Coadsorption Of Anions Research Articles

Effects of Anion Coadsorption on the Self-Assembly of 11-Acryloylamino Undecanoic Acid on an Au(111) Electrode.

11-acryloylamino undecanoic acid (AAUA) is a versatile polymerizable surfactant that has been applied to coat medical devices, and these applications can benefit from a fundamental understanding of its interaction with a metal substrate. Cyclic voltammetry and in situ scanning tunneling microscopy (STM) were used to examine the adsorption configuration of AAUA molecules on an ordered Au(111) electrode and their mutual interactions, as AAUA was adsorbed from a methanol dosing solution. In addition to the van der Waals force between the aliphatic groups, the hydrogen bonding between the carboxylic acid and acrylamide groups was also important to guide the spatial arrangement of AAUA admolecules on the Au electrode. The -COOH group of AAUA admolecule likely dissociated in neutral media to -COO-, which formed hydrogen bonds with H2PO4 - in phosphate buffer solution (PBS). This interaction between the AAUA admolecules and ions in the electrolyte resulted in different electrochemical characteristics observed in phosphate buffer solution (PBS) and potassium sulfate (K2SO4). Molecular-resolution STM imaging revealed distinctly different AAUA spatial structures on the Au electrode in PBS and K2SO4. Shifting the potential positively to 0.5 V (versus Ag/AgCl) led to lifting of the reconstructed Au(111) to the (1 × 1) phase and the dissolution of the ordered AAUA film, suggesting that the orientation of the AAUA admolecule was altered. The ordered AAUA adlayer could be partially recovered by shifting the potential negatively.

Au(111) Surface Oxidation Kinetics Probed by Electrocatalytic Oxidation of Formic Acid

The kinetics of surface oxidation for Au(111) in 0.1 M HCOOH + 0.1 M HClO4 was studied at 20 °C utilizing the electrocatalytic formic acid oxidation reaction (FAOR). In the surface oxidation region, cyclic voltammetry shows a distinct dependence of FAOR on scan rate revealing simultaneous deactivation of active surface sites. The surface oxide is essentially inactive for the FAOR. A series of current-time curves was recorded after potential steps into the potential range between 0.65 and 1.00 V vs saturated Mercury-mercurous sulfate electrode and analyzed within a three-step surface oxidation model, which successively involves (i) adsorption at surface defects, (ii) adsorption on well-defined Au(111) terraces, and (iii) two-dimensional nucleation-and-growth on terraces. Although Au surface oxidation is by far more complex, the system can be described in terms of the intrinsic activity of the Au(111) surface and the oxide coverage The chronoamperometric measurements can be modeled without considering the interaction of the oxide species with the FAOR and the co-adsorption of anions. The potential dependence of the kinetic parameters is used to obtain theoretical current-time curves for FAOR on Au(111). This study sheds light on the “fast FAOR” and contributes to the understanding of degradation phenomena in electrocatalysis.

The adsorption and electropolymerization of terthiophene on Au(1 1 1) electrode – Probed by in situ STM

• In situ STM imaging terthiophene molecule adsorbed on an ordered Au(1 1 1) electrode in sulfuric and perchloric acid. • Dominant effect of potential control on the adsorption and oxidation of terthiophene admolecule. • Terthiophene admolecules accompanied by bisulfate anions to form highly ordered adlattices. • Revelation of adsorption configuration and spatial arrangement of TT on Au(1 1 1) electrode. • STM visualization of oligothiophenes produced by anodization. In situ scanning tunneling microscopy (STM) was used to examine terthiophene (TT) adsorbed on an ordered Au(1 1 1) electrode in 0.1 M H 2 SO 4 and HClO 4 , as a model system to gain a fundamental understanding of the effects of potential control on the coadsorption of anion and the oxidation of a thiophene - bearing entity. TT was irreversibly adsorbed onto an Au(1 1 1) bead crystal from a dosing solution made of ethanol at room temperature. The cyclic voltammogram (CV) recorded with this sample in 0.1 M H 2 SO 4 showed a series of well-defined peaks, resulting from the restructuring of the TT adlayer, coupled with anion coadsorption and the phase transition of the Au(1 1 1) electrode. In contrast, the CV obtained in 0.1 M HClO 4 was mostly featureless before TT admolecules were irreversibly oxidized at E > 0.7 V (vs Ag/AgCl). TT molecule was adsorbed most horizontally on Au(1 1 1) electrode in H 2 SO 4 , but displaced by bisulfate anion at positive potential. Anodization of this TT/Au(1 1 1) electrode triggered the oxidation and subsequent coupling of TT monomers to yield oligothiophenes, whose internal structures and dimensions were revealed by STM in pH1 and 5 sulfate media. The as – produced oligothiophenes appeared as winding ribbons spanning ∼ 200 and ∼ 40 Å in pH 5 and 1 sulfate media. Their internal structures, particularly the trans and cis conformations of neighboring thiophenes units, were visualized by molecular resolution STM.

(Invited) A Thermodynamic Approach to the First Steps of Metal Deposition

The current response of an electrochemical system to a potential pertubation often contains contributions from many parallel processes. A prominent example is Cu underpotential deposition on Au surfaces, where the Cu deposition is accompanied by the coadsorption of anions. Conventional electrochemical methods like cyclic voltammetry mostly obtain information on the total charge and can therefore not a priori discriminate between the involved processes. Electrochemical microcalorimetry may provide additional, independent information, which may help to identify such processes. By measuring the reversibly exchanged heat during an electrochemical reaction, one directly obtains the reaction entropy of the electrochemical process, including all side reactions. Knowledge of the reaction entropy, together with information from conventional electrochemical methods may thus help to disentangle the elementary steps of an electrochemical process. In this contribution we will give examples for: 1. Anion adsorption on metal surfaces with and without metal underpotential deposition. The dependence of the entropy of the adsorption process for anion adsorption on Au(111) surfaces shows a characteristic variation with potential for all studied anions, which points to an interplay between the configurational entropy of the anion layer and structuring of water at the electrode surface. The entropy data will be corroborated by surface plasmon resonance experiments, which obtained information on the reaction volume of the processes.2. Metal deposition and surface layer formation from metal complexes. The deposition of Ag from Ag-cyanide complexes will be discussed for different CN- Entropy data support the formation of a passivating AgCN layer from [Ag(CN)2]- at low CN- concentrations. The deposition of metallic Ag probably proceeds via direct reduction of [Ag(CN)2]- at all cyanide concentrations.3. The steps of metal deposition from multi valent ions. The elementary steps of Cu deposition from aqueous Cu2+ solutions are studied by time-resolved heat measurements of the deposition process. The data point to a two-step process where the Cu+ - Cu2+ reaction is an electron transfer process in solution, independent of the ion transfer process to metallic Cu at the electrode.

Monovalent – divalent cation competition at the muscovite mica surface: Experiment and theory

HypothesisIon adsorption on mineral surfaces depends on several factors, such as the mineral surface structure and the valency, size and hydration of the ion. In order to understand competitive adsorption at mineral surfaces, experimental techniques are required that can probe multiple ionic species at the same time. By comparing adsorption of two different cations, it should be possible to derive the factors governing ion adsorption. Divalent cations are expected to bind stronger to the negatively-charged muscovite surface than monovalent cations. ExperimentsHere, the competition between the monovalent Cs+ and the divalent Ca2+ cation for adsorption at the muscovite mica basal plane was investigated using surface X-ray diffraction. Using an extended surface complexation model, we simultaneously fit the measured cation coverages and net surface charges reported in literature. FindingsIn order to reproduce those complementary data sets, both cation adsorption and anion coadsorption were included in the surface complexation model. Moreover, the intrinsic muscovite surface charge and the maximum of available adsorption sites had to be reduced compared to existing literature values. Competition experiments revealed that the affinity of Cs+ for the muscovite surface is larger than the affinity of Ca2+, showing that hydration forces are more important than electrostatics.

Generation of hydroxyl radicals from reactions between a dimethoxyhydroquinone and iron oxide nanoparticles

The hydroxyl radical (·OH) is a powerful oxidant that is produced in a wide range of environments via the Fenton reaction (Fe2+ + H2O2 → Fe3+ + ·OH + OH-). The reactants are formed from the reduction of Fe3+ and O2, which may be promoted by organic reductants, such as hydroquinones. The aim of this study was to investigate the extent of ·OH formation in reactions between 2,6-dimethoxyhydroquinone (2,6-DMHQ) and iron oxide nanoparticles. We further compared the reactivities of ferrihydrite and goethite and investigated the effects of the O2 concentration and pH on the generation of ·OH. The main finding was that the reactions between 2,6-DMHQ and iron oxide nanoparticles generated substantial amounts of ·OH under certain conditions via parallel reductive dissolution and catalytic oxidation reactions. The presence of O2 was essential for the catalytic oxidation of 2,6-DMHQ and the generation of H2O2. Moreover, the higher reduction potential of ferrihydrite relative to that of goethite made the former species more susceptible to reductive dissolution, which favored the production of ·OH. The results highlighted the effects of surface charge and ligand competition on the 2,6-DMHQ oxidation processes and showed that the co-adsorption of anions can promote the generation of ·OH.

(Invited) Microcalorimetric Investigation of Cu Underpotential and Bulk Deposition – a Direct Access to Anionic Side Processes and Reaction Steps

Measuring the heat during electrochemical reactions provides direct information on the reaction entropy of the electrochemical processes. These include also charge neutral side processes, e.g., coadsorption of anions with metal ions or ordering processes of the solvent at the interface, which will not show up in the electrochemical charge balance. Electrochemical microcalorimetry hence provides complementary information to the current voltage relationship as measured by conventional electrochemical techniques and may allow for the identification of side processes often obscured in electrochemical systems (1, 2). In this presentation we will present two examples, i) we will investigate anionic side processes during the Cu UPD formation on Au(111) in the presence of sulfate (3) and ii) we will address the reaction steps and the role of Cu+during the Cu-bulk deposition. For Cu UPD it is well accepted that Cu deposition is accompanied by strong coadsorption of anions. From the course of the reaction entropy and the charge balance during the UPD process we determined the potential dependent anion coverages. From experiments with pH variation it followed that sulfate and bisulfate become adsorbed with roughly equal amounts. In addition we found highly positive reaction entropy upon the completion of the first metal monolayer, which signals a charge neutral substitution process of adsorbed anions by oxygenated species. During pulsed Cu-bulk deposition we found that the heat evolution continues after the current pulse, i.e., without external current flow (see figure). This directly signals Cu deposition from Cu+ followed by a slow reestablishment of the Cu+/Cu2+ equilibrium, in agreement with the reaction mechanism of Mattsson and Bockris, proposed on the basis of the Tafel plot (4).

Quantum\u2013continuum simulation of underpotential deposition at electrified metal\u2013solution interfaces

The underpotential deposition of transition metal ions is a critical step in many electrosynthetic approaches. While underpotential deposition has been intensively studied at the atomic level, first-principles calculations in vacuum can strongly underestimate the stability of underpotentially deposited metals. It has been shown recently that the consideration of co-adsorbed anions can deliver more reliable descriptions of underpotential deposition reactions; however, the influence of additional key environmental factors such as the electrification of the interface under applied voltage and the activities of the ions in solution have yet to be investigated. In this work, copper underpotential deposition on gold is studied under realistic electrochemical conditions using a quantum–continuum model of the electrochemical interface. We report here on the influence of surface electrification, concentration effects, and anion co-adsorption on the stability of the copper underpotential deposition layer on the gold (100) surface.

Counterion Effects on Electrolyte Interactions with Gold Nanoparticles

Electrolyte interactions with nanoparticles (NPs) at the solid/liquid interfaces are highly complicated as the charged species can be directly adsorbed onto the NP surfaces, confined in the diffusion layer immediately surrounding the NPs, and dispersed in bulk solutions. Existing studies on electrolyte interactions with NPs are based primarily on the electrical double layer theory that focuses mainly on electrolyte interactions with NPs with fixed pre-existing charges. Demonstrated herein is a comprehensive study of counterion effects during the electrolyte bindings to gold nanoparticles (AuNPs), including halide-induced AuNP aggregation and fusion, quantitative cation and anion coadsorption, selective cation and anion displacement on AuNPs, and surface-enhanced Raman spectroscopic features of the ionic species adsorbed onto AuNP surfaces. In contradiction to previous reports that electrolyte effects are anion-specific, we demonstrated that cations can play a dominant role in the halide-induced AuNP aggre...

Cu Underpotential Deposition on Ru(0001)

The underpotential deposition (UPD) has attracted research attention for many years due to its fundamental and practical importance[i]. Development of scanning probe methods (STM and AFM), and X-ray scattering techniques has enabled mechanistic in situ studies for many substrate-adlayer systems with great insight in details of UPD adlayer formation and structure, alloying and anion co-adsorption[ii]. However, in some cases, the noble metal surfaces with complex reactivity and interaction with water based electrolytes have rendered UPD studies a very difficult endeavor. One example is Ru surface for which a just few UPD studies have been reported[iii]-,[iv],[v],[vi],[vii]. Our talk presents a study of Cu UPD on Ru(0001). The Ru(0001) surface was prepared by annealing at 1400 C for one hour in Ar+H2 atmosphere (96:4), Figure 1. After cooling the Ru surface was protected by CO-saturated 0.1 M HClO4 solution and transferred to the electrochemical cell or STM cell for further studies. The example of the cyclic voltammetry and insitu STM results are shown in Figure 1. The potential has been gradually changed towards the region where Cu UPD on Ru(0001) occurs and high resolution/quality STM images were recorded. The morphological change of Ru(0001) surface due to Cu UPD process were correlated with CV data and characteristic cathodic and anodic peaks in CV were associated with particular features/process occurring during Cu UPD deposition and/or stripping. The Cu UPD process and adlayer structure on Ru(0001) is compared with corresponding Cu UPD process on other closed pack surfaces such as Au(111) and Pt(111). The apparent similarities and differences were identified and discussed during the talk. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. Figure 1. Cyclic voltammetry for Cu UPD on Ru(0001), scan rate 10 mV/s. Solution: 0.1 M HClO4 + 10-3 M Cu2+. Inset shows Ru crystal during inductive annealing. [i] S. Trasatti, J. Electroanal. Chem. 33, 351 (1971), [ii] E. Budevski, G. Staikov and W. J. Lorenz in Electrochemical Phase Formation and Growth, ed. R.C. Alkire et al. VCH, Berlin (1996), see Ref. herein. [iii] M. Vukmirovic, in Modern Aspects of Electrochemistry, vol. 42, ed. C. Vayenas et al. Springer, NY (2008). [iv] N. Bogolowski, S. Huxter, A. –E. Abd-El-Latif,G.A. Attard, H. Baltruschat, Journal of Electroanalytical Chemistry, 646, 68 (2010). [v] D. K. Gebregziabiher,Y-G. Kim, C. Thambidurai, V. Ivanova, P-H. Haumesser, J.L. Stickney, Journal of Crystal Growth, 312 1271 (2010). [vi] Y. Zhang, L. Huang, T. N. Arunagiri, O. Ojeda, S. Flores, O. Chyan, R. M. Wallace, Electrochemical and Solid-State Letters, 7 C107 (2004). [vii] M. A. Quiroz, L. Salgado, Y. Meas, Electrochimica Acta, 33 435 (1988). Figure 1

Underpotential deposition: From planar surfaces to nanoparticles

Abstract An overview is given of selected theoretical, experimental and computer simulation research on thermodynamic modeling applied to the metal underpotential deposition. Focus is made mainly on the last 20 years. The upd -theory on planar surfaces is revisited and the thermodynamic framework is extended to consider underpotential deposition on nanoparticles and to include anion coadsorption, solvation and double layer charging. Results from molecular dynamics and Monte Carlo simulations are shown for systems of experimental interest. At the end some perspectives for further advanced modeling of the present problem are given.

Diverse Secondary Interactions between Ions Exchanged into the Resin Phase and Their Analytical Applications

The research activities by the author's group to elucidate the chemical states of ions within the ion exchange resin phase are summarized. The resin with the higher exchange capacity has the smaller space available for ion exchange, and the higher cross linking degree interferes more with swelling of the resin. As a result, diverse secondary interactions between exchanged ions are observed on the resins of high exchange capacities and high cross linking degrees: the van der Waals contact results in incomplete exchange or enhanced dehydration of ions, hydrogen bond formation between acidic anions, and coadsorption of anions with metal ions. Contribution of the simple ion exchange mechanism to the reactions of iminodiactate-type chelating resins with metal ions in the acidic media is quantitatively discussed. The resulting complexes were successfully applied to preconcentration and separation of anions.

Formation of zinc–oxianion complex adlayer by underpotential deposition of Zn on Au(1 1 1) electrode: Preferential formation of zinc monohydrogen phosphate complex in weakly acidic solutions

The underpotential deposition (upd) of zinc was studied on Au(111) electrode in weakly acidic solutions by voltammetry. The difference of anion co-adsorption strength was investigated on upd Zn in the solutions containing phosphate, sulfate, perchlorate, and halides. The order of anion co-adsorption strength was found to be phosphate>sulfate, phosphate≫perchlorate, and phosphate≫halides. We present the electrochemical evidence that no co-adsorption of halide progresses on upd Zn at E>−0.7V (vs. SCE) in spite of the relatively high adsorbability of halide on Au(111). In 0.1M KH2PO4 (pH=4.4), the amount of charge density of the upd Zn stripping corresponded to that of 1/3 monolayer of the Zn adlayer formed as a result of mono-electron transfer between −0.35V and −0.6V. The coverage was coincident with that inferred from the (√3×√3)R30° STM image reported by us in the same condition. The formation of zinc–oxianion (phosphate and sulfate) surface complexes by Zn upd was proposed base on the difference of complex formation constants among the related zinc–anion complexes. Identification of the zinc–phosphate surface complex was thermodynamically carried out around pH=4 with a Nernst equation, where the shifts of Zn upd peak potential were investigated with the concentration changes of Zn2+, H2PO4−, and H+ under an imaginarily reversible condition. The formation of ZnHPO4 surface complex was concluded in the Zn upd with mono-electron transfer and explained a recent XAS analysis, which demonstrate each phosphate anion bridging three upd Zn atoms.

Ferrocene-terminated alkanethiol self-assembled monolayers: An electrochemical and in situ surface-enhanced infra-red absorption spectroscopy study

The effect of anions on the redox behavior and structure of 11-ferrocenyl-1-undecanethiol (FcC11) monolayers (SAM) on Au(111) single crystal and Au(111-25nm) thin film electrodes was investigated in 0.1M solutions of HPF6, HClO4, HBF4, HNO3, and H2SO4 by cyclic voltammetry (CV) and in situ surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS). We demonstrate that the FcC11 redox peaks shift toward positive potentials and broaden with increasing hydrophilicity of the anions. In situ surface-enhanced IR-spectroscopy (SEIRAS) provided direct access for the incorporation of anions into the oxidized adlayer. The coadsorption of anions is accompanied by the penetration of water molecules. The latter effect is particularly pronounced in aqueous HNO3 and H2SO4 electrolytes. The adlayer permeability increases with increasing hydrophilicity of the anions. We also found that even the neutral (reduced) FcC11 SAM is permeable for water molecules. Based on the property of interfacial water to reorient upon charge inversion, we propose a spectroscopic approach for estimating the potential of zero total charge of the FcC11-modified Au(111) electrodes in aqueous electrolytes.

In depth analysis of complex interfacial processes: in situ electrochemical characterization of deposition of atomic layers of Cu, Pb and Te on Pd electrodes

A combination of cyclic voltammetry, electrogravimetry, and electrochemical impedance spectroscopy has been used to characterize, in situ, the underpotential deposition (UPD) of atomic layers of Cu, Pb and Te on Pd electrode surfaces. This approach provides co-adsorption and competitive adsorption of anions to be measured and quantified during the UPD processes, highlighting the complex competitive processes that can e.g. hinder the design of new catalysts. The formed Cu, Pb and Te atomic layers on the Pd electrode showed no evidence of anion co-adsorption or surface alloying effects, which indicates that these systems, when formed in a perchlorate medium, could act as building blocks for catalysts. The mode of deposition was found to vary greatly for each overlayer. Cu was found to form a compact monolayer on the Pd surface, while Te formed a bilayer structure on the Pd surface, of which ∼1/4 of a monolayer was found to be irreversibly adsorbed. The formation of Pb overlayers was complicated by background UPD of hydrogen and its absorption to the underlying Pd substrate. While perchloric acid media are suitable for the formation of the overlayer, catalytic application of the formed Pb-layers would require a higher pH to negate such processes.

A model for underpotential deposition in the presence of anions

A simple model to study the effect of on top coadsorption of anions in underpotential deposition is formulated. It considers a lattice-gas model with pair potential interactions between nearest neighbors. As test system, the electrodeposition of silver on gold is studied by means of grand canonical Monte Carlo simulations. The influence of anions on the adsorption isotherms is analyzed. It is found that as the interaction between silver atoms and anions increases, the monolayer adsorbs at more negative chemical potentials. For large interactions between silver atoms and anions, a expanded structure occurs for the silver monolayer.

Heat Effects upon Electrochemical Copper Deposition on Polycrystalline Gold

Heat effects upon Cu deposition on polycrystalline Au surfaces from sulphuric acid electrolytes were calorimetrically measured. By combination of pyroelectric temperature detection at the backside of a thin electrode foil with pulsed electrochemistry, sensitivities to electrochemical conversions of a few percent of a Cu monolayer (ML), corresponding to about 1 microJ cm(-2) were achieved. We compared the heat evolution upon Cu under potential deposition (UPD), Cu deposition onto a fully developed Cu UPD layer and bulk Cu deposition onto a 300 ML thick Cu layer on Au. The heat effects were measured dependent on the amplitudes of the applied potential steps, that is, the driving forces of the respective reactions. From the differences of the heat effects among the Cu deposition processes, we deduced implications on the reaction mechanisms. For Cu UPD, the heat effects were explicable by the deposition of 1.3 Cu atoms per two electrons flowing to the electrode accompanied by sulphate coadsorption, similar to Cu UPD from sulphuric acid solutions on Au(111). Upon Cu deposition on a Cu UPD layer the heat effects signal considerable anion coadsorption up to the deposition of about 0.5 ML of Cu. At higher conversions the deposition mechanism gradually changes towards bulk Cu deposition, accompanied by reduction of the sulphate coverage.

Effect of Temperature Variation on the Under-Potential Deposition of Copper on Pt(111) in Aqueous H2SO4

The under-potential deposition of copper on Pt(111) from aqueous 0.05 M H2SO4 + 5 mM CuSO4·5H2O is studied in the 273 ≤ T ≤ 333 K range using cyclic voltammetry (CV). The CV transients always reveal two cathodic peaks (CI and CII) for the entire temperature range; there is only one anodic peak (AI) in the case of 273 ≤ T < 298 K and the second one appears in the case of T ≥ 298 K. In the case of 273 ≤ T < 298 K, the cathodic and anodic peaks shift toward lower potentials upon T increase, while in the case of T ≥ 298 K, there is no observable peak displacement. The charge density associated with the CuUPD deposition and stripping does not reveal any temperature-dependence. The results demonstrate that T variation does not lead to charge-density redistribution between the peaks, but the charge density of the first peak (CI and AI) in the deposition-stripping profiles is always greater than that of the second peak (CII and AII). The experimentally determined charge density associated with the Cu deposition and stripping is lower than the value expected for one epitaxial CuUPD layer on Pt(111). This difference is due to the coadsorption of anions that occurs concurrently with UPD Cu.

Interaction and Structure of Surfaces Coated by Poly(vinyl amines) of Different Line Charge Densities

Interactions between preadsorbed films of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed films were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a flat configuration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous films. As revealed by reflectivity measurements, such preadsorbed PVA films are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA films have very similar diffuse layer charge densities of about 1.5 mC/m2. Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reflectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the films is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the films behave differently. While the highly charged PVA shows unspecific adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances.

Zn 2+ and Sr 2+ adsorption at the TiO 2 (110)–electrolyte interface: Influence of ionic strength, coverage, and anions

The X-ray standing wave technique was used to probe the sensitivity of Zn 2+ and Sr 2+ ion adsorption to changes in both the adsorbed ion coverage and the background electrolyte species and concentrations at the rutile ( α-TiO 2) (110)–aqueous interface. Measurements were made with various background electrolytes (NaCl, NaTr, RbCl, NaBr) at concentrations as high as 1 m. The results demonstrate that Zn 2+ and Sr 2+ reside primarily in the condensed layer and that the ion heights above the Ti–O surface plane are insensitive to ionic strength and the choice of background electrolyte (with <0.1 Å changes over the full compositional range). The lack of any specific anion coadsorption upon probing with Br −, coupled with the insensitivity of Zn 2+ and Sr 2+ cation heights to changes in the background electrolyte, implies that anions do not play a significant role in the adsorption of these divalent metal ions to the rutile (110) surface. Absolute ion coverage measurements for Zn 2+ and Sr 2+ show a maximum Stern-layer coverage of ∼0.5 monolayer, with no significant variation in height as a function of Stern-layer coverage. These observations are discussed in the context of Gouy–Chapman–Stern models of the electrical double layer developed from macroscopic sorption and pH-titration studies of rutile powder suspensions. Direct comparison between these experimental observations and the MUltiSIte Complexation (MUSIC) model predictions of cation surface coverage as a function of ionic strength revealed good agreement between measured and predicted surface coverages with no adjustable parameters.