Maria Jurkiewicz-Herbich

Standard states and standard adsorption Gibbs energy of substances at electrode interfaces

Abstract The physical meaning of ΔG°ad is discussed in terms of choice of standard states, form of the isotherm equation, true adsorption equilibrium reaction and contributions from interactions in the solution bulk. It is shown that the quantitative value of ΔG°ad may be misleading if the above factors are not taken into account properly. The particular case of thiourea adsorption on Hg is discussed in detail. It is recommended that surface and bulk concentrations are expressed as mole fractions whenever possible so as to minimize problems of ΔG°ad interconversion among different isotherms.

The temperature effect on thermodynamic parameters in the adsorption of chloride ions at the mercury/aqueous HC1 solution

Summary The adsorption of Cl− ions on mercury from aqueous HCl solutions was studied as a function of concentration (0.1–1.4 M) and temperature (25, 40 and 60°C) using measurements of differential capacity and of interfacial tension. It was found from the temperature variation of the Esin-Markov coefficient and of the adsorbed charge, q′Cl−, that the adsorption of Cl− ions on mercury decreases with increasing temperature. Some parameters characterizing the structure of adsorbed ionic layers at various temperatures were determined on the basis of Parsonss thermodynamic analysis. The effect of temperature on the components of the inner layer capacity and on the (x2−x1)/x1 thickness ratio were discussed. In addition, some data concerning the entropy of adsorption of Cl− ions on mercury were presented.

Adsorption study of amino acids on a polycrystalline gold electrode

Abstract The adsorption of serine (Ser), tyrosine (Tyr) and histidine (His) on a gold electrode from 0.1 M LiClO 4 aqueous solutions was investigated. The adsorbates studied are the natural amino acids that play an important role in various biological processes. The experimental data obtained from ac impedance measurements were analysed to determine the dependence of adsorption parameters, i.e. the standard Gibbs energy of adsorption (Δ G 0 ) and parameter of interactions in the adsorbed layer on the electrode potential and pH of the solution. The Gibbs energy of adsorption was calculated by fitting Henrys and Frumkins isotherms to the experimental data. It was found that adsorption of amino acids on the gold electrode decreases in the order, Tyr≥His>Ser. The parameters of adsorption of amino acids on the gold electrode were compared with the corresponding data for Hg electrode.

Structure of the mercury/water + methanol mixtures interface in a thiourea-containing system

Abstract The adsorption of thiourea at mercury from water+methanol mixtures containing 0.1 M KF and various mole fractions of methanol, was studied on the basis of previously published differential capacity results and the potential shift produced by concentration changes of thiourea. The data were analysed to obtain the surface pressure and the surface excess of thiourea respectively, as a function of charge and bulk thiourea concentration. Then, using the methods previously developed by Parsons, Frumkin isotherm constants and electrostatic properties of the inner layer were determined. The parameters are compared with those published by Parsons et al. for the adsorption of thiourea from pure water and methanolic solutions.

Adsorption of nicotinic acid at the mercury—solution interface

Abstract Differential capacity measurements are reported for the adsorption of nicotinic acid (Nc) at the mercury electrode in water using buffered solutions at various pH and aqueous solutions with KF as a supporting electrolyte. The data were analysed to obtain the surface pressure and the surface excess of Nc respectively, as a function of charge and bulk Nc concentrations. Then, using the methods previously developed by Parsons, the Frumkin isotherm constants were determined and compared with those published for the adsorption of pyridine. From results obtained, a structural picture of the adsorption layer has been outlined.

Adsorption of thiourea on mercury in water + methanol mixtures

Abstract The adsorption of thiourea on mercury has been investigated by capacitance measurements in three water + methanol mixtures, having various mole fractions of methanol. The back integration method was employed to obtain the curves of surface charge densities at the mercury surface for different concentrations of thiourea in the solution. As a test of specific adsorption in the systems studied the Esin-Markov plots were used.

The supporting electrolyte effect at the mercury/ethanolic solution interface as found from thiourea experiments

Abstract The fact that different supporting electrolytes tend to complicate comparisons of the adsorption characteristics of thiourea (TU) was the subject of the present study. In the systems investigated the inner layer structure at the mercury/ethanolic solution interface has been described using TU in the absence and in the presence of coadsorbed anions. All this data suggests that in the systems studied, only KPF 6 should be used as in inert supporting electrolyte to determine the inner layer structure of the solvent.

Adsorption of nicotinamide and nipecotamide from aqueous solutions at the mercury electrode

Differential capacity measurements are reported for the adsorption of nicotinamide (NcA) and nipecotamide (NpA) at the mercury electrode in aqueous solutions containing KF as a supporting electrolyte. The experimental data were analysed to obtain the surface excess and adsorption parameters of NcA and NpA as a function of the electrode charge density and the bulk concentration of the adsorbate. The results obtained indicate that NcA molecules are adsorbed flat on the mercury surface. The adsorbed layers for both substances contain mixtures of adsorbate and water molecules. The role of the π electrons of the pyridine ring in the adsorption process and the charge dependence of the standard Gibbs energy are discussed.

Surface and interfacial behaviour of nicotinic acid, nicotinamide and nipecotamide

Abstract The results of an adsorption study of Nc, NcA and NpA at the free surface of water are compared with those previously obtained at the uncharged Hg/solution interface. The saturation surface concentrations of adsorbates differ at the two interfaces. For Nc and NcA at the air/solution interface the surface concentration is smaller than that of NpA (i.e. NpA achieves full saturation coverage, but Nc and NcA do not), whereas the opposite effect is observed at the uncharged Hg surface. The lateral interaction has been found to be essentially attractive at the free surface, whereas such an interaction is repulsive at the Hg surface; this is interpreted in terms of hydrogen bonding of the polar group to neighbouring water molecules in the adsorption layer. The former probably results from the easier replacement of water molecules from the surface layer. This effect however, decreases in the cases of Nc and NcA (particularly in the presence of 0.1 M KF) as referred to NpA molecules.

Analysis of the differential capacity of the mercury/simple alcohol solution interface

Abstract A method of analysing the differential capacity developed by Amokrane and Badiali is applied to investigations of the structure of the interface between mercury and simple aliphatic alcohols. Capacitance data for methanol, ethanol, n-propanol and n-butanol are discussed. The solvent contribution C s is obtained by subtracting the contribution C M due to the metal from the inner layer capacitance C i according to the procedure proposed by Amokrane and Badiali. The C S values are found to be very similar for all aliphatic alcohols studied. The C S -σ M curves have a similar shape with a single maximum at a positive charge density and a characteristic inflection at a negative charge density. The effect of ions in the various supporting electrolytes on the C S -σ M profiles was also studied.