共查询到20条相似文献,搜索用时 15 毫秒
1.
Secil Tekin-Celebi Ali Osman Solak Zafer Ustundag Serkan Demirci 《Chemical Papers》2012,66(12):1146-1156
Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)6]3?/4? redox probe solutions at different pH; this allowed us to estimate the surface pK a values. Acidity constants for Pt-BA surface were found to be pK a (3.09 ± 0.25), (4.89 ± 0.11), and (3.91 ± 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK a (3.16 ± 0.45), (4.24 ± 0.40), and (5.64 ± 0.12). The Pt-BA surface pK a values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK a values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK a of bulk pABA. 相似文献
2.
Abiman P Wildgoose GG Crossley A Jones JH Compton RG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(34):9663-9667
The pKa value of protonated Jeffamine (bis(3-aminopropyl) terminated polyethylene glycol) in solution and attached as a monolayer to graphite surfaces has been determined using potentiometric titration. The protonated Jeffamine was found to have a pKa value of 9.7 in solution at 25 degrees C, whereas this value decreases to 7.1 when it is attached to a graphite surface. Potentiometric titrations from 25 to 40 degrees C allowed us to determine the surface pKa of the protonated Jeffamine at each temperature studied and hence to determine the enthalpy, entropy and Gibbs energy changes associated with the deprotonation of the amino-terminated surface bound Jeffamine groups. It was found that the enthalpic contribution is negligibly small and the evaluation of these thermodynamic parameters controlling the shift in surface pKa value indicates that this process is controlled by entropic contribution arising from the ordering/disordering of solvent molecules at the carbon-water interface. This suggests that the long chain Jeffamine molecules are oriented on the carbon surface rather than existing in the bulk solution. 相似文献
3.
The well-known failure of the Hammett Ho function to describe the prototropic reaction isotherms of aromatic carboxamides can be corrected by taking account of the hydration requirements of the prototropic reactions of the carboxamides relative to the hydration requirements of the dissociations of the primary amines used as indicators to establish the Ho scale. This approach has been successfully applied to benzamide and the naphthamides. For up to a 4:1 molar ratio of water to sulfuric acid, the amide acidity scale, HA and the Ho scale are simply related by HA = Ho — 2 log aw , where aw is the activity of water. 相似文献
4.
Wei HZ van de Ven TG Omanovic S Zeng YW 《Langmuir : the ACS journal of surfaces and colloids》2008,24(21):12375-12384
The interactive behavior of flavin adenine dinucleotide (FAD) with a bare glassy carbon electrode (GCE) and a Ru-modified GCE was investigated. The reduction of FAD at a GCE/ruthenium-modified GCE surface is a quasi-reversible, surface-controlled process, and our data implied that the attachment of FAD onto the surface is caused by nonspecific adsorption instead of covalent linkage, in which the adenine ring of FAD adopts a flat orientation on the GCE surface in neutral and dilute solutions in order to maximize the pi-pi stacking with the carbon surface and reorients to a perpendicular orientation as the surface gets more crowded. FAD desorption during the exchange with nicotinamide adenine dinucleotide (NAD+) is one order of magnitude slower than desorption in the absence of NAD+, which indicates a strong interaction between FAD and NAD+. General knowledge of the interactive behavior of NAD+ on a FAD-adsorbed GCE provides useful information for the design of a modified electrode surface for the generation of NADH from NAD+. 相似文献
5.
Currentless deposition of precious metals, Au, Pt, Ir, Ru, Rh and Pd, on glassy carbon surfaces in LiCl-KCl eutectic at 450°C has been reported. When the electrodes are simply immersed in the melt solutions containing the metal ions, metal deposits equivalent to several atomic layers are obtained within a few minutes. Chronopotentiometric stripping of deposited metals and the decays of open-circuit potentials during the soaking experiments strongly suggest that the deposition is related to the surface redox properties of carbon substrates. Interpretation of the deposition process based on the mixed potential concept is presented, and the substrate reactivity derived from the deposited amount is ascribed to the surface redox reactions involving bulk O2? ions, superficial water and surface functional groups. 相似文献
6.
The electrochemical properties of FAD adsorbed on graphite were studied with cyclic voltammetry. A film with a surface coverage of up to 2×10?8 mol cm?2 was formed after 4 h. The film-covered electrodes were stable for long times and they were investigated in buffers without any FAD. A single well-behaved wave was observed, k0≌1 s?1. The pH dependence of E1/2 was studied. On glassy carbon, platinum and gold much less FAD was adsorbed and it was removed with one rinse. 相似文献
7.
Synthetic monoclinic and tetragonal vanadium-doped zirconias (VZrO2) with vanadium loading ranging from 0.5 to 15 mol% are used to modify glassy carbon and graphite/polyester composite electrodes able to detect oxygen and hydrogen peroxide in neutral aqueous media. Electrodes modified by monoclinic VZrO2 decrease the overpotential for the reduction of oxygen and hydrogen peroxide in neutral and alkaline media and enhance their reduction currents with respect to unmodified carbon electrodes. This is associated to seven-coordinated vanadium centers isomorphously substituting zirconium ones in the ZrO2 lattice. The catalytic effect shows site-selectivity, since it is almost entirely absent in tetragonal VZrO2 in which eight-coordinated vanadium sites exists. Under optimized conditions using differential pulse detection mode, the height of the cathodic catalytic current peak is directly proportional to the hydrogen peroxide concentration over the concentration range 5-400 μM with a sensitivity of 170 μA/mM at pH 10.0. The detection limit (3σ) is calculated as 0.9 μM. 相似文献
8.
Vase KH Holm AH Pedersen SU Daasbjerg K 《Langmuir : the ACS journal of surfaces and colloids》2005,21(18):8085-8089
A new versatile method has been developed for the electrochemically assisted grafting of carbon materials. The approach is based on the reduction of iodonium salts and allows the immobilization not only of aryl groups, such as phenyl or nitrophenyl, but also of alkynyl groups under mild conditions. In particular, the immobilization of alkynyl groups is important because such grafting cannot be accomplished using any other known reductive procedure. The electrochemical properties of the grafted surfaces with estimated coverages of (4-6) x 10(-)(10) mol cm(-)(2) are investigated against the ferrocene and Fe(CN)(6)(3)(-) solution probes. The analysis of the surfaces is carried out by means of cyclic voltammetry and X-ray photoelectron spectroscopy. 相似文献
9.
Based on the literature values, the pK1 and pK2 for pyrazine and its six methyl derivatives, the correlation with electron density on the nitrogen atoms N1 and N4 was found. The effect of methyl group was proved to be additive. The total electron density, π-electron density, and dipole moments for compounds under study were determined by the CNDO /2-MO method. 相似文献
10.
Andrew Garton William T. K. Stevenson 《Journal of polymer science. Part A, Polymer chemistry》1988,26(2):541-557
Dispersions of carbon blacks and chopped carbon fibers in epoxy resins may be characterized by infrared internal reflection spectroscopy (IR–IRS). The high surface areas of the carbons (often ≥ 100 m2/g) ensure a high degree of interfacial contact between the carbon surface and the polymer. The crosslinking kinetics and final crosslinked state of an anhydride-cured epoxy resin are shown to be affected strongly by the carbons through adsorption of the tertiary amine catalyst at oxidation sites on the carbon surface. Oxidative treatments of the carbons (nitric acid oxidation, air oxidation) increase the effect on the crosslinking chemistry. Carbon dioxide treatment of the carbons, which produces a basic surface, reduces the effect on the crosslinking. The effects on the crosslinking kinetics were confirmed by differential scanning calorimetry (DSC). The relevance of these results to the characterization of the interphase in carbon fiber/epoxy composites is discussed. 相似文献
11.
Blanco SE Almandoz MC Ferretti FH 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(1-2):93-102
In this paper we determine the overlapping pK(a) values of resorcinol in water, applying a UV-Vis spectroscopic method that uses absorbance diagrams. On the other hand, in order to explain the pK(a) values obtained, we also investigate the molecular conformations and solute-solvent interactions of the resorcinate anions, using ab initio and density functional theory methods. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-donor capability, are proposed. The mentioned reactions and equilibria constituted the indispensable theoretical basis to calculate the acidity constants of resorcinol. Basis sets at the HF/6-31 + G(d) and B3LYP/6-31 + G(d) levels of theory were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the resorcinate anions and water molecules. In this way, it was determined that in alkaline aqueous solutions the monoanion and dianion of resorcinol are solvated with two and four molecules of water, respectively. The agreement between the experimentally determined pK(a) values and those reported in the literature demonstrates the applicability and accurateness of the spectroscopic method here used. On the other hand, the agreement between the experimental and theoretically calculated pK(a) values provides solid support for the acid-base reactions proposed in this work. 相似文献
12.
Contrary to earlier observations a characteristic luminescence due to the C6H5NH+3 ion was observed. The ion at 80 K, gives a fluorescence similar to that of benzene and a phosphorescence similar to that of aniline. The benzene type fluorescence changes to the aniline type on warming. It is suggested that an excited C6H5NH+3 ion giving rise to the fluorescence becomes dissociated in the excited state. Computations for a change in the pKa values for excited singlet and triplet states of C6H5NH+3 ion also suggest that is a larger probability of losing the proton in the excited state. 相似文献
13.
Doshi DA Shah PB Singh S Branson ED Malanoski AP Watkins EB Majewski J van Swol F Brinker CJ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(17):7805-7811
Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhydrophobic (SH) surface submerged in water. A low-temperature, low-pressure technique was used to prepare a rough, highly porous organosilica aerogel-like film. UV/ozone treatments were used to control the surface coverage of hydrophobic organic ligands on the silica framework, allowing the contact angle with water to be continuously varied over the range of 160 degrees (superhydrophobic) to <10 degrees (hydrophilic). NR shows that the superhydrophobic nature of the surface prevents infiltration of water into the porous film. Atomic force microscopy and density functional theory simulations are used in combination to interpret the NR results and help establish the location, width, and nature of the SH film-water interface. 相似文献
14.
Andrew Garton William T. K. Stevenson S. P. Wang 《Journal of polymer science. Part A, Polymer chemistry》1988,26(5):1377-1391
The effects of carbon blacks, chopped carbon fibers, and crushed carbon fibers on the crosslinking chemistry of a diglycidyl epoxy resin/m-phenylenediamine system were examined by infrared (IR) spectroscopy and differential scanning calorimetry (DSC). The carbon and graphite surfaces were given oxidizing and reducing treatment to simulate the surface treatment of carbon fibers used in the manufacture of composites. The oxidized carbon surfaces initially accelerated epoxy–amine reactions but inhibited the later stages of the reaction such that the final extent of cure was reduced. The oxidized carbons also preferentially adsorbed the amine curing agent, resulting in a stoichiometric imbalance at the interface. 相似文献
15.
The X-ray crystal structure of a Pt(II) complex of composition trans-[(NH(3))(2)Pt(1,9-DimeA) (1,9-DimeAH)](ClO(4))(3) (2) with 1,9-DimeA = 1,9-dimethyladenine and 1,9-DimeAH(+) = 1,9-dimethyladeninium) is presented. Complex 2 forms upon deprotonation of one of the exocyclic amino groups of the adeninium ligands in trans-[(NH(3))(2)Pt(1,9-DimeAH)(2)](ClO(4))(4) (1), where the two nucleobases are in a head-tail arrangement. The low pK(a1) of 1 (4.1 +/- 0.2) is a consequence of a combination of the effects of metal coordination to N7 of the purine base and efficient stabilization of the deprotonated species. This feature is supported by the results of the structure determination of 2, which displays a head-head orientation of the two bases and intramolecular H-bonding between the imine group of 1,9-DimeA and the amino group of 1,9-DimeAH. In the fully deprotonated species trans-[(NH(3))(2)Pt(1,9-DimeA)(2)](ClO(4))(2) (3), the two nucleobases are again in a head-tail arrangement. The findings are of relevance with regard to the concept of "shifted pK(a) values" of nucleobases. This concept is applied to rationalize acid-base catalysis reactions involving nucleobases of DNA and RNA which occur in the near-physiological pH range. 相似文献
16.
Polyphenolic acids are a complex group of compounds that have attracted enormous attention in the last few years because of their biological properties. In this work, the proportion of organic modifier and the pH of acetonitrile-water mixtures used as mobile phases were optimized in order to separate a series of polyphenolic compounds. The linear solvation energy relationship formalism based on the single solvent polarity parameter, E(T)N was used to predict their chromatographic behavior as a function of the percentage of acetonitrile in the eluent. Moreover, the correlation established between retention and the pH of the aqueous-organic mobile phase was used to optimize the pH of the mobile phase. The optimized mobile phase is composed of acetonitrile and formic acid buffer adjusted to pH 4.25, with 12% (v/v) acetonitrile. Also, the pKa values of polyphenolic acids in acetonitrile-water mixtures were determined using chromatographic data, and in order to validate the optimized conditions, a series of polyphenolic compounds was studied in strawberries. 相似文献
17.
Peter van de Weijer Hans Thijsse Douwe van der Meer 《Magnetic resonance in chemistry : MRC》1976,8(4):187-191
The pH dependence of the 13C chemical shifts (δ) of the diazanaphthalenes has been recorded. From this dependence the pKa values have been determined using the Henderson-Hasselbach equation. The change in 13C chemical shifts under the influence of nitrogen protonation (Δδ) has been predicted using the Δδ values of quinoline and isoquinoline. The correlation between observed and expected Δδ values of the symmetric diazanaphthalenes is very good. Assuming these changes in chemical shifts to be of general validity, the site of protonation in the asymmetric diazanaphthalenes has been determined by comparison of the expected Δδ values for α- and β-nitrogen protonation with the observed ones. The site of protonation for 1,6- and 1,7-naphthyridine is the β-nitrogen atom, whereas for cinnoline both monoprotonated species are present in a significant amount. 相似文献
18.
The conversion of the plant photoreceptor phytochrome from an inactive (Pr) to an active form (Pfr) is accomplished by a red-light induced Z --> E photoisomerization of its phytochromobilin chromophore. In recent years, the question whether the photoactivation involves a change in chromophore protonation state has been the subject of many experimental studies. Here, we have used quantum chemical methods to calculate relative ground and excited-state pKa values of the different pyrrole moieties of phytochromobilin in a protein-like environment. Assuming (based on experimental data) a Pr ZaZsZa chromophore and considering isomerizations at C15 and C5, it is found that moieties B and C are the strongest acids both in the ground state and in the bright first singlet excited state, which is rationalized in simple geometric and electronic terms. It is also shown that neither light absorption nor isomerization increases the acidity of phytochromobilin relative to the reference Pr state with all pyrrolenic nitrogens protonated. Hence, provided that the subset of chromophore geometries under investigation is biologically relevant, there appears to be no intrinsic driving force for a proton-transfer event. In a series of benchmark calculations, the performance of ab initio and time-dependent density functional theory methods for excited-state studies of phytochromobilin is evaluated in light of available experimental data. 相似文献
19.
J. J. Van Luppen J. A. Lepoivre R. A. Dommisse F. C. Alderweireldt 《Magnetic resonance in chemistry : MRC》1979,12(7):399-404
1H and 13C chemical shift parameters are given for 4-piperidones and their derivatives in buffered aqueous solution. The 13C shift increments of methyl substituents on the nitrogen atom are discussed. The pH-shift dependence is studied in detail and pKa values are given for ketone forms and hydration products. The hydration equilibria are measured as a function of pH and temperature. 相似文献
20.
In the present work, models describing the effect of the pH on the chromatographic and electrophoretic behaviour for polyprotic peptides were compared. The proposed models can be simultaneously used for determination of dissociation constants and selection of the optimum pH for the separation of peptides, in water and acetonitrile-water mixtures widely used in liquid chromatography and in capillary electrophoresis. The models use the pH value measured in the acetonitrile-water mixture instead of the pH value in water and take into account the effect of the activity coefficients. They permit the determination of the acidity constants in the aqueous and hydro-organic mobile phase from chromatographic retention and electrophoretic migration measurements, respectively. The values obtained by both proposed techniques agree with the potentiometric values previously determined. The suitability of the proposed models for predicting chromatographic and electrophoretic behaviour of compounds studied from a limited number of experimental data was also compared. The separation between solutes by both techniques in a complex mixture can be easily predicted, making simple and rapid pH selection to achieve optimum separation. 相似文献