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Alamethicin was reconstituted into 1,2-di-O-phytanyl-sn-glycero-3-phosphocholine (DPhPC) bilayer supported at the gold (111) electrode surface using a combination of Langmuir-Blodgett/Langmuir-Schaefer deposition techniques. Chronocoulometry was employed to describe the behavior of the bilayer as a function of the electrode potential. Electrochemical impedance spectroscopy (EIS) was used to determine the membrane resistivity and capacitance. The photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements provided complementary information about the conformation and orientation of the alamethicin and DPhPC molecules in the adsorbed bilayer. The electrochemistry results demonstrated that the bilayer is stable at the electrode surface at potentials between 0.0 V and − 0.5 V vs Epzc. At more negative potentials, a progressive electro-dewetting of the membrane from the electrode surface takes place. The membrane is detached from the electrode surface when (E-Epzc) is more negative than − 1.1 V. The PM-IRRAS results illustrate that alamethicin peptides assume a surface state where their helices assume a large angle with respect to the surface normal at potentials close to Epzc. When negative potentials are applied, the alamethicin molecules are inserted into the membrane. Peptide insertion is complete at (E-Epzc) ~ − 0.5 V. The membrane resistivity determined from the ESI data indicates that alamethicin molecules adopt an open channel state when the helices are inserted into the membrane and a closed channel state (non-conductive) at E close to Epzc where they assume a flat orientation on the surface. The IR spectra collected during the negative and positive potential scans demonstrated that the open and closed ion channel states are potential controlled and reversible. In contrast to the significant changes in alamethicin orientation, the tilt angle of the acyl chains of DPhPC molecules show little to no change with the electrode potential. The data presented in this paper provides insight on the voltage-gating behavior of the alamethicin ion channels incorporated into phospholipid bilayers.

Alamethicin was reconstituted into DPhPC/DPTL tethered bilayer lipid membranes (tBLMs) to gain a better understanding of ion pore formation. The properties of the ion-conductive alamethicin pores were investigated by electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) as a function of the peptide concentration within the membrane. At low peptide concentrations (≤ 5%), the alamethicin-incorporated tBLM displays the highest membrane resistance since the alamethicin molecules adopt a surface state where the tilt angles of the helices are large with respect to the surface normal. At alamethicin concentrations ranging from 10 to 20 mol%, the tilt angles of the helices decrease to lower values signifying that the alamethicin molecules are inserting into the tBLM. Consequently, the membrane resistivity is also lowered indicating that the peptides are forming ion conductive channels. The results show an excellent correlation between molecular information concerning the orientation and conformation of alamethicin molecules obtained from the PM-IRRAS measurements and electrochemical properties of the membranes measured by EIS. These findings reveal that the alamethicin concentration within the film is directly related to the ion conductance across membrane.

In situ ATR-SEIRAS experiments were carried out to describe water structure at the surface of multilayers of Pd modified nanopatterned gold film electrodes in contact with aqueous 0.1 M HClO4 solution. The results show that the surface water becomes more ordered when the thickness of the Pd film increases. The “ice-like water” band becomes stronger and for the electrode with 5 ML Pd@Au, the ratio of “ice-like water” to “liquid-like water” becomes larger than that in bulk water. Concomitantly, the population of “monomers” or small water clusters diminishes. All surface water bands increase when hydrogen adsorption takes place. The bending band corresponding to “monomers” or “multimers” water is observed chiefly at the potentials of hydrogen adsorption.

A mixed monolayer of 1,2-dipalmitoyl-sn-glycero-3-(cytidine diphosphate) nucleolipid (DG-CDP) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was transferred from the air–water interface onto an gold (1 1 1) electrode surface using the Langmuir-Schaeffer (horizontal touch) technique. Compression isotherms were recorded for the monolayer spread at the air–water interface. These measurements demonstrated that the mixture composition 7:3 of DG-CDP:DPPC displayed reduced repulsive interactions between polar heads and ideal packing of acyl chains. Chronocoulometric experiments were performed to demonstrate stability of this film at the gold–solution interface. Photon polarization modulation infrared reflection absorption spectroscopy showed that the tilt angle of the acyl chains is slightly higher, and the chains are less twisted in the mixed than in the pure nucleolipid monolayer. In the mixed film, the tilt of the cytosine moiety (the sensing element of the monolayer) assumes a large angle with respect to the surface normal, even when the monolayer becomes detached from the gold surface. The mixed monolayer has improved properties for future applications for detection of guanine, its complementary base. Ministerio de Ciencia e Innovación CTQ2014-57515-C2-1-R, RED2018-102412-T Junta de Andalucía FQM202 Natural Sciences and Engineering Research Council of Canada (NSERC) RG-03958

The charge number per adsorbed molecule and surface concentration of 2,3-di- O -phytanyl- sn -glycero-1-octaethyleneglycol-D,L-α-lipoic acid ester (DPOL) at a Au(111) electrode surface was investigated by chronocoulometry using two different film deposition methods. First, a known amount of the thiolipid was transferred from the air-water interface of a Langmuir trough onto the gold electrode surface via a Langmuir-Blodgett (LB) deposition. The charge density measurements for this monolayer system were used to determine the charge number per adsorbed DPOL molecule (electrosorption valency). The observed electrosorption valency values of the adsorbed DPOL film were much lower than the expected number of transferred electrons for a simple reductive desorption process. In the second deposition method, charge densities were measured for the electrode covered by a self-assembled DPOL monolayer. The electrosorption valency values determined from the LB DPOL film were used to calculate the packing density of the DPOL molecules within the self-assembled monolayer. The surface concentration of the molecules within the thiol monolayer with octaethylene glycol chains gave similar results to a related thiolipid with tetraethylene glycol chains (DPTL). This new finding indicates that both of these molecules (DPTL and DPOL) assume a brush conformation in densely packed self-assembled monolayers.

Reduction of oxygen catalyzed by laccase was studied by surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with electrochemical techniques. Laccase molecules were adsorbed on modified carbon nanotubes (CNTs). SEIRAS provided spectra of Amide I band of adsorbed laccase. Fourier self-deconvolution (FSD) and two-dimensional correlation spectroscopy (2D COS) techniques were employed to reveal the sub-band structure of the Amide I band. The analysis demonstrated that laccase adsorbed on CNTs remains its native state both in the oxidized and reduced states. The detailed analysis showed that change of the oxidation state leads to a small change of the secondary structure of the protein. This change is comparable to the change observed for similar blue-copper oxidases in solution during redox titration. The results of this study demonstrated that CNT's immobilized laccase is an excellent catalyst of oxygen reduction. This study illustrates power of quantitative analysis of IR data to provide information about three-dimensional structure of surface immobilized proteins.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) was employed to study structure of water in a phospholipid bilayer deposited at a gold electrode surface. The technique employs attenuated total reflection (ATR) and an enhancement of the electric field of the IR photon that decays steeply with distance from the metal surface. These conditions allow one to subtract the background from the bulk water and to determine spectra of water in the bilayer or in a confined space between the metal surface and the bilayer. The IR data demonstrated that three types of water are present in the supported bilayer. At potentials close to zero charge the polar heads of the phospholipid molecules retain hydration water. At intermediate charge densities water penetrating deeply into the bilayer and multimers of water molecules were detected in the bilayer. At charge densities more negative than −20 μC/cm2 the bilayer is lifted from the metal surface and liquid like water appears in the space separating the bilayer from the metal.

The effect of amiloride, an acid-sensing ion channel (ASIC) blocker, on the formation of alamethicin ion channels in tethered zwitterionic phospholipid bilayers was investigated using electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). EIS measurements show a decrease in conductivity indicating that ion transported across the alamethicin-rich phospholipid membrane is inhibited by the presence of amiloride. The PM-IRRAS spectra indicate that amiloride has no effect on the secondary structure of the alamethicin peptide. The IR data suggests that amiloride does not block ion translocation through the alamethicin ion pore but prevents the insertion of alamethicin peptides into the biomembrane suppressing the formation of ion channels. The mechanism responsible for amiloride inhibition is explained in terms of the electrostatic interactions between the molecule and membrane surface. Amiloride is positively charged at physiological pH levels invoking a repulsive force on the positive pole of the alamethicin molecular dipole. These repulsive interactions prevent the insertion of the alamethicin peptides into the hydrophobic core of the lipid bilayer. The present paper offers a molecular view of the interactions that occur between amiloride, an ion channel inhibitor, with the lipids and ion channel forming peptides in model tBLMs.

Natural Sciences and Engineering Research Council of Canada; National Natural Science Foundation of China [20873116]

Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) were employed to measure leaching current of gold in an alkaline thiosulfate solution. First, the leaching current was determined with the help of RDE by measuring currents around the open circuit potential (ocp) and applying the mixed potential theory. The leaching current measured by this method may have contribution from another parasitic reaction such as oxidation of the leaching reagent. Therefore, a second method that employs RRDE was applied to verify that the leaching current measured at the RDE represents the gold dissolution reaction. In this approach gold was used for the disk and the ring was made of a glassy carbon. The gold ions produced at the disk during the leaching reaction were reduced at the ring. The collection efficiency allowed then to calculate the rate of gold ions production at the disk from the measured ring current. The leaching currents measured using this procedure were in excellent agreement with currents measured using RDE and the mixed potential theory confirming that the leaching currents measured by the first method were free of parasitic reactions and could be used as a measure of gold dissolution rate.

Differential capacitance, chronocoulometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of DMPC + cholesterol (70:30 mol%) phospholipid bilayers prepared using combined Langmuir–Blodgett/Langmuir–Schaefer (LB/LS) deposition. The electrochemical measurements indicate that incorporation of cholesterol into the membrane improves the quality of the bilayer if both leaflets contain cholesterol. Phospholipid bilayers containing DMPC and cholesterol prepared using LB/LS deposition were found to have smaller tilt angles (∼27°) than bilayers prepared by vesicle spreading (∼52°), indicating that LB/LS deposition is a superior method for preparing supported phospholipid bilayers at a gold electrode surface.

Electrochemical methods have been used to provide a thermodynamic description of the adsorption of citric acid (H 3 A) and dihydrogen citrate (H 2 A − ) on a Au(1 1 1) surface. We compare the adsorption of citric acid species from an electrolyte (pH 1) corresponding to the fully-protonated tri-acid species and an electrolyte (pH 3) corresponding to an approximately equimolar mixture of H 3 A and H 2 A − . Our results indicate a stronger adsorption of the monovalent ion compared to the neutral citric acid although both sets of experiments reveal similar charge numbers per adsorbed ion species. The latter values are dependent on the electrical variable (electrode potential or charge density) and approach a value of −3 at the most positive potentials (charge densities) studied for both sets of data (pH 1 and pH 3). This indicates that the adsorbed species is the fully de-protonated citrate ion, A −3 . From our chronocoulometry data we determine the outer Helmholtz potential for the citrate covered gold surface which is significantly larger for the system at pH 3 compared to pH 1. The results of this study on a two-dimensional gold surface can be extended to three-dimensional citrate-stabilized gold colloids, and rationalize the pH induced coagulation of colloidal sols.

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the voltammograms of a Pt(1 1 1) electrode in KBr solutions with an excess of a supporting electrolyte (0.1 M HClO4 and 0.1 M KClO4 + 10?3 M HClO4 + xM KBr where x varied between 5 × 10?4 and 1 × 10?2). The surface Gibbs excess, the Gibbs energy of adsorption and the charge number at a constant electrode potential and a constant chemical potential have been determined. The effect of pH on the magnitude of these parameters has been evaluated. The thermodynamic parameters for bromide have been compared to the parameters determined from previous thermodynamic studies of (bi)sulfate, chloride and OH adsorption at the Pt(1 1 1) electrode surface. As expected, bromide adsorption is stronger than for the other anions and halide adsorption seems to be limited by close packing of the adlayer. The calculated charge number values suggest that Br adsorption involves a full charge transfer, as in the case of chloride

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1 ? x) M HClO4 + x M KClO4 + 2.5 × 10?3 M NaCl with a constant ionic strength and variable pH. The total charge densities were calculated by integration of cyclic voltammetry curves. The adsorption of chloride present in the electrolyte blocked hydrogen and OH adsorption at potentials 0.3 < E < 0.7 V (SHE). Consequently, the differential capacity, charge density and surface energy become pH independent in that potential range providing pH independent integration constants for the integration of CVs. For E < 0.3 V (SHE), a complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen were determined without a need to introduce any arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin–Markov coefficient) for adsorbed hydrogen were calculated using the Gibbs excess data. The charge numbers are equal to ?1, indicating that the adsorbed species is a totally discharged hydrogen atom. For 0.1 M HClO4 + 2.5 × 10?3 M NaCl solution Gibbs excesses of both hydrogen and chloride were determined. In a narrow potential range 0.2 < E < 0.3 V (SHE) chloride and hydrogen atoms are simultaneously adsorbed at the electrode surface. We have demonstrated that their adsorption has a competitive character

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in NaCl solutions with an excess of an inert electrolyte (0.1 M HClO 4 ). A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, charge numbers at constant electrode potential and constant chemical potential for chloride adsorption at the Pt(1 1 1) surface have been determined. Our results show that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.5 ML and is only 10% smaller than the coverage expected for a close packed monolayer of chlorine atoms. We demonstrated that at negative potentials, where Cl and hydrogen adsorption overlaps, the adsorption has a competitive character.

The behaviour of a self-assembled monolayer of 2-mercaptobenzimidazole (MBI) at the Au(1 1 1) electrode has been examined using cyclic voltammetry and in situ FTIR spectroscopy. The charge associated with the reductive desorption is pH independent while the oxidative partial redeposition charge increases when the pH is lowered. This is due to differences between the nature and the solubility of the MBI desorption product. In alkaline and neutral media MBI desorbs as the thiolate. In contrast, in acidic solutions the thiol is the desorbed product. Subtractively normalized interfacial reflection Fourier transform absorption spectroscopy (SNIFTIRS) has been applied to investigate the MBI monolayer in contact with aqueous solutions of different pH. The SNIFTIRS data are in agreement with the electrochemical results. Moreover, quantitative analysis of the IR data provided evidence that adsorbed MBI molecules assume a tilted orientation with an angle of 60 ±5° between the C2 axis of the molecule and the direction normal to the gold surface.

The adsorption of citrate on Au(1 1 1) from citric acid containing electrolytes has been quantified using in situ subtractively normalised interfacial Fourier transform infrared spectroscopy (SNIFTIRS). These spectra, together with spectral simulations, provide evidence that citric acid, which contains 3 carboxylic acid groups, deprotonates fully upon adsorption. Spectral simulations are consistent with a limiting coverage of about 3 × 10 −10 mol cm −2 and the surface bound carboxylate groups adopting a tilted orientation. A model for the molecular adsorption is proposed.

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1−x) M HClO4+x M KClO4 with a constant ionic strength and a variable hydronium ion concentration. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen and OH were determined without the need to introduce an arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. We found that the maximum surface concentration of H amounts to ∼11×1014 ions cm−2 or to a coverage 0.73 monolayer (ML) in good agreement with previous estimates. The maximum surface coverage of OH is equal to ∼8×1014 ions cm−2 or to a coverage ∼0.53 ML. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin–Markov coefficient) for adsorbed H and OH were calculated using the Gibbs excess data. The charge numbers are equal to ∼1 for H and to ∼−1 for OH. These values suggest that the adsorbed species is hydrogen atom and discharged OH species.

Neutron reflectometry (NR) has been employed to study the structure and composition of thin films formed by 4-pentadecylpyridine (C15-4Py) at a gold electrode surface. The thickness and the water content of films of C15-4Py have been measured as a function of the potential applied to the electrode. At positive potentials, where condensed film is formed, the monolayer contains defects that are filled with water. At very negative potentials, the film is desorbed from the electrode surface. NR has demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface as a thicker and water rich film. # 2003 Elsevier B.V. All rights reserved.

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(111) electrode in solutions of H 2 SO 4 with an excess of an inert electrolyte (0.1 M HClO 4 ). A high quality Pt(111) single crystal electrode with a low defect density was employed in this study. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The best results were obtained when the charge was the independent variable. These results show that the maximum Gibbs excess of (bi)sulfate attains a value of ∼3×10 14 ions cm −2 , which corresponds to the packing density of ∼0.2 monolayer. This number agrees well with the results of a radioactive labeling method (Kolics and Wieckowski, J. Phys. Chem. Sect. B 105 (2001) 2588) and with recent STM studies (Itaya, Prog. Surf. Sci. 58 (1998) 121; Funtikov, Linke, Stimming and Vogel, Surf. Sci. 324 (1995) L343). The calculated Esin–Markov coefficient and electrosorption valencies exhibit an important dependence on the electrode potential (or charge).

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was applied to determine the acid dissociation constant for bisulfate adsorbed at the Pt(111) electrode surface. The experiments were carried out in NaF+HF buffer solutions with pH varying between 0.4 and 6.1. The SNIFTIR spectra were used to identify adsorbed bisulfate and sulfate ions and to monitor changes in the concentration of the adsorbed species with pH. The pKa for the adsorbed bisulfate was determined from the changes of the intensity of IR bands assigned to sulfate and bisulfate with pH. The values of pKa depend on the electrode potential and their values range between 3.3 and 4.7. They are significantly higher than the value of pKa in the bulk of the solution. The magnitude of pKa indicates that bisulfate is the predominantly adsorbed species at pH 4.7.

In situ subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of pyridine at the Au(110) electrode surface. The IR spectra provide direct spectroscopic evidence that pyridine molecules adsorb, bonded through the nitrogen atom to the Au(110) surface, in the whole range of electrode potentials. IR spectroscopy was further used to study the effect of the electrode potential on the tilt angle (angle between the C2 axis of the molecule and the direction normal to the surface) of the adsorbed molecules. The IR data consistently show that the tilt angle decreases progressively with increasing electrode potential (charge on the metal). The change of the tilt angle may be described in terms of the electric field–dipole interaction and is caused by the dielectric saturation phenomenon. At low charge density, where the field is small, the film of N-bonded pyridine molecules is less rigid and the molecules exhibit a waving motion. This motion is restrained significantly in the presence of a large electrostatic field (high charge density at the metal). The monolayer of adsorbed molecules can be considered as being effectively frozen at sufficiently high charge densities. © 2002 Elsevier Science B.V. All rights reserved.

We present a simulation of subtractively normalized interfacial Fourier transform infrared spectra (SNIFTIRS) for the case of pyridine adsorbed at the Au(111) electrode. From experimental data and SNIFTIRS simulation the tilt angles can be evaluated as a function of the electrode potential for pyridine molecules adsorbed on terrace and defect sites, providing a detailed view of the site and potential dependent molecular orientation.

The adsorption of l -phenylalanine (Phe) at the Au(111) electrode surface has been studied using electrochemical techniques and subtractively normalized interfacial Fourier transform infrared (SNIFTIR) techniques. The electrochemical measurements of cyclic voltammetry, differential capacity and chronocoulometry were used to determine Gibbs energies of adsorption and the reference (E1) and sample (E2) potentials to be used in the spectroscopic measurements. The vibrational spectra have been used to determine: (i) the orientation of the molecule at the surface as a function of potential; (ii) the dependence of the band intensity on the surface coverage; (iii) the character of surface coordination, and (iv) the oxidation of adsorbed Phe molecules at positive potentials. The adsorption of Phe is characterized by ΔG values ranging from −18 to −37 kJ mol−1 that are characteristic for a weak chemisorption of small aromatic molecules. The electrochemical and SNIFTIR measurements indicated that adsorbed Phe molecules change orientation as a function of applied potential. At the negatively charged surface Phe is predominantly adsorbed in the neutral form of the amino acid. At potentials positive to the pzc, adsorption occurs predominantly in the zwitterionic form with the COO− group directed towards the surface and the ammonium group towards the solution. At more positive potentials electrocatalytic oxidation of Phe occurs and is marked by the appearance of the CO2 asymmetric stretch band in the FTIR spectrum. Thus, relative to pzc, Phe is weakly chemisorbed at negative potentials, changes orientation at potentials close to the pzc and is oxidized at positive potentials.

Chronocoulometry and a thermodynamic analysis of charge density data were used to describe chloride adsorption at the Pt(111) electrode surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per adsorbed chloride ion at a constant potential (electrosorption valency) and at constant chloride concentration (reciprocal of the Esin–Markov coefficient) were determined. Chloride forms a chemisorption bond with platinum. The charge numbers at constant potential and at constant chloride concentration are close to unity. This result suggests that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.43 ML. This number agrees well with coverage determined in recent surface X-ray scattering experiments [1] . Adsorption of chloride at Pt(111) is potential and pH dependent. We have demonstrated that at negative potentials, co-adsorption of Cl and hydrogen atoms has a synergistic character. In contrast the adsorption of chloride and OH at positive potentials has a competitive nature.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been used to examine (i) the orientation and coordination of benzoate on the Au(111) electrode; (ii) the dependence of the IR band intensity on the surface coverage; and (iii) the chemical stability of benzoate at the Au(111) surface. The results indicate that orientation of adsorbed benzoate changes from a nearly flat co-ordination at a negatively charged surface to a tilted position at positive charge densities. The transition in the orientation of the adsorbed molecule occurs in the vicinity of the potential of zero charge in the solution investigated. The integrated IR band intensities correlate very well with the surface concentration of benzoate obtained from chronocoulometric measurements. An overall agreement between the spectroscopic and electrochemical results is excellent. The spectroscopic data demonstrate that no decomposition or oxidation of benzoate occurred at the Au(111) electrode surface over the potential range investigated.

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at the Au(111) electrode surface. The vibrational spectra have been used to study the character of surface coordination and the stability of adsorbed 4-CNPy molecules. Our studies show that 4-CNPy molecules are totally desorbed from the Au(111) surface at potentials lower than −0.7 V versus SCE and that they adsorb at the gold surface at more positive potentials. At potentials lower than 0.05 V versus SCE, the adsorption has a non-dissociative character. The 4-CNPy molecules are initially oriented flat (π-bonded) on the electrode surface and reorient from the flat to a vertical state when the electrode potential approaches 0 V (SCE). When the potential is greater than 0.05 V versus SCE, the character of 4-CNPy adsorption becomes dissociative and the adsorbed molecules partially hydrolyze to form isonicotinamide (INA). The IR spectra acquired at very positive potentials indicate that 4-CNPy is oxidized at E>0.6 V versus SCE.

The thermodynamics of solid electrodes are discussed in light of the recent measurements of surface stress. Interfacial tension and surface stress are not even approximately equal, and they generally exhibit a different dependence on the electrode potential. The variation of the interfacial strain with potential is small so that the Lippmann equation for a solid is practically the same as for a liquid electrode. Changes in the interfacial tension can be obtained by integrating the charge density over the potential.

In situ X-ray scattering and chronocoulometric experiments were performed to assess the influence of adsorption of pyridine, 2,2′-bipyridine and uracil on the driving force for the (p×√3)→(1×1) transition of the Au(111) surface. We have shown that the overall driving force is a combination of the driving force due to charge and the driving force due to the adsorbate. We have estimated the magnitude of the two driving forces and have given the upper and lower limits to this estimate. Our results show that the two driving forces are of comparable magnitude and that the interpretation of the surface reconstruction phenomena given in terms of either purely charge or a purely adsorbate effect is an oversimplification.

In situ X-ray scattering studies of the Au(111) electrode surface have been carried out in KClO4 solutions containing uracil, pyridine or 2,2′-bipyridine using grazing-incident-angle diffraction. At sufficiently negative potentials, the (p × √3) surface reconstruction forms with a stripe separation p = 23. The formation of a two-dimensional condensed uracil film or a complete monolayer of planar oriented pyridine or 2,2′-bipyridine hinders the lifting of the reconstruction. Whereas the stripe separation increases continuously over a 0.6 V range in the base electrolyte, the presence of the organic compounds blocks this process considerably. Only a small, step-like increase in p is observed, where the adlayer is disordered. There exists a 0.7 V hysteresis between the lifting and the subsequent formation of the reconstruction. The (p × √3) phase reappears at potentials where a ‘gas-like’ adlayer is formed once again. Cyclic diffractograms and potential step measurements have been employed to study the time dependence of these substrate transitions for the system uracil | Au(111).

The adsorption of an insoluble monolayer of 12-(9-anthroyloxy) stearic acid (12-AS) onto a single crystal gold electrode has been described. The spreading of the insoluble surfactant onto the metal | solution interface of the gold electrode was initially investigated with the help of cyclic voltammetry and differential capacity. Electrochemical studies indicate that the film of 12-AS molecules spreads onto the metal surface at potentials close to the potential of zero charge (pzc) and desorbs from the electrode surface at potentials that are sufficiently negative. The spreading of the film and its subsequent desorption are repeatable. The 12-AS molecule is a surfactant dye which may be used to perform spectroelectrochemical experiments. Electroreflectance spectroscopy, fluorescence spectroscopy and measurements of the light scattered by the desorbed surfactant molecules were employed to determine the mechanism by which molecules of the insoluble surfactant spread onto and desorb from the metal | solution interface of the gold electrode. With the help of the spectroelectrochemical experiments we have demonstrated that the repeatable potential-induced desorption and adsorption (spreading) of the insoluble molecules involves formation of micelles (or similar molecular organizations) in the subsurface region and spreading of the micelles onto the electrode surface.

Thermodynamic analysis of charge density data was performed to describe sulfate adsorption at the Au(111) surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed sulfate ion and the Esin-Markov coefficients were determined. The thermodynamic data indicate that sulfate adsorption at the Au(111) electrode has an ionic character and is driven by the interaction of the ion with its image charge on the metal. The surface dipole formed by the ion and its image is heavily screened by the metal and surface solvent molecules. The ionic character of sulfate adsorption at the Au(111) electrode is consistent with the results of independent second harmonic generation experiments. The second harmonic generation data indicate that adsorption of sulfate affects little the electronic structure of the metal surface.

Electrochemical and surface X-ray scattering data are compared for the Au(111) electrode in 0.01 M KBr solution in 0.1 M KClO4. The irreversible peaks in the voltammogram are well correlated with the scattered X-ray intensity from the reconstructed surface in both sweep directions. Surface charge and bromide coverage measured by the chronocoulometric technique are also correlated with the X-ray intensity. The reconstruction lifts over a potential region where the surface charge changes from − 10 to 20 μC cm−2 and the bromide coverage increases from 10% to 25% of its saturated coverage.

Cyclic voltammetry, differential capacity and chronocoulometry have been employed to study pyrazine adsorption quantitatively at the Au(111) | aqueous solution interface. The adsorption isotherms, Gibbs energies of adsorption and the electrosorption valency for pyrazine adsorption at the Au(111) surface have been determined. The results indicate that pyrazine adsorption at gold has the character of weak chemisorption which involves either the interaction of π orbitals of the aromatic ring with free electrons at the metal surface or a mixing of the n orbital at the nitrogen atom with d electronic states in the metal. Consequently the pyrazine molecule may assume either the π-bonded flat or the N-bonded vertical surface coordinations. The flat orientation is stabilized at the negatively charged surface, the vertical orientation is stabilized at the positively charged surface. A reorientational phase transition in the monolayer of adsorbed pyrazine was observed at the potential of zero charge.

Chronocoulometry was employed to measure charge density at the Au(111) electrode surface. Thermodynamic analysis of the charge density data for ionic adsorption from a solution with an excess of a supporting electrolyte is described and used to study SO 4 2− adsorption at the Au(111) electrode from a 0.1 M HClO 4 solution. The Gibbs excesses determined from the charge density data are compared with the Gibbs excesses determined from radiochemical measurements using 35 S-labelled sulphate solutions. Very good agreement between the Gibbs excesses determined by the two techniques is observed. Essin-Markov coefficients for SO 4 2− adsorption from two series of solutions were determined: (i) at constant pH and variable K 2 SO 4 concentration; (ii) at constant K 2 SO 4 concentration and variable pH. The Essin-Markov coefficients were used to identify the nature of the adsorbed species (HSO 4 − or SO 4 2− ). Our results indicate that SO 4 2− ion is the adsorbed species even if HSO 4 − predominates in the bulk of the solution. Chronocoulometry and radiochemistry were used to study SO 4 2− adsorption at the Au(111) electrode. Very good agreement between the Gibbs excesses determined from chronocoulometric and radiochemical measurements was observed. This agreement is an important verification of the reliability of these two in-situ techniques for measuring surface coverages. Although the two techniques give the same physical quantity, which is the Gibbs excess of adsorbed ions, each of them probes different properties of the interface and therefore has different limitations. The quantity measured by chronocoulometry is the electrode charge density. The surface concentration is calculated from σ M employing one integration step and one differentiation step and assuming the validity of the electrocapillary equation. Therefore the chronocoulometric technique can be applied only within the double-layer region of the gold electrode. However, since both the Gibbs excess and the charge density are measured simultaneously, this technique provides not only the surface coverages but also an additional electrical variable needed for the interpretation of the coverage data. In contrast, radiochemistry provides information about the Gibbs excess from direct counts of the radioactivity when the electrode with adsorbed radiolabelled material is pressed against the scintillator window. Therefore this technique can be used to study adsorption on both reactive and non-reactive materials. However, the technique is restricted by the efficiency in squeezing out the solution containing radioactive species from the thin layer between the electrode surface and the scintillator window. When the count rates from solution trapped in that gap are high with respect to surface counts, the adsorption measurements are difficult and ultimately become impossible to conduct. In fact, the method cannot be employed for adsorption measurements when the bulk concentration of labelled solute is higher than 10 −2 M [1]. In addition, the radiochemical technique provides information about the Gibbs excess only.

Adsorption of Cl − at the Au(111) electrode in the presence of underpotentially deposited copper adatoms is described. It is shown that the underpotential deposition of copper promotes adsorption of Cl − , and at high or intermediate underpotentials a mixed overlayer which consists of copper adatoms and coadsorbed Cl − ions is formed. At intermediate underpotentials, the surface concentration of Cl − in the mixed overlayer amounts to 8 × 10 14 ions cm −2 which is only 10% less than the concentration of Cl − in the close-packed monolayer of Cl − . This surface concentration of Cl − is independent of the concentration of Cl − and Cu 2+ in the bulk. In solutions of low Cl − or Cu 2+ concentrations and at low underpotentials the surface concentration of Cl − decreases when the copper coverage approaches a monolayer. In solutions of high copper and chloride concentrations and low underpotentials the surface concentration of Cl − increases above the value of 8 × 10 14 ions cm −2 . The increase in the surface concentration of Cl − is observed when the copper coverage exceeds one monolayer. Therefore it may suggest formation of copper clusters or a trilayer.

Chronocoulometry, radiochemistry and Raman spectroscopy were used to study pyridine adsorption at a polycrystalline gold electrode. Fair agreement between the surface concentrations determined from chronocoulometric experiments and radiochemical measurements was observed. The chronocoulometric technique proved to be more accurate than the radiochemical method; however, it is restricted to ideally (or nearly ideally) polarizable electrodes. In contrast, the radiochemical technique is more universal and can be employed to study adsorption at both reactive and non-reactive electrode surfaces. Raman spectroscopy gave a signal which, because of the dipole depolarization effect, displays a non-linear dependence on the surface concentration of the adsorbed molecules for coverages greater than 2 3 monolayer. Therefore Raman spectroscopy has a limited value for quantitative analysis of adsorbed species.

Electrochemistry, Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) have been employed to study the stability of the Au(210) electrode surface in the presence of adsorbed pyridine. The experiments have been performed in an ultrahigh vacuum (UHV)-electrochemistry apparatus that allows for a fast transfer of the sample between an UHV chamber equipped with LEED and AES equipment and an electrochemical chamber in which electrochemical experiments can be performed. The “dry emersion” technique has been employed to emerse the electrode at a controlled potential and to transfer it into the UHV chamber for analysis of its surface composition using AES and its surface structure by LEED. Experiments performed in a weakly adsorbing electrolyte and in 10−3 M pyridine solution have shown that the Au(210) electrode surface displays a remarkable stability at the metal-solution interface. Its surface structure is apparently unchanged when the electrode charge density varies between −30 μC cm−2 and + 30 μC cm−2, both in a pure weakly adsorbing electrolyte and in the presence of the chemisorbed organic molecule.

This paper describes first equilibrium studies of partitioning an insoluble surfactant (octadecanoic acid) between the gas—solution and solid—solution interfaces. A novel experimental approach is described in which a Wilhelmy plate is used to measure the film pressure of the insoluble organic monolayer spread at the gas—solution interface while simultaneously the film pressure of surfactants that spread from the gas—solution interface to the metal—solution interface of the electrode is monitored in situ by electrochemical means. Using as an example gold single-crystal electrodes, it is demonstrated that the equilibrium between the two-dimensional phases is strongly dependent not only on the electrode potential but also on the morphology of the solid electrode surface. The significance of these experiments for the choice of optimum conditions for transfer of insoluble monolayers from the gas solution onto the solid surface using the Langmuir—Blodgett technique is discussed.

It will be shown that the main source of second harmonic generation (SHG) (including isotropic and anisotropic contributions) arises from an intermediate layer covering the same region where electrochemical processes take place. Therefore, SHG exhibits an enormous potential for surface studies and is very useful as an in situ spectroscopic technique in electrochemistry. For instance, the formation and lifting of surface reconstruction can be monitored in situ under the influence of both potential and adsorption of ions or small organic molecules. For the Au(111) surface, the Au(111) - (√3 × 23) reconstruction leads to a symmetry change of the type C 3v → C s . At the Au(100) electrode, reconstruction gives rise to a significant contribution of a three-fold symmetry to the observed anisotropy pattern and a complex potential dependence of the anisotropy is observed. In addition, adsorption and potential controlled changes of the symmetry pattern of the SHG intensity can be separated from the effect of (re)charging the electrode. The variation in the static, interfacial electric field alters the surface electron distribution and, hence, their net polarizability in the z direction. This effect can be used to study—during the course of increasing coverage—possible phase transitions within the adsorbate layer.

Adsorption of pyridine at an Au(210) single-crystal electrode has been investigated using chronocoulometry, cyclic voltammetry and phase-sensitive ac voltammetry. The adsorption parameters, such as the relative Gibbs surface excesses, the Gibbs energies of adsorption, the electrosorption valencies and the potential drop across the inner layer, have been calculated. These results suggest that pyridine molecules assume a vertical, N-bonded, orientation at the Au(210) surface. The full coverage Gibbs energy of adsorption at the potential of maximum adsorption is approximately equal to −63 kJ mol−1, which is a value typical of weak chemisorption. The adsorption of pyridine at the (210) surface was compared with adsorption at the (311), (110), (100) and (111) planes of gold.

The adsorption of benzonitrile (BN) at a gold (111) single crystal electrode has been investigated quantitatively using chronocoulometry. The procedure described in this paper differs slightly from previous work in that experiments were performed not only in neutral pH solutions but also at pH 2.4 in order to shift the potential corresponding to the oxidation of the metal surface toward more positive values and hence extend the double layer region effectively. The following adsorption parameters: film pressure, Gibbs surface excess, Gibbs energy of adsorption and electrosorption valency, were determined as functions of the electrode potential and the charge density for concentrations of BN up to 0.03 M (close to saturation). The values of the relative Gibbs excess indicate that adsorbed BN molecules are oriented parallel to the electrode surface at negative and moderately positive potentials and low bulk concentrations. The Gibbs energy at maximum adsorption is equal to −27 kJ mol −1 which suggests that BN is chemisorbed weakly on Au(111). The results presented here are compared with those obtained from other studies of BN adsorption on gold and mercury.

The energetics of pyridine adsorption onto a Au(311) single-crystal electrode surface have been investigated using chronocoulometry. Adsorption parameters such as the relative Gibbs surface excesses, the Gibbs energies of adsorption and electrosorption valencies have been determined. The data have been analysed in terms of both the electrode potential and surface charge density. Over the potential region investigated (−0.8 V to +0.6 V), pyridine has been found to adsorb in the vertical orientation with the nitrogen end of the pyridine molecule facing the Au(311) surface. The limiting surface concentration for this orientation was found to be 6.6±0.3×10−10 mol cm−2. The Gibbs energies of adsorption were calculated using the Henry's law isotherm. At the potential of maximum adsorption, ΔGmax0 was found to be equal to −44 kJ mol−1. The absolute value for ΔGmax0 is fairly large, indicating that pyridine is weakly chemisorbed onto the Au(311) electrode surface.