Equilibrium aspects of polycation adsorption on silica surface: how the adsorbed layer responds to changes in bulk solution

Adsorption of cationic high molecular weight polyacrylamides (CPAM) (M(w) is about 800 kDa) with different fractions of cationic units tau = 0.09 and tau = 0.018 onto silica surface was studied over a wide range of pH (4-9) and KCl concentration (c(s) = 10(-3)-10(-1) M) by in-situ null ellipsometry. We discuss how the adsorbed layer depends on the bulk conditions as well as kinetically responds to changes in solution conditions. The adsorbed amount Gamma of CPAM increases with pH for all studied electrolyte concentrations until a plateau Gamma is reached at pH > 6. At low pH we observed an increase in adsorbed amount with electrolyte concentration. At high pH there is no remarkable influence of added salt on the values of the adsorbed amount. The thickness of adsorbed polymer layers, obtained by ellipsometry, increases with electrolyte concentration and decreases with pH. At low c(s) and high pH the polyelectrolyte adsorbs in a flat conformation. An overcompensation of the surface charge (charge reversal) by the adsorbed polyelectrolyte is observed at high c(s) and low pH. To reveal the reversibility of the polyelectrolyte adsorption with respect to the adsorbed amount and layer thickness, parameters such as polyelectrolyte concentration (c(p)), c(s), and pH were changed during the experiment. Generally, similar adsorbed layer properties were obtained independent of whether adsorption was obtained directly to initially bare surface or by changing pH, c(s), or the concentration of polyelectrolyte solution in the presence of a preadsorbed layer, provided that the coverage of the preadsorbed layer was low. Once a steady state of the measured parameters (Gamma, d) was reached, experimental conditions were restored to the original values and corresponding changes in Gamma and adsorbed layer thickness were recorded. For initially low surface coverage it was impossible to restore the layer properties, and in this case we always ended up with higher coverage than the initial values. For initial high surface coverage it was usually possible to restore the initial layer properties. Thus, we concluded that polyelectrolyte appears only partially reversible to changes in the solution conditions due the slow rearrangement process within the adsorbed layer.     

Effect of the electrostatic interaction on the redox reaction of positively charged cytochrome C adsorbed on the negatively charged surfaces of acid-terminated alkanethiol monolayers on a Au(111) electrode

The electrochemical properties of cytochrome c (cyt c) adsorbed on mixed self-assembled monolayers (SAMs) of 2-mercaptoethanesulfonate (MES)/2-mercaptoethanol (MEL) are compared with those on single-component SAMs of MES, MEL, and mercaptopropionic acid (MPA), using cyclic voltammetry and potential-modulated UV-vis reflectance spectroscopy. The rate constant of electron transfer (ET), k(et), of cyt c adsorbed on the SAM of MPA decreases from 1450 +/- 210 s(-1) at pH 7 to 890 +/- 100 s(-1) at pH 9. In contrast, the value of k(et) of cyt c on the SAM of MES is pH-independent at 100 +/- 15 s(-1). Those facts suggest that a large negative charge density on the SAM surface slows down the ET between cyt c and the electrode. The surface charge density of the SAM affects also the amount of electroactive cyt c, Gamma(e), which decreases from 10.0 +/- 1.0 to 5.3 +/- 1.1 pmol cm(-2) with increasing pH from 7 to 9 on the SAM of MPA. Similarly, the k(et) of cyt c adsorbed on the mixed SAMs of MES/MEL sharply decreases from 900 +/- 300 s(-1) to 110 s(-1) as the surface mole fraction of MES increases beyond 0.5, suggesting the presence of a negative surface charge threshold beyond which the rate of ET of cyt c is dramatically lowered. The decrease in the k(et) on the SAMs at high negative charge densities probably results from the confinement of adsorbed cyt c by the strong electrostatic force to an orientation that is not optimal for the ET reaction.     

Properties of pH-responsive mixed aggregates of polystyrene-block-poly(L-lysine) and nonionic surfactant in solution and adsorbed at a solid surface

Poly(styrene) 388- block-poly( l-lysine) 138 could be dispersed in water with the aid of the nonionic surfactant C 12E 6. Light scattering and direct imaging techniques show that the copolymer/surfactant aggregates are polydisperse spherical micelles. The rather broad size distribution can be attributed to the glassy state of the polystyrene core of the micelles hampering equilibration. Nevertheless, the poly( l-lysine) block remains pH sensitive in these mixed aggregates and circular dichroism measurements show that poly( l-lysine) block adopts a random coil conformation at low pH and an beta-sheet conformation at pH > or = 11 without any change in the micellar shape. Samples prepared by evaporation of drops of the solutions on graphite wafers exhibit different wetting patterns depending on the polypeptide conformation as indicated by atomic force microscopy.     

Thermal stability limits of proteins in solution and adsorbed on a hydrophobic surface

A coarse-grained Monte Carlo simulation is used to study thermal denaturation of small proteins in an infinitely dilute solution and adsorbed on a flat hydrophobic surface. Intermolecular interactions are modeled using the Miyazawa-Jernigan (MJ) knowledge-based potential for implicit solvent with the BULDG hydrophobicity scale. We analyze the thermal behavior of lysozyme for its prevalence of α-helices, fibronectin for its prevalence of β-sheets, and a short single helical peptide. Protein dimensions and contact maps are studied in detail before and during isothermal adsorption and heating. The MJ potential is shown to correctly predict the native conformation in solution under standard conditions, and the anticipated thermal stabilization of adsorbed proteins is observed when compared with heating in solution. The helix of the peptide is found to be much less stable thermally than the helices of lysozyme, reinforcing the importance of long-range forces in defining the protein structure. Contact map analysis of the adsorbed proteins shows correlation between the hydrophobicity of the secondary structure and their thermal stability on the surface.     

Fuzzy modeling applied to optical and surface properties of a ferrocenylsiloxane polyamide solution

A fuzzy model was designed to predict changes in surface tension and maximum absorbance due to self-assembly in a DMF solution of poly{1,1′-ferrocene-diamide-[1,3-bis(propylene) tetramethyl-disiloxane} as a function of temperature and concentration. The building of fuzzy rule-based inference systems appears as a grey-box because it allows interpretation of the knowledge contained in the model as well as its improvement with a-priori knowledge. The method provides accurate results and increases the efficiency of utilizing the available information in the model. Small mean squared errors (0.0064 for absorbance and 0.79 for surface tension) and strong correlations between experiment and simulated results (0.93 and 0.97, respectively) were found during model validation. The results showed that it is feasible to apply a Mamdani fuzzy inference system to the estimation of optical and surface properties of a ferrocenylsiloxane polyamide solution.     

The electrostatic interaction between a charged sphere and an oppositely charged planar surface and its application to protein adsorption

To calculate the electrostatic interaction between a charged sphere and a charged surface under the condition of constant charge density on the two surfaces is difficult. The theory presented in this paper provides an approximate solution to this problem when the charge of the two bodies is of opposite sign. The proposed calculation model is based on a solution of the Poisson–Boltzmann (P–B) equation for two oppositely charged planar surfaces to which the approximate integration procedure developed by Deryaguin is applied. The obtained expression is rather simple and is in good agreement with retention data for a protein in ion exchange chromatography. The developed model is physically more sound than the previously developed ‘slab’ model for protein retention. Under the experimental conditions of ion exchange chromatography of proteins, the two models give comparable numerical values for the ionic strength dependence of retention.     

From small charged molecules to oligomers: a semiempirical approach to the modeling of actual mobility in free solution

According to Stokes' treatment, the ionic mobility of particles, which are small with respect to Debye length, is usually considered to be proportional to the nominal charge and inversely proportional to the hydrodynamic radius. Experimentally, it is well known, however, that the ionic mobility of a small multicharged molecule does not depend linearly on its nominal charge in a wide range. This behavior can be accounted for by a condensation of the charge or a modification of the friction coefficient with the charge. This paper presents a semiempirical modeling of the actual mobility based on the assumption of additivity of frictional contributions pertaining to the uncharged molecular backbone and to each charged or uncharged moiety. Condensation of the charge was not considered. The model first appeared to be suitable for multicharged analytes having a characteristic dimension smaller than the Debye length, such as benzene polycarboxylic acids and polysulfated disaccharides. This approach was then adapted to account for the actual mobilities of singly and evenly charged oligomers (N-mers) having a dimension smaller than or similar to the Debye length. Rather good experimental agreement was obtained for polyalanines and polyglycines (N < or = 6), fatty acid homologs, fully sulfonated polystyrene oligomers (N < or = 13), and polycytidines (N < or = 10). Especially the influence of the polymerization degree on the mobility of oligomers having identical charge densities was clarified. It is also shown that the electrophoretic contribution to the overall friction coefficient increases linearly with the nominal charge but hardly depends on the chemical nature of the charged moieties. This model should be of interest to evaluate the role of various physicochemical phenomena (hydrodynamic and electrophoretic frictions, hydrodynamic coupling, charge condensation) involved in the migration of charged oligomers.     

A millimeter-sized negatively charged polymer embedded with molybdenum disulfide nanosheets for efficient removal of Pb(II) from aqueous solution

Molybdenum disulfide (MoS2) has excellent trapping ability for lead ions whereas its micro-/nanoscale size has greatly impeded its practical applications in the...     

1H NMR study of phase transition of uncharged and negatively charged poly(N-isopropylmethacrylamide) in D2O solutions

¹H NMR spectroscopy has been applied to the analysis of dynamic-structural changes during temperature-induced phase transition of non-ionized poly(N-isopropylmethacrylamide) (PIPMAm) and ionized copolymers of N-isopropylmethacrylamide with sodium methacrylate, all in D₂O solutions with various polymer concentrations (c = 0.1-10 wt.-%) and ionic comonomer mole fractions (i = 0-10 mole %). It was found that the formation of compact globular-like structures during the phase transition is independent of polymer concentration for non-ionized samples; the presence of negative charges on the polymer chains leads to a dependence of the phase transition temperature on c and i. Virtually all PIPMAm segments are in globular-like structures for low polymer concentrations; for c ⩾ 1 wt.-%, this holds only for low content i of the ionic comonomer. An increase in c and i leads to a decrease in the fraction of polymer segments in globular-like structures; for samples with highest values of c and i, the phase transition was not observed.     

Adsorption of barium and calcium chloride onto negatively charged alpha-Fe(2)O(3) particles

Adsorption of cations (Na(+), Ca(2+), Ba(2+)) onto negatively charged (pH 10.4) hematite (alpha-Fe(2)O(3)) particles has been studied. The oxide material was carefully prepared in order to obtain monodisperse suspensions of well-crystallized, quasi-spherical particles (50 nm in diameter). The isoelectric point (IEP) is located at pH 8.5. Adsorption of barium ions onto oxide particles was carried out and the electrophoretic mobility was measured throughout the adsorption experiment. Comparison with calcium adsorption at full coverage reveals a higher uptake of Ba(2+). In both cases it shows also that chloride ions coadsorb with M(2) ions. Simultaneous uptake of the positive and negative ions explains why the electrophoretic mobility does not reverse to cationic migration. A theoretical study of the surface speciation has been carried out, using the MuSiC model. It reveals the presence of negative as well as positive sites on both sides of the point of zero charge (PZC) of the hematite particles, which may explain the coadsorption of Ba(2+) and Cl(-) at pH 10.4. The effective charge of the oxide particles, calculated from the electrophoretic mobility, is in very good agreement with the results found with the MuSiC modelization and the chloride/barium adsorption ratio. It also verifies the theory of ionic condensation. Calorimetric measurements gave a negative heat for the overall reaction occurring when Ba(2+)/Cl(-) ions adsorb onto hematite. Despite the fact that anions (Cl(-) and OH(-)) adsorption onto mineral oxides is an exothermic phenomenon, it is likely that barium and calcium adsorption is endothermic, denoting the formation of an inner-sphere complex as reported in the literature.     

Role of ion ligands in the attachment of poly(ethylene oxide) to a charged surface

Using a surface force balance we demonstrate unambiguously that high-molecular-weight poly(ethylene oxide) (PEO) does not adsorb onto mica from purified water with no added salt, a surprising observation in view of its strong adsorption on mica from aqueous 0.1 M KNO3 solution. Analysis of the force profiles, together with the known complexation of PEO with metal ions, suggests that the polymer attachment to the negatively charged surface is mediated by the hydrated potassium ion acting as a ligand.     

Dynamooptical and electrooptical properties of poly(methylphenylsiloxane) in solution and bulk

The dynamooptical, electrooptical, and hydrodynamic properties of a low-molecular-mass poly(methylphenylsiloxane) containing 33% phenyl radicals (with respect to the total amount of side groups) in dilute solutions and in bulk are studied. The size of macromolecules, as well as the molecular mass of the polymer, its shear optical coefficients Δn/Δτ = (0.29 ± 0.3) × 10^?10 (in decalin) and (0.43 ± 0.03) × 10^?10 cm s²/g (in bulk), and the specific Kerr constants K = (2.30 ± 0.02) × 10^?12 (in benzene), (2.23 ± 0.02) × 10^?12 (in decalin), and (2.24 ± 0.09) × 10^?12 cm⁵/[g (300 V)²] (in bulk), are estimated and compared with the corresponding characteristics of poly(dimethylsiloxane). The effect of solvents on the intramolecular mobility, optical anisotropy, and dipole structure of polymer macromolecules is considered.     

STM observation of a ruthenium dye adsorbed on a TiO2(110) surface

Individual Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2 molecules, commonly known as N3, adsorbed on a TiO2 surface were visualized with a scanning tunneling microscope (STM) operated in ultrahigh vacuum. A TiO2(110)-(1 x 1) crystal was taken out from the vacuum vessel and immersed into an acetonitrile solution of N3. A monolayer of pivalate ((CH3)3CCOO-) ions was used to protect the (1 x 1) surface from contamination during the wetting process of the N3 adsorption. The N3 molecules adsorbed on the flat terraces protruded by 0.65 nm from the pivalate monolayer. The image height difference of the admolecules could be understood with the assumption that the N3 molecules anchor to the TiO2 surface via two carboxyl groups. The number density of the N3 molecules on the steps was higher than that on the terraces. The poorly coordinated Ti atoms exposed at the step edges form preferential sites where the carboxyl groups can approach, due to a lower steric obstacle or because the structure of the adsorbed N3 molecules suffers less distortion.     

Molecular dynamics simulation studies of the transport and adsorption of a charged macromolecule onto a charged adsorbent solid surface immersed in an electrolytic solution

Molecular dynamics simulations were performed in order to study the transport and adsorption of a charged macromolecule (desmopressin) onto a charged solid surface in an electrolytic solution. The strong Coulombic interaction from the charged solid surface represents the major force for accelerating, orienting, entrapping in the electrical double layer, and adsorbing the macromolecule onto the charged solid surface. The macromolecule is flattened as it approaches the charged surface, giving rise to a stronger surface exclusion effect that shields surface sites. When adsorbed, the macromolecule is restrained by a surface interaction more than one hundred times stronger than the thermal energy, of which 99.8% results from the strong dominant Coulombic interaction, and trapped by a hydration layer adjacent to the surface. This leads to zero lateral displacement of the adsorbed macromolecule and indicates that surface diffusion is a physically implausible mechanism in similar systems. Explicit solvent is required for realistic representation of the macromolecular structure and the surface interaction energy. The adsorbed macromolecule also decreased the electrostatic potential gradient perpendicular to the charged solid surface and introduced additional electrostatic potential gradients laterally. The results obtained from the molecular dynamics simulations confirm the importance of electrophoretic migration and support the physical mechanisms used in a macroscopic continuum model that predicts an overshoot in the concentration of a charged macromolecule in the adsorbed phase under certain conditions of pH and ionic strength.     

Poly(acrylic acid)-poly(ethylene glycol) layers on positively charged surface coatings: molecular structure, protein resistance, and application to single protein deposition

A new copolymer (PAA-PEG2000) has been designed, consisting of a negatively charged poly(acrylic acid) (PAA) backbone to which poly(ethylene glycol) (PEG) side chains with a molecular weight of about 2 kDa were grafted in a molecular ratio of 3:10. It readily adsorbs to positively charged surfaces and may be considered to be the anionic counterpart of PEG-grafted poly(l-lysine) (PLL-PEG), which was first described by Kenausis et al. and is widely used to render negatively charged surfaces protein-resistant. The synthesis of PAA-PEG2000 can be carried out in aqueous solution at room temperature and does not require any sophisticated techniques such as handling in an inert gas atmosphere. Using ellipsometry and infrared reflection absorption spectroscopy (IRRAS), the film structure has been carefully analyzed for copolymer adsorption onto three different positively charged surfaces, namely, thin layers of poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and (3-aminopropyl)triethoxysilane (APTES). Besides the film thickness, the conformation of the PEG chains and their orientation with respect to the surface normal appear to be important parameters for the protein resistance of the films. Although PAA-PEG2000 adsorbed to PAH and PEI renders the surfaces inert, only partial protein resistance has been observed if the copolymer is deposited on APTES. In a model application, we have generated heterogeneous surfaces composed of isolated small Au nanoparticles (AuNP's) embedded in a protein-resistant layer of PAA-PEG2000 and demonstrated that the AuNP's can serve as adsorption sites for single protein species. In the future, these nanopatterned surfaces may be used for the investigation of isolated proteins.     

Theoretical study of vibration-phonon coupling of H adsorbed on a Si(100) surface

In this paper a perturbation-theory study of vibrational lifetimes for the bending and stretching modes of hydrogen adsorbed on a Si(100) surface is presented. The hydrogen-silicon interaction is treated with a semiempirical bond-order potential. Calculations are performed for H-Si clusters of different sizes. The finite lifetime is due to vibration-phonon coupling, which is assumed to be linear or bilinear in the phonon and nonlinear in the H-Si stretching and bending modes. Lifetimes and vibrational transition rates are evaluated with one- and two-phonon processes taken into account. Temperature effects are also discussed. In agreement with the experiment and previous theoretical treatment it is found that the H-Si (upsilon(s) = 1) stretching vibration decays on a nanosecond timescale, whereas for the H-Si (upsilon(b) = 1) bending mode a picosecond decay is predicted. For higher-excited vibrations, simple scaling laws are found if the excitation energies are not too large. The relaxation mechanisms for the excited H-Si stretching and the H-Si bending modes are analyzed in detail.     

The structure of n-fold negatively charged C60 (n = 1, 2,…,6)

Restricted Hartree–Fock calculations were performed to optimize the structure of C₆₀ and its negatively charged ions. The length of the short bond was found to increase lineraly with the charge, whereas the longer bond steadily decreases, resulting in a difference of only 0.0266 Å between the two bond types. Icosahedral symmetry was retained in all calculations. The state with the highest spin multiplicity is calculated to be the lowest in energy for all ions. © 1993 John Wiley & Sons, Inc.     

The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova