Competitive chemisorption between pairs of cinchona alkaloids and related compounds from solution onto platinum surfaces

Quinoline-derived compounds exhibit the following relative chemisorption strengths from CCl4 solutions onto platinum surfaces, as determined by in-situ infrared spectroscopy: quinine, quinidine > cinchonidine > cinchonine > 6-methoxyquinoline > lepidine > quinoline. This sequence explains nonlinear enantioselectivity effects with cinchona chiral modifiers in hydrogenation catalysis.     

Time-lapse STM studies of diastereomeric cinchona alkaloids on platinum metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.     

Influence of the molecular architecture on the adsorption onto solid surfaces: comb-like polymers

The processes of adsorption of grafted copolymers onto negatively charged surfaces were studied using a dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The control parameters in the study of the adsorption are the existence or absence on the molecular architecture of grafted polyethyleneglycol (PEG) chains with different lengths and the chemical nature of the main chain, poly(allylamine) (PAH) or poly(L-lysine) (PLL). It was found out that the adsorption kinetics of the polymers showed a complex behavior. The total adsorbed amount depends on the architecture of the polymer chains (length of the PEG chains), on the polymer concentration and on the chemical nature of the main chain. The comparison of the thicknesses of the adsorbed layers obtained from D-QCM and from ellipsometry allowed calculation of the water content of the layers that is intimately related to the grafting length. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. It is shown that the adsorption of polymers with a charged backbone is not driven exclusively by the electrostatic interactions, but the entropic contributions as a result of the trapping of water in the layer structure are of fundamental importance.     

Role of the solvent in the adsorption-desorption equilibrium of cinchona alkaloids between solution and a platinum surface: correlations among solvent polarity, cinchona solubility, and catalytic performance

The role that the nature of the solvent plays in defining the extent of cinchona alkaloid adsorption-desorption equilibrium on platinum surfaces has been studied both by testing their solubility in 54 different solvents and by probing the stability of adsorbed cinchona in the presence of those solvents. The solubilities vary by as much as 5-6 orders of magnitude, display volcano-type correlations with solvent polarity and dielectric constant, and follow a cinchonine < cinchonidine < quinine, quinidine sequence. The adsorption-desorption equilibrium shifts toward the solution with increasing dissolving power of the solvent. The relevance of these results to the behavior of cinchona as chiral modifiers in hydrogenation catalysis is discussed.     

Multilayer adsorption from multicomponent liquid mixtures on solid surfaces

Multilayer adsorption of multicomponent liquid mixtures on homogeneous and heterogeneous solid surfaces is discussed. Heterogeneity effects of the adsorbent surface have been taken into account in calculation of mole fractions of the components in the first adsorbed layer. The model calculations, illustrating multilayer and heterogeneity effects, have been performed for adsorption of ternary liquid mixtures.

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Mehrschicht-Adsorption von flüssigen Mehrkomponentenmischungen auf festen Oberflächen

Zusammenfassung Es wird die Mehrschichtadsorption auf homogenen und heterogenen Oberflächen diskutiert. Bei der Berechnung der Molbrüche der Komponenten der ersten adsorbierten Schicht wurden Heterogenitäts-Effekte berücksichtigt. Die Modellrechnungen wurden an ternären flüssigen Mischungen vorgenommen.

     

Kinetics of dissociative adsorption on stepped surfaces of platinum

The kinetics of dissociative adsorption of oxygen was studied by the Monte Carlo method for a model which supposed that absorption occurs with a high rate only on steps and that O_ads may migrate from steps onto terraces and backward. At the relatively low activation energy of diffusion from steps onto terraces E_dif∼ 75 kJ/mol (E_dif is lower by 4 kJ/mol in the back direction), the function log(s(θ)), wheres is the sticking coefficient and θ is the surface coverage by oxygen atoms, is almost linear at 300 K; that is, the Roginskii-Elovich adsorption equation is obeyed. If it is supposed that the bonding energy of adatoms on terraces is slightly higher than that on steps, and the sticking coefficient is constant and equal to s₀ up to high degrees of coverage as well as in the model of the kinetics of adsorption with a precursor state. Deceased.     

New results on the mass spectra of cinchona alkaloids

The electrospray ionization (ESI) mass spectra of 16 cinchona alkaloid compounds were studied for the first time. The electron ionization (EI) spectra of 22 cinchona alkaloids were also recorded, 14 of which had not been examined previously. In the case of EI the characteristic direction of the fragmentation is the scission of the C8-C9 bond. Under EI the cleavage of the C4'-C9 bond occurs only in the case of hydrogenated cinchona alkaloids, whereas the C9-O bond cleavage can be observed in the case of ester and ether derivatives. At a low capillary exit voltage (CapEx) in the ESI measurements there is no fragmentation, and only the [M + H](+) and in some cases the double protonated [M + 2H](2+) ions can be detected. On increasing the CapEx the characteristic primary direction is the cleavage of the C9-O bond, which was observed in the case of epialkaloids and esterified or etherified cinchona derivatives, respectively. Copyright 2000 John Wiley & Sons, Ltd.     

Heats of adsorption of hydrocarbons on solid surfaces

The integral heats of the interphase interaction of liquid aliphatic and aromatic hydrocarbons with fibers and powders of polyacrylonitrile and polypropylene, clay, and sandstone at 293 and 303 K were evaluated microcalorimetrically. The relationships between the heat of adsorption of hydrocarbons and the sorbate : adsorbent mass ratio were obtained. The heat and time needed for the interaction of hydrocarbons with the solid surfaces to reach equilibrium depend mainly on the adsorbent structure.     

Competitive adsorption from multicomponent non-electrolytic liquid mixtures on heterogeneous solid surfaces

Summary A simple model for competitive adsorption from multicomponent non-electrolytic liquid mixtures on energetically heterogeneous solid surfaces is discussed in terms of statistical thermodynamics. Integral equations are derived for the fundamental thermodynamic quantities that characterize competitive adsorption at the liquid-solid interface. Extensive model studies are presented in order to illustrate the influence of adsorbent heterogeneity on the behaviour of these thermodynamic quantities.on leave from Faculty of Chemistry, M. Curie-Sklodowska University, PL-20031 Lublin, Poland     

On the mechanism of surfactant adsorption on solid surfaces: free-energy investigations

The adsorption free-energy of surfactant on solid surfaces has been calculated by molecular dynamics (MD) simulation for a model surfactant/solvent system. The umbrella-sampling with the weight histogram analysis method (WHAM) was applied. The entropic and enthalpic contributions to the full potential of mean force (PMF) were obtained to evaluate the detailed thermodynamics of surfactant adsorption in solid/liquid interfaces. Although we observed that this surfactant adsorption process is driven mainly by a favorable enthalpy change, a highly unfavorable entropic contribution still existed. By decomposing the free energy (including its entropic and enthalpic components) into the solvent-induced contribution and the surfactant-wall term, the effect of surface and solvent on the adsorption free-energy has been distinguished. The contribution to the PMF from the surface effect is thermodynamically favorable, whereas the solvent term displays an obviously unfavorable component with a monotonic increase as the surfactant approaches to the surface. The impact of various interactions from the surfaces (both solvent-philic and solvent-phobic) and the solvent on the adsorption PMF of surfactant has been compared and discussed. Compared to the solvent-philic surface, the solvent-phobic surface generates more stable site for the surfactant adsorption. However, the full PMF profile for the solvent-phobic system shows a clear positive maximum value at the bulk-interface transition region, which leads to a considerable long-range free-energy barrier to the surfactant adsorption. These results have been analyzed in terms of the local interfacial structures. In summary, this comprehensive study is expected to reveal the microscopic interaction mechanisms determining the surfactant adsorption on solid surfaces.     

Structure dependence of NO adsorption and dissociation on platinum surfaces

The influence of surface structure on NO chemisorption and dissociation on Pt[100]-(1x1), Pt[211], and Pt[410] has been studied using density functional theory slab calculations with the generalized gradient corrections. The presence of steps on Pt[211] strengthens the NO-surface chemisorption bond, but the barrier for NO dissociation remains high. On the other hand, the steps on Pt[410] help to stabilize the N and O adatoms that form upon dissociation and the transition state. The calculated barrier of 80.2 kJ/mol on Pt[410] is in good agreement with experiment. These results show that both the presence of steps and the nature of the steps are important to activate NO. An ensemble of square-arranged Pt atoms has been identified as an important feature in activating the N-O bond.     

Statistical thermodynamics of adsorption from multicomponent liquid mixtures on heterogeneous solid surfaces

The statistical thermodynamics of adsorption from multicomponent liquid mixtures on heterogeneous solid surfaces is discussed by assuming the cell adsorption model and ideal adsorbed phase.Two integral representations for the adsorption isotherm are proposed: one based onn-dimensional energy distribution function (i.e., each adsorption site is characterized by adsorption energies of all components), and the other based on distribution of differences of adsorption energies ofn-1 components in relation to adsorption energy of the chosen component (i.e., each adsorption site is characterized byn-1 differences of adsorption energies of the components in relation to adsorption energy of the chosen component).The expressions for differential adsorption heat for adsorption from binary liquid mixtures have been derived from both integral equations.

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Statistische Thermodynamik der Adsorption aus flüssigen Mehrkomponentenmischungen auf heterogenen festen Oberflächen

Zusammenfassung Die statistische Thermodynamik der Adsorption von aus mehreren Komponenten bestehenden flüssigen Mischungen auf heterogenen festen Oberflächen wird für das Modell der Zellenadsorption bei ideal adsorbierter Phase diskutiert.Zwei Integraldarstellungen der Adsorptionsisotherme werden vorgeschlagen: eine auf einen-dimensionale Verteilungsfunktion der Energie gestützte (das heißt, jede Adsorptionsstelle wird durch Adsorptionsenergien von allen Komponenten charakterisiert); die andere basiert auf der Verteilung der Unterschiede von Adsorptionsenergien dern-1-Komponenten in bezug auf die Adsorptionsenergie der ausgewählten Komponente (das heißt, jede Adsorptionsstelle wird durchn-1-Unterschiede charakterisiert. Formeln für differentiale Adsorptionswärmen für die Adsorption aus binären flüssigen Mischungen sind von beiden Integral-Gleichungen abgeleitet worden.

     

Adsorption studies of polyethers Part II: adsorption onto hydrophilic surfaces

The adsorption of BAB-type triblock copolymers (B=poly(ethylene oxide); A=poly(propylene oxide)) from aqueous solution onto hydrophilic silica particles is described with particular reference to the role of the copolymer composition. The adsorbed amount and the layer thickness were determined by the standard depletion method and photon correlation spectroscopy, respectively. Snowtex-YL silica was used as the adsorbent. The results show an increase in the adsorbed amount with increasing molar masses of both PEO and PPO blocks. The adsorbed layer thickness is found to depend strongly on PEO block mass. Both these parameters (adsorbed amount and hydrodynamic layer thickness) show a maximum as a function of the mole fraction of the PPO block present in the copolymer. The conformation of the adsorbed layer is determined by the surface–copolymer interaction; principally by the interaction of the hydrophilic PEO block with the silica surface. A good qualitative agreement of the experimental results with theoretical predictions and self-consistent mean field calculations has been found.     

Surfactant adsorption on solid surfaces: recognition between heterogeneous surfaces and adsorbed surfactant aggregates

We study the possibility of the recognition of surface heterogeneities with surfactant adsorption by performing Monte Carlo simulations. It is found that when each patch size of a heterogeneous surface is capable of being commensurate with the size of aggregates adsorbed on the constituent homogeneous surfaces, the adsorption isotherm of the system will display both adsorption characteristics for each homogeneous surface. Otherwise, one or more adsorption characteristics will be spoiled or destroyed. Therefore, the adsorption isotherm of surfactants on a heterogeneous surface provides a signal of recognition.     

Packing of molecules on solid surfaces in physical adsorption

The packing of molecules in a monolayer under physical adsorption conditions has been analyzed.

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Role of adsorption in liquid chromatography of macromolecules and potential of liquid chromatography in assessing adsorption of macromolecules onto solid surfaces

Many liquid chromatographic (LC) separations of macromolecules are influenced or directly based on adsorption of solutes on column packing. In the case of well known size exclusion chromatography (SEC), adsorption effects are usually unwanted and therefore suppressed. Still they appear in many SEC systems and may badly affect precision of results obtained. In other LC methods applicable to high polymers, adsorption is deliberately combined with exclusion. The aim is to discriminate complex polymer systems which exhibit more than one single distribution of their molecular characteristics. The main goals of such combinations include either a controlled increase or a full suppression of separation selectivity according to one molecular characteristics. Most important so far known exclusion-adsorption compensation methods allowing to suppress dependence of LC retention volumes on polymer molar mass are reviewed. The discussion is accomplished with a presentation of newly developed full adsorption - desorption (FAD) method which can be combined with various LC procedures. A very useful combination represents the on-line FAD/SEC procedure which enables also to study adsorption and desorption phenomena in the systems solid surface - solvent - macromolecules.     

Application of the transfer-matrix method to equilibria of polymer reactions and adsorption from solution

The transfer-matrix method is used to describe the occupation of a lattice of sites in equilibrium with bulk solution, thus giving a model for adsorption from solution (or binding or reaction). Numerical illustration is given using the zeroth approximation of the quasi-crystalline model for the bulk phase. Application of the method to two-dimensional systems involving orientation effects and phase changes is briefly discussed.     

Enhanced adsorption of roxarsone onto humic acid modified goethite from aqueous solution

The adsorption of roxarsone (ROX) on the humic acid modified goethite (HA-α-FeOOH) was evaluated for several potential environmental factors. Results showed that 1) ROX had a higher adsorption capacity onto HA-α-FeOOH than unmodified α-FeOOH; 2) the adsorption of ROX increased with a decrease in pH; 3) the high ionic strength significantly inhibited the adsorption capacity of HA-α-FeOOH; and 4) a higher temperature yielded greater adsorption, since the process for ROX to be adsorbed by HA-α-FeOOH was a spontaneous endothermic reaction. The maximum adsorption capacity of ROX was found to be 80.71?mg?·?g^?1, when the temperature was 308?K. Meanwhile, the inhibitory effects of ionic strength and PO₄^3? on the adsorption of ROX onto HA-α-FeOOH were enhanced with an increase in concentration. In addition, the adsorption equilibrium data obeyed Langmuir isotherm model and the kinetic data were well described by the pseudo-second-order kinetic model. From the infrared spectra of HA-α-FeOOH, it could be deduced that the ROX adsorption onto HA-α-FeOOH was achieved via the ion exchange between the arsenic acid and the carboxyl group on adsorbent, as well as the formation of As-O-Fe bond between Fe-O and arsenic acid ions.