Collagen adsorption and structure on polymer surfaces observed by atomic force microscopy

The structure and adsorption patterns of type I and type III collagen were imaged on various polymer substrates with atomic force microscopy. Type I collagen had higher adsorption on polystyrene than on a series of polymethacrylates and formed a network of tightly, interwoven strands. Upon adsorption to different polymethacrylates, with varying side chain lengths, the collagen molecules formed long, branching fibrils. Types I and III collagen had different adsorption patterns, in some cases, on the identical substrate material. For example, instead of forming a tightly packed network, type III forms long, branching fibers on the polystyrene surface. On other materials, such as poly(n-butyl methacrylate), the two types of collagen showed similar adsorption pattern and structure. Adsorbed collagen was also imaged on various blends of polystyrene and polymethacrylates to determine how the polymer surface chemical structure and surface topography mediates protein adsorption.     

Electrostatic model calculations on multiple adsorption at NaCl surfaces

A previously proposed electrostatic model for physisorption at ionic solids is extended to multiple adsorption of small molecules. A new algorithm is developed to avoid interpenetration of the interacting systems. The geometry optimization procedure is described. Ab initio calculations are used for the application of the method to the adsorption of CO and CO₂ at NaCl(100) surfaces simulated by Na₂₅Cl₂₅ clusters. The orientation of the adsorbate molecules in dependence on the cumulative atomic multipole moments (CAMMs) of the cluster atoms is discussed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 685–693, 1997     

The adsorption and activation of formic acid on different anatase TiO_2 surfaces

Formic acid photodegradation is one of the most important reactions in organic pollution control, and helps to improve the hydrogen generation efficiency in titanium dioxide catalyzed water photodecomposition. Based on density functional theory and Reax FF molecular dynamics, the adsorption, diffusion and activation of formic acid on the different anatase TiO_2(101),(001),(010) surfaces are investigated.The result shows that the adsorption of COOH on anatase TiO_2 surface shrinks the energy gap between the dehydrogenation intermediate COOH and HCOO. On the anatase TiO_2(101) surface, the formic acid breaks the O–H bond at the first step with activation energy 0.24 eV, and the consequent break of α-H become much easier with activation energy 0.77 eV. The dissociation of α-H is the determination step of the HCOOH decomposition.     

Tip–surface interactions in noncontact atomic force microscopy on reactive surfaces

The imaging process in noncontact atomic force microscopy (AFM) is studied on a number of reactive surfaces, namely, the Takayanagi reconstructed Si(111), InP(110), and GaAs(110). We show that on these surfaces, the short-range dangling-bond type of interaction between the tip and the surface is decisive in achieving atomic resolution. The short-range tip–surface interaction is modeled in the density functional theory within the GGA approximation. We show that we can achieve quantitative agreement with the experimental data in the commonly used frequency modulation technique for AFM surface corrugation with a very simple model for the tip geometry treating the tip–surface interaction in the perturbation theory. The nature of the short-range tip–surface interaction on the three surfaces is considered and the consequences thereof for the experiments is discussed.     

A model for the cooperative adsorption of surfactant mixtures on solid surfaces

Adsorption of surfactant mixtures on solids is of considerable theoretical and practical importance. In this study, cooperative adsorption of surfactant mixtures of nonyl phenol ethoxylated decyl ether (NP-10) and n-dodecyl-beta-D-maltoside (DM) on silica and alumina has been investigated as a function of the distribution of individual surfactants between solution and solid surface. In the mixed adsorption process, DM is identified to be the "active" adsorbing component and NP is the "passive" co-adsorbing one in the process of adsorption on alumina, while their roles are reversed on silica. A modified model has been proposed to quantify the adsorption behavior of surfactant mixtures and to obtain information in terms of aggregation number and standard free energy for surface aggregation. This model is the first model applied to the aggregation of the surfactant mixture at the solid/solution interfaces.     

First-principles study of C adsorption, O adsorption, and CO dissociation on flat and stepped Ni surfaces

The adsorption of atomic oxygen and carbon was studied with plane wave density functional theory on four Ni surfaces, Ni(110), Ni(111), Ni(210), and Ni(531). Various adsorption sites on these surfaces are examined in order to identify the most favorable adsorption site for each atomic species. The dependence of surface bonding on adsorbate coverage is also investigated. Adsorption energies and structural information are obtained and compared with existing experimental results for Ni(110) and Ni(111). In addition, activation barriers to CO dissociation have been determined on Ni(111) and Ni(531) by locating the transition states for these processes. Our results indicate that the binding energies of C are comparatively stronger on stepped surfaces than on flat surfaces, and the energy barriers associated with CO dissociation strongly favor reactions occurring near surface steps.     

Uranyl adsorption on solvated edge surfaces of pyrophyllite: a DFT model study

In a computational study we addressed the adsorption of uranyl UO(2)(2+) on solvated (110) and (010) edge surfaces of pyrophyllite, applying a density functional approach to periodic slab models. We explored bidentate adsorption complexes on various partially deprotonated adsorption sites: octahedral Al(O,OH), tetrahedral Si(O,OH), and mixed AlO-SiO. Aluminol sites were determined to be most favorable on the (110) surface of pyrophyllite, while on the (010) surface mixed AlO-SiO sites are preferred. The structural parameters of all low-energy complexes on both surfaces agree rather well with EXAFS results for the structurally similar mineral montmorillonite. We calculate the average U-O distance to surface and aqua ligand oxygen atoms to increase with the increasing coordination number of uranyl whereas EXAFS results indicate the opposite trend. According to our results, several adsorption species, with different coordination numbers on different edge faces, may coexist on clay minerals. This computational finding rationalizes why earlier spectroscopic studies indicated the existence of more than one adsorption species, whereas a single type of adsorption complex was suggested from most EXAFS results.     

Quantum mechanical model for the dissociative adsorption of diatomic molecules on metal surfaces

A quantum mechanical nonadiabatic theory of dissociative adsorption of diatomic molecules X2 on metal surface is presented. The following reaction coordinates are used to construct crossing diabatic potential energy surfaces (PES): the distance y between the atoms of the X2 molecule, the distance x of the X2 molecular axis from the surface, the set of coordinates describing possible displacements of metal atoms under adsorption. Expression for the rate constant is derived using the model potentials describing vibrations along these coordinates. The calculated dependency of the rate constant W on the reaction heat DeltaE is compared with that in classical approximation. It is shown that quantum effects lead to a weaker dependence of W on DeltaE as compared to that for classical one.     

Statistical mechanics of a two-dimensional lattice model of benzene adsorption in zeolites

In this Letter, we present a mean field calculation of the statistical mechanics of a lattice model of benzene adsorption in the quasi two-dimensional network of pores in zeolites. A lattice fluid model is introduced with monomer states to represent molecules standing perpendicular to the principle axis of the pore, dimer states to represent molecules lying flat against the pore wall, and vacant sites or holes. For a wide range of interaction parameters the model gives steps in adsorption isotherms similar to those observed experimentally for benzene adsorption in silicalite. Our treatment attributes the experimentally observed steps in the level of adsorption with rising pressure, to orientational transitions amongst molecules in the adsorbed phase with two possible ground states arrangements of the benzene molecules in the zeolite pores energetically competing with each other.     

Direction specific interactions of 1,4-dicarboxylic acid with calcite surfaces

Interactions of succinic acid (SUC) with the {104} cleavage faces of calcite show a strong preference in crystallographic directions. In situ atomic force microscopy revealed that the morphology of etch pits on the crystal surfaces experienced a transition from the common rhombus to a hexagon upon the introduction of SUC. The pit shape further evolved from 6-sided to 7-sided and eventually to 5-sided with increasing concentrations of SUC. Analysis indicates that the morphology changes may result from SUC preferentially binding to the [42] and [010] edges of the (104) plane to selectively slow down their step speed.     

Chemical force microscopy investigation of phosphate adsorption on the surfaces of iron(III) oxyhydroxide particles

Phosphate-modified AFM tips were prepared by the deposition of self-assembled monolayers (SAMs) of bis(11-thioundecyl) phosphate on Au-coated silicon nitride cantilevers. The properties of the PO(2)H-terminated SAMs were investigated by studying the pH-dependent force interactions of the tips with phosphate- and carboxylic acid-terminated SAM control surfaces. The PO(2)H functional groups had a pK(a) of approximately 5.0. A chemical force microscopy (CFM) study was conducted on the interactions between the probes and the surfaces of hydrous ferric oxide particles prepared in our laboratory by hydrolytic precipitation from FeCl(3). The forces between PO(2)H probes and the hydrous ferric oxide surfaces were seen to exhibit a strong pH dependence, with maximum attractive forces occurring for pH values between 5 and 8. The effects of postprecipitation of the hydrous ferric oxide colloids with orthophosphate, H(2)PO(4)(-), dimethylphosphate, (CH(3)O)(2)PO(2)H (DMP), and tannic acid (TA) on the adhesive interactions between the PO(2)H tips and the solids were also investigated. Attenuated total reflectance infrared spectroscopy (ATR-IR) was used to verify the presence of surface-adsorbed species and zeta potentiometric measurements to determine surface charge on the colloids. We show that the method of chemical force titration using phosphate-terminated tips can differentiate between these various colloids and that it shows promise as a general method for studying this environmentally important class of compounds.     

Antibody adsorption and orientation on hydrophobic surfaces

The orientation of a monoclonal, anti-streptavidin human IgG1 antibody on a model hydrophobic, CH(3)-terminated surface (1-dodecanethiol self-assembled monolayer on gold) was studied by monitoring the mechanical coupling between the adsorbed layer and the surface as well as the binding of molecular probes to the antibodies. In this study, the streptavidin antigen was used as a probe for the Fab portions of the antibody, while bacteria-derived Protein G' was used as a probe for the Fc region. Bovine serum albumin (BSA) acted as a blocking protein. Monolayer coverage occurred around 468 ng/cm(2). Below 100 ng/cm(2), antibodies were found to adsorb flat-on, tightly coupled to the surface and unable to capture their antigen, whereas the Fc region was able to bind Protein G'. At half-monolayer coverage, there was a transition in the mechanism of adsorption to allow for vertically oriented antibodies, as evidenced by the binding of both Protein G' and streptavidin as well as looser mechanical coupling with the surface. Monolayer coverage was characterized by a reduced level in probe binding per antibody and an even less rigid coupling to the surface.     

A model for the free energy of adsorption on low energy surfaces.

In constructing a generalized thermodynamics for the fluid-vapor-solid equilibrium in poorly wetted systems the specific free energy of adsorption at saturation vapor pressure is a basic and elusive term. If the adsorbed phase is modeled as a two dimensional gas, systems for which a complete spectrum of data is available can serve as an empirical basis for constructing and testing adsorption-contact angle relationships. From the extension of such relationship other often inassessible terms can be estimated. Such a construct is reported here and extended to the estimation of the excess adsorption entropy at saturation vapor pressure in non-wetting systems     

Heterogeneous extended langmuir model with multiregion surfaces for adsorption of mixtures

The multiregion or multisite adsorption theory is applied to the heterogeneous extended Langmuir (HEL) model for predicting adsorption from mixtures. A new model, multiregion HEL (MR-HEL), is derived. MR-HEL is thermodynamically consistent. It uses the same three parameters for each component of the mixture as in the HEL model. Examples, including eight binary and one ternary systems, show that both MR-HEL and HEL yield satisfactory results for relatively ideal systems with like components. For nonideal and highly nonideal mixtures, however, MR-HEL reduces the total average deviation for the predicted amount adsorbed for each component by more than 50% in comparison with the original HEL model. The improvement by MR-HEL is significant. Moreover, the new model predicts not only an azeotrope for binary system CO(2)+C(3)H(8) that shows strong nonideal behavior but also the correct azeotropic composition.     

Conformational analysis of colchicine and isocolchicine by molecular mechanics

The structures of isocolchicine ( ) and colchicine ( ) have been calculated using the MMX routine. The low energy conformations for isocolchicine and colchicine fit well with x-ray crystallographic data. The B ring atropisomer of isocolchicine, which can be spectroscopically observed, is calculated to be <1 kcal/mole higher in energy than . The boat-boat inversion conformer of colchicine, which has been predicted to be important in the binding of to tubulin, is also calculated. The B ring geometry of this isomer does not differ to the extent previously predicted.     

Characterization of specific interactions capacity of solid surfaces by adsorption of alkanes and alkenes. Part II: Adsorption on crystalline silica layer surfaces

Summary Adsorption of branched octanes and linear hydrocarbons on crystalline lamellar silica surfaces has been studied by inverse gas chromatography at infinite dilution. Taking the adsorption of the n-alkanes as a reference, the influence of the double bond on the hydrocarbon adsorption phenomena has been demonstrated. Thermodynamical parameters have been calculated which permit conclusions to be made on the adsorption mechanisms of lamellar materials.     

Ab initio modeling of protein/biomaterial interactions: glycine adsorption at hydroxyapatite surfaces

How does glycine adsorb at hydroxyapatite surfaces? Ab initio simulations based on periodic B3LYP GTO calculations reveal the detailed mechanism of binding to the (001) and (010) surfaces by shedding light on how acid and basic amino acid residues of proteins interact with hydroxyapatite based biomaterials.     

Thermodynamics and statistical mechanics of multilayer adsorption

The multilayer adsorption models of Brunauer-Emmett-Teller and Guggenheim-Anderson-de Boer are reconsidered. The relationship between the fitting parameters and the physical parameters of the equation is discussed. The preexponential factors of the parameters are shown to be in general far different from unity, contrary to a widespread use. A thermodynamical derivation illuminates the hypothesis on which the multilayer sorption equation is dependent and frees it from too restrictive hypothesis usually taken as necessary for its validity. Equations are derived for the number fraction of sorption sites occupied by different numbers of molecules. The Guggenheim-Anderson-de Boer equation is shown to imply incomplete occupation (jamming) of the first sorption layer at saturation.