1,721,038 research outputs found
Layer-by-layer PMIRRAS characterization of DMPC bilayers deposited on a Au(111) electrode surface
No full textA combination of Langmuir-Blodgett and Langmuir-Schaefer techniques was employed to deposit 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers at a gold electrode surface. One leaflet consisted of hydrogen-substituted acyl chains, and the second leaflet was composed of molecules with deuterium-substituted acyl chains. This architecture allowed for layer-by-layer analysis of the structure of the bilayer. Photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to determine the conformation and orientation of the acyl chains of DMPC molecules in the individual leaflets as a function of the potential applied to the gold electrode. The bilayer is adsorbed onto the metal surface when the field applied to the membrane does not exceed 108 V/m. When adsorbed, the bottom leaflet is in contact with a hydrophobic metal surface, and the top leaflet is interacting with the aqueous solution. The asymmetry of the environment has an effect on the orientation of the DMPC molecules in each leaflet. The tilt angle of the acyl chains of the DMPC molecules in the bottom leaflet that is in contact with the gold is ~10° smaller than that observed for the top leaflet that is exposed to the solution. These studies provide direct evidence that the structure of a phospholipid bilayer deposited at an electrode surface is affected by interaction with the metal
Thermodynamic studies of bromide adsorption at the Pt(111) electrode surface perchloric acid solutions: Comparison with other anions
No full textThe 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 chlorid
Determination of the Gibbs excess of H adsorbed at a Pt(111) electrode surface in the presence of co-adsorbed chloride
No full textThe 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 characte
Thermodynamic studies of chloride adsorption at the Pt(111) electrode surface from 0.1 M HClO4 solution
No full textThe 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 HClO4). 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 characte
Thermodynamic approach to the double layer capacity of a Pt(1 1 1) electrode in perchloric acid solutions
No full textA thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(1 1 1) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 − x) M KClO4 + x M HClO4. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 ± 5 μF/cm2, in good agreement with previous estimates reported in the literature by using other approaches
Atomic force microscopy and electrochemical impedance spectroscopy studies of alamethicin embedded into a biomimetic membrane
Full text linkThis thesis is an investigation of the formation of pores by alamethicin molecules in negatively charged bilayers. Unilamellar vesicles of neutral 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and negatively charged 1,2-dimyristoyl-sn-glycero-3-phosphorylglycerol (DMPG) in the absence and presence of alamethicin were fused onto the β-thioglucose modified gold surface. Atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) were employed to investigate the resulting floating bilayer lipid membranes (fBLMs). A direct visualization of the alamethicin pores was obtained from molecular resolution AFM images, revealing that alamethicin forms porous nanoclusters with an average pore diameter of 2.3 ± 0.3 nm. When alamethicin was inserted into the fBLM, a significant decrease in membrane resistivity was observed, indicating that the peptides are forming ion conducting pores. This thesis will also describe the voltage-gated behaviour of alamethicin in a fBLM composed of a mixture of negatively charged egg-PG and neutral DMPC phospholipids. Egg-PG was substituted for DMPG to improve membrane fluidity. At open circuit potential (OCP), the AFM images show that alamethicin molecules inserted into the bilayer form porous nanocluster aggregates within the phospholipid matrix with average pore diameters of 2 ± 0.2 nm. The EIS data collected at OCP show that presence of alamethicin decreases the membrane resistivity by about one order of magnitude. When negative potentials are applied to the electrode, high resolution AFM images show that there is a smooth bilayer on the surface at OCP (0.1 V vs Ag/AgCl), and the bilayer is buckled as the potential stepped to -0.4 V vs Ag/AgCl. The EIS data show that the membrane resistivity changes from 897 ± 85 kΩ cm2 to 153 ± 24 kΩ cm2, when the potential is stepped from 0.1 V to -0.4 V vs Ag/AgCl. The effect of amiloride, an ion channel blocker, on the alamethicin pore formation was investigated. AFM images of DMPC/egg-PG/alamethicin/amiloride show amiloride makes changes on the surface structure of the bilayer and EIS measurements show that amiloride causes a dramatic increase in membrane resistance from 0.89 ± 0.08 MΩ cm2 to 10.6 ± 3.1 MΩ cm2, confirming that amiloride has ion channel inhibiting properties
Adsorption and aggregation of sodium dodecyl sulfate on Au(111) electrode surfaces
No full textElectrochemical measurements, scanning probe microscopy and neutron reflectivity experiments have been combined to present a complete description of the adsorption and aggregation of sodium dodecyl sulfate at the Au(111) interface. The various states of adsorption are found to be a function of bulk solution surfactant concentration and the charge on the electrode surface. At low surfactant concentrations, SDS adsorbs to form two-dimensional arrays of "dimer pairs". At higher concentrations, additional SDS molecules adsorb on the surface resulting in the formation of hemimicellar surfactant aggregates. This work presents the first direct images of the potential controlled phase transition between the hemimicellar and condensed states of a dodecyl sulfate (SDS) film at the Au(111) electrode surface. The adsorbed SDS forms stripe-shaped hemimicellar aggregates at small or moderate charge densities at the electrode. Adsorbed SDS molecules are ordered and form a long range two-dimensional lattice. It is proposed that each unit cell contains two flat-lying SDS molecules. (Abstract shortened by UMI.
Study of the Surface Chemistry and Dissolution Rate of Gold in ThiosulfateSolutions with Organic Additives
Full text linkGold nanorods were fabricated by electrodeposition of gold in porous alumina templates. Thesesubstrates were characterized by scanning electron microscopy (SEM), specular reflectancespectroscopy, and surface enhanced Raman spectroscopy (SERS). Reflectance measurements revealedminima at 550 nm and 750 nm and these were assigned to transverse plasmon modes. SERSexperiments with 4‐aminothiophenol determined that the surface enhancement factor was 105 – 106.These SERS substrates were then used for the study of the surface chemistry of gold in solutionsof thiosulfate and its decomposition products. It was established that there is initial adsorption ofthiosulfate followed by formation of surface bound trithionate and tetrathionate. At extended times(3 hours) there is an increase in the surface concentration of gold sulfide and elemental sulfur which isthe result of thiosulfate decomposition. This passive layer prevents further interaction of gold with theleaching solution.The organic compounds 2‐thiouracil, 3‐mercaptopropionic acid, and L‐cysteine were studied foruse as additives in thiosulfate leaching solutions. It was found that 2‐thiouracil quickly formed a selfassembledmonolayer (SAM) and this minimized the interaction of thiosulfate with the gold surface. 3‐mercaptopropionic acid also showed evidence of SAM formation but thiosulfate and its decompositionproducts could also be detected with SERS. When L‐cysteine was used as an additive the SERS spectrawere largely featureless except for clear signs of gold‐sulfur bonds. It was proposed that this was due toadsorption of L‐cysteine which has a low scattering cross section.Leaching rates for thiosulfate solutions with and without the additives were determined byinductively coupled plasma with optical emission spectroscopy, ellipsometry, and UV‐Vis spectroscopy.Thiosulfate had the highest rate of 1.2 nm gold / 24 hours though the organic additives were similar.When these rates were adjusted to consider only the first 30 minutes of exposure they were similar toreported values.Barrick Gold Corporatio
Electrochemical studies of adsorption of insoluble pyridine surfactants and their mixtures at a gold(111) electrode
Full text linkThe adsorption of insoluble surfactants, 4-pentadecyl-pyridine (C15-4Py), 10-decyl-9-(2-(4-pyridyl)ethyl) anthracene (DPEA), and their mixtures of 10 and 25 mol % DPEA in C15-4Py have been studied using cyclic voltammetry, cyclic ac voltammetry and chronocoulometry. It was found that a monolayer of surfactants is transferred from the gas-solution (G/S) to the metal-solution (M/S) interface using the single horizontal touching method. This monolayer is unstable and transforms into a bilayer which has properties similar to the bilayer obtained by the double horizontal touching technique. Hence the equilibrium between surfactants at the G/S and at the M/S interfaces is established. Pure DPEA is more stable in the monolayer state. The following aspects of the mechanism of adsorption of insoluble pyridine surfactants were determined: (1) In surfactant bilayers, spontaneous transport of the molecules of the surfactant from the overlayer to the underlayer takes place upon the potential-induced reorientation of the pyridine heads of the surfactants. The reverse process requires an activation energy and hence is shifted negatively with respect to that, observed in the anodic scan. The activation energy and the reorientation rate depend on the difference in the packing density of the surfactant underlayer before and after the reorientation. (2) Addition of DPEA to C15-4Py decreases the packing density of the adsorbed film resulting in the decrease of the activation energy and an increase in the rate of the reorientation of pyridine heads. (3) When desorbed, surfactants form micelle-like aggregates that can adsorb charged ions and experience electrostatic attraction to the oppositely charged electrode
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