the importance of considering the nature of ENP coatings as well as organic molecule adsorption onto particle and collector surfaces to avoid underestimating ENP mobility in natural and engineered aquatic environments.

Electronic structure and imaging contrast of graphene moiré on metals

Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption sites for molecules or metal clusters on graphene/Ir(111).

Interpretation of valence band photoemission spectra at organic-metal interfaces

Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as "interface states" having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.

Nonequilibrium Adsorption and Reorientation Dynamics of Molecules at Electrode/Electrolyte Interfaces Probed via Real-Time Second Harmonic Generation

Nonequilibrium adsorption and subsequent reorientation of organic molecules at electrode/electrolyte interfaces are important steps in electrochemical reactions and other interfacial processes, yet real-time quantitative characterization and monitoring of these processes, particularly for the reorientation step, are still challenging experimentally. Herein, we investigated the nonequilibrium adsorption process of 4-(4-(diethylamino)styryl)-N-methyl-pyridinium iodide (D289) molecules from acetonitrile solution onto a polycrystalline platinum electrode surface using real-time second harmonic generation (SHG) in combination with the potential step method. The time-dependent SHG curves exhibit two distinct regimes, which were interpreted with a two-step adsorption model consisting of a fast adsorption and a slow reorientation step for D289 on the surface. D289 was assumed to initially adsorb in an orientation perpendicular to the surface and then reorient to a parallel orientation. We derived a quantitative mathematical expression containing a biexponential function to fit the temporal SHG curves and obtain the rate constants for the two steps. The rate constants for fast adsorption and the slower reorientation processes show similar potential-dependent behavior: the rate decreases with an increase in the negative potential. We further proposed a molecular mechanism involving the displacement of D289 and CH3CN molecules adsorbed on the electrode interface to explain this potential-dependent behavior. On the basis of such analysis, we obtained a detailed picture of the adsorption of D289 molecules on the Pt electrode/CH3CN electrolyte, which consists of three consecutive steps: diffusion, adsorption, and reorientation. The results of this study may shed light on adsorption mechanisms at the electrode/electrolyte interface as well as at biological and other functional material interfaces.

A dynamic, luminescent and entangled MOF as a qualitative sensor for volatile organic solvents and a quantitative monitor for acetonitrile vapour

A new luminescent entangled (srs nets with two-fold interpenetration, linked via short Cu⋯Cu distances of ca. 2.8 Å) MOF, namely, [(CuCN)3L·(guest)x]n (L = 2,6-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)pyridine; x = 1 or 2), which contains microporous channels, is synthesized. This MOF material shows structural dynamics upon adsorption of various volatile organic solvent molecules, accompanied by reversible guest responsive colour changes of the fluorescence emission that are discernible by the naked eyes. The vacuated form of this material can also exhibit qualitative solvate-optical sensing behaviours toward various mixed solvents. Moreover, a quantitative real-time monitoring device is constructed that implements the determination of acetonitrile vapour pressure through optical response, which causes a drastic emission change from blue to yellow. The detailed structural elucidation provides structure–property understanding for the host–guest responsive dynamics.

Piperazine-functionalized ordered mesoporous polymer as highly active and reusable organocatalyst for water-medium organic synthesis

A piperazine-functionalized mesoporous polymer (PP-MPs) was synthesized through a one-step surfactant-directed phenol–formaldehyde–piperazinecarbaldehyde oligomer self-assembly approach. This novel PP-MP catalyst exhibited highly catalytic reactivity and selectivity for water-medium Knoevenagel reaction, water-medium [3 + 3] cycloaddition reaction and intermolecular cross-conjugated addition reaction. The excellent performance could be attributed to its high dispersion of piperazine active sites and ordered mesoporous structure, which effectively diminished the steric hindrance and diffusion limitation. Moreover, its pure organic framework could enhance surface hydrophobicity, which promoted organic reactant molecule adsorption, especially in water. Meanwhile, piperazine functional groups embedded in the polymeric framework could inhibit the leaching of active species and thus resulted in the excellent durability without significant loss of activity for recovery and recycling at least four times.

Selective Adsorption Properties of Cationic Metal–Organic Frameworks Based on Imidazolic Linker

In this work we propose an approach for the controlled synthesis of cationic frameworks by the use of imidazolium salts as linker molecules. The imidazolium salt linker H2ImidCl was prepared in five steps and converted with zinc and copper nitrate, respectively, to give the two topologically different cationic frameworks [Cu(Imid)(H2O)]+ and [Zn4(Imid)5]3+. The framework charge is compensated by anions located in the pores. [Cu(Imid)(H2O)]+ shows a selective adsorption of organic molecules either bearing a carboxylic group or having an ionic nature. Furthermore, adsorbed molecules featuring a carboxylic group are captured inside the pores, whereas ionic substances can be reversibly adsorbed and desorbed, which was followed by UV/vis spectroscopy.

Analysis of the origin of lateral interactions in the adsorption of small organic molecules on oxide surfaces

A decomposition scheme is proposed to analyze the physical contributions to the decrease in the binding energy of chemisorbed species with increasing coverage. This scheme is applied to the acetaldehyde–TiO2 (110) rutile system as a model for other small organic molecule—oxide surface systems. Different density functional theory (DFT) functionals have been employed at both low-medium and high coverages to understand how the different theoretical descriptions of the various terms influence the adsorbate–surface interaction. At low coverages, it is found that the localized adsorbate to surface electron donation is the fundamental physical process that influences the adsorbate–surface interaction. This results shows that while it is usually assumed that only pairwise adsorbate–adsorbate interactions influence the adsorption energy, the progressive modification of the surface properties (surface reduction in this case) may also play a significative role. The DFT+U functional results, in this case, in the best agreement with the experimental binding energy, and the inclusion of the dispersive forces results in largely overestimated adsorption energies. At higher coverages, the pure GGA and GGA+U functionals overestimate the repulsive terms and the computed binding energy is well below the experimental data. The inclusion of the dispersive forces is required to correctly reproduce the experimental results. The contributions of the different physical terms are also analyzed.

A Review on Reducing Graphene Oxide for Band Gap Engineering

Exfoliation, i.e. individual separation of carbon sheets, is of great interest to produce single-layered graphene nanosheets. Chemical or thermal treatments are popular approaches to exfoliate graphite chunks. In general, these conventional methods are assisted with intercalation via covalent or non-covalent functionalization, expansion, and swelling, adsorption of organic molecules in gas phase, solid nanoparticle insertion or direct molecular exfoliation. However, direct covalent modification of graphene is challenging and the zero-band gap of graphene limits its use in field-effect transistors in nanoelectronics. Therefore, the use of a band-gap tunable p-type semiconducting reduced graphene oxide (rGO) is an alternative route. There are several approaches to tune its band gap, including tailoring the chemistry. Critical parameters include the control of oxygen amount determined by the degree and time of oxidation and reduction conditions (e. g. temperature), often leading to nonstoichiometry. This short review therefore highlights the production of rGO focusing primarily on the effect of thermal treatmenton the nature and the role of oxygen during thermal exfoliation of GO. The impact of oxygen functionalization on the modulation of the band gap is also reviewed for chemically and thermally reduced GO, as well as chemically treated rGO followed by a thermal exfoliation.

Adsorption of nucleic acid bases on magnesium oxide (MgO)

AbstractThe adsorption of organic molecules on mineral matrices might have played a fundamental role in processes that led to the emergence of life. We investigated the adsorption properties of the nucleobases adenine, cytosine, uracil and hypoxanthine on magnesium oxide (MgO), determining the single solute batch equilibrium adsorption isotherms. Langmuir-type isotherms were fitted to data, assuming a rapid reversible equilibration of adsorption, demonstrated effectively through desorption experiments. The Langmuir equilibrium adsorption constantKand the amount of the solute per unit of adsorbent mass necessary to complete the monolayerbwere calculated. The results indicate that MgO is a good adsorbent for nucleobases (adenine > uracil > hypoxantine > cytosine), suggesting a role of metal oxides in concentrating biomolecules in prebiotic conditions that might have favoured the passage from geochemistry to biochemistry.

Adsorption of Organic Molecules on Kaolinite from the Exchange-Hole Dipole Moment Dispersion Model.

Intermolecular interactions between organic molecules and clay minerals are important in a wide range of chemical applications, ranging from oil-sands petroleum extraction to environmental chemistry and catalysis. The binding energies between each of benzene, n-hexane, pyridine, 2-propanol, and water and the kaolinite surface are calculated using density functional theory with the exchange-hole dipole moment dispersion model. The dominant noncovalent interactions are found to be hydrogen bonding for pyridine, 2-propanol, and water, OH-π interactions for benzene, and CH-O interactions for n-hexane. All molecules considered are more strongly bound to the hydrophilic alumina face, rather than the hydrophobic siloxane face, of kaolinte.