Adsorption parameters in solutions of ethyl esters of aliphatic acids

Based on the regression analysis of electrocapillary curves (ECC) in solutions of ethyl esters of aliphatic acids (from acetic to valeric), it is shown that the adsorption of these compounds on a mercury electrode from aqueous solutions corresponds in the first approximation to the model of two parallel capacitors. Moreover, to describe experimental ECC, the knowledge of only five main parameters of this model is necessary. The dependence of the adsorption parameters found on the number of carbon atoms in an ester molecule is analyzed.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 406–411.Original Russian Text Copyright © 2005 by Damaskin.     

Charge dependence of adsorption energy of ions at electrodes

Analysis of the energetic and geometric characteristics of the inner part of the electrical double layer has been carried out in the presence of the specific adsorption of ions accompanied by change in the dimensions and dielectric properties of the inner layer. On the basis of the formulae for the electrochemical potential of the adsorbed ion on the electrode surface and for the components of the inner layer capacity, the equations of the Frumkin isotherm and of its parameters have been derived. It has been shown that, in general, the Frumkin isotherm is non-linear at the given electrode charge and the charge dependence of the adsorption equilibrium constant has a parabolic form. The results of the corresponding theoretical calculations have been compared with the experimental data obtained by the study of the specific adsorption of tetra-alkylammonium cations on a bismuth electrode in ethanolic and aqueous solutions. Good agreement of the experimental results with those calculated theoretically confirm the validity of the equations derived.     

Regression analysis of electrocapillary curves of mercury electrode in solutions of secondary butyl alcohol

A regression technique is used for analyzing electrocapillary curves (ECC) in secondary butanol solutions, which were published earlier. It was shown that, in contrast to similar analysis of electrocapillary curves for a number of other compounds, if we want to describe the adsorption of 2-BuOH within the framework of the model of two parallel capacitors, the linear dependence of the parameter of intermolecular interaction on the electrode potential should be taken into account. This result stems from the very high accuracy of interface tension measurements in the system under study. It is found that, in terms of the Frumkin isotherm and the model of two parallel capacitors, the interpretation of data on the adsorption of 2-BuOH has a much simpler physical meaning as compared with the strictly thermodynamic approach used earlier.     

Calculating adsorption parameters of organic substances from the capacitance curves in the absence of information on the electrode zero-charge potential in a supporting electrolyte solution

The possibility of estimating the potential of zero charge (PZC) of an electrode in a supporting electrolyte solution (SES) from the differential capacitance vs. potential dependences measured in SES and in the presence of different amounts of organic substances is analyzed by using model calculations. It is shown that the electrode PZC in SES cannot be uniquely determined in the model of two parallel capacitors and Frumkin isotherm which are commonly used for describing the organic substance adsorption. It is shown that all adsorption parameters, with the exception of the adsorption potential drop of the organic substance, can be calculated from capacitance curves in the absence of information on the electrode PZC. From the analysis carried out it follows that, under given conditions, the potential of the organic-substance maximum adsorption is an adsorption parameter on the same scale as is used for differential-capacitance measurements.     

Parameters of the double electric layer and n-butanol adsorption on lead in methanesulfonate solutions

A differential capacity measurement showed that the zero charge potential of a lead electrode in a sodium methanesulfonate solution was ?0.69 V. Like sulfate anions, methanesulfonate anions had low specific adsorbability on lead. The adsorption of n-butanol from a methanesulfonate electrolyte on a lead electrode was approximated by the Frumkin isotherm equation. The parameters of n-butanol adsorption in a methanesulfonate solution were close to those reported for fluoride and sulfate solutions in the literature.     

Adsorption of polyvinylimidazole onto kaolinite

The adsorption of polyvinylimidazole (PVI) onto kaolinite from aqueous solutions has been investigated systematically as a function of parameters such as calcination temperature of kaolinite, pH, ionic strength, and temperature. According to the experimental results, the adsorption of PVI increases with pH from 8.50 to 11.50, temperature from 25 to 55 degrees C, and ionic strength from 0 to 0.1 mol L(-1). The kaolinite sample calcined at 600 degrees C has a maximum adsorption capacity. Adsorption isotherms of PVI onto kaolinite have been determined and correlated with common isotherm equations such as Langmuir and Freundlich isotherm models. The Langmuir isotherm model appeared to fit the isotherm data better than the Freundlich isotherm model. The physical properties of this adsorbent are consistent with the parameters obtained from the isotherm equations. Furthermore, the zeta potentials of kaolinite suspensions have been measured in aqueous solutions of different PVI concentrations and pH. From the experimental results, (i) pH strongly alters the zeta potential of kaolinite; (ii) kaolinite has an isoelectric point at about pH 2.35 in water and about pH 8.75 in 249.9 ppm PVI concentration; (iii) PVI changes the interface charge from negative to positive for kaolinite. The study of temperature effect has been quantified by calculating various thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy changes. The dimensionless separation factor (RL) has shown that kaolinite can be used for adsorption of PVI from aqueous solutions.     

Refining adsorption parameters of n-butyl alcohol at the interface of mercury electrode with aqueous NaF and Na2SO4 solutions by correcting the adsorbate concentration

As an adjunct to the regression analysis of differential capacitance curves, which allows refining the adsorption parameters, a program, which takes into account possible errors in volume concentrations of organic substances, is developed. Using this procedure, the earlier data on the differential capacitance of a mercury electrode in aqueous solutions of normal butanol (1-BuOH) containing either 0.1 M NaF or 0.05 and 0.5 M Na₂SO₄ as the supporting electrolyte are analyzed. This allows obtaining the most accurate values of adsorption parameters for the systems mentioned above within the framework of the model of two parallel capacitors and the Frumkin isotherm. It is shown that, when a linear dependence of the intermolecular interaction parameter on the electrode potential is taken into account, the standard deviation of experimental capacitance values from those calculated using the mentioned model is 6.8–8.8%, which points to very high accuracy of this phenomenological model.     

Regression technique for analyzing electrocapillary curves

Based on the Nelder-Mead simplex technique, a program for the regression analysis of electrocapillary curves (ECC), which is aimed at determination of adsorption parameters of simple organic substances, is developed. This method is tested by the example of theoretically calculated ECC strictly corresponding to the model of two parallel capacitors. Then, the method is used for the regression analysis of ECC on a mercury electrode in 0.916 M NaF solutions with different additions of 5-chlor-1-penthanol. The adsorption parameters found for this system are in a good agreement with those obtained earlier by the regression analysis of differential capacitance curves.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 285–290.Original Russian Text Copyright © 2005 by Damaskin, Safonov.     

Effect of Potential on Coadsorption of Two Organic Substances in the Frumkin Model at a Maximum Possible Number of Solutions for the Relevant Set of Isotherms

Effect of the electrode potential on the coadsorption of two organic substances is studied using a set of two mixed Frumkin isotherms and a model of three parallel capacitors in conditions where in the absence of an electric field this system has seven solutions. It is shown that only four of these solutions may correspond to realizable adsorption states. Effect of the electrode potential on the number of possible solutions of the isotherm set, the number of realizable adsorption states, as well as on the composition and properties of a surface layer is considered.     

Electrical double layer and adsorption of iodide ions at the Bi | acetonitrile interface

The adsorption of I^? ions on the Bi(111) single crystal plane from solutions in acetonitrile has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained using the mixed electrolyte method applied for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of ions adsorption has been calculated using the virial adsorption isotherm. It was found that under comparable conditions the results obtained at constant electrode potential and at constant electrode charge are coincident and the Gibbs energy of I^? anion adsorption increases in the sequence of solvents: methanol < ethanol < propylene carbonate < acetonitrile. The electrosorption valency has been calculated and it was found that this parameter increases in the sequence propylene carbonate < ethanol < methanol < acetonitrile.     

The behaviour of allyl thiourea at a mercury-solution interface as an example of adsorption in several states with partial electron transfer

The superficial excesses determined from electrode charge measurements have been studied at constant potential or charge. The usual tests suggest a change in the orientation of the adsorbed molecule. It is shown theoretically that the Frumkin isotherm still offers an useful approximation when two adsorbed states are considered. In this case, two at least of the three adjustable parameters, the apparent adsorption energy, and the apparent molecular area, change with the electrical variable, being function of the adsorption energies for both states and also of the corresponding molecular areas. Consistent results are obtained from the consideration of isotherms using this approximation and from examination of the apparent adsorption valency.     

Electrical double layer and adsorption of iodide ions at the Bi|ethylene carbonate interface

The adsorption of I^? anions on the Bi(111) single-crystal plane from solutions in ethylene carbonate has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained by integration of differential capacitance curves, and the Gibbs excess has been calculated using the mixed-electrolyte method applied for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of I^? anion adsorption has been calculated using the virial adsorption isotherm. It was found that under comparable conditions, the results obtained at constant electrode potential and at constant electrode charge were coincident. The Gibbs energy of I^? anion adsorption and the calculated electrosorption valency value were found to be very close to these values obtained in propylene carbonate.     

New Analysis of Data on the Differential Capacitance of a Mercury Electrode during the Adsorption of tert-Butanol

A regression analysis of data on the differential capacitance of a mercury electrode in aqueous solutions of tert-butanol, which were published previously, is performed. It is shown that, in contradistinction to an analogous analysis of electrocapillary curves (ECC), in order to describe the adsorption of this compound in the framework of a model of two parallel capacitors, it is necessary to take into account the linear dependence of the parameter of an intermolecular interaction on the electrode potential. It is established on the basis of modeling calculations that, in order to obtain the same result in the case of a regression analysis of ECC, the relative accuracy of measurements of the interfacial tension must be two orders of magnitude the relative accuracy of measurements of the differential capacitance.     

Temperature dependence of adsorption parameters of n-butanol and n-valeric acid at their adsorption on mercury electrode from aqueous solutions

Adsorption parameters that enter into the Frumkin isotherm and the model of two parallel capacitors are calculated based on the data of the regression analysis of differential capacitance curves of a mercury electrode in solutions of n-butanol and n-valeric acid at 25, 50, and 75°C. The analysis of the temperature dependences of these parameters allowed free energies, entropies, and enthalpies of adsorption to be found. It is shown that the hydrophobic effect, which is associated with the increase in the enthalpy of liquid water when adsorbate molecules leave it, makes a substantial contribution into the adsorption free energy of studied compounds.     

Adsorption of iodide ions on the Cd(0001) single crystal plane in methanol

The adsorption of I⁻ ions on the Cd(0001) single crystal plane from solutions in methanol has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained using the mixed-electrolyte method for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of adsorption of ions has been calculated using the different modifications of the virial adsorption isotherm. It was found that under comparable conditions, the results obtained at constant electrode potential and at constant electrode charge are coincident and the adsorption of I⁻ anion increases in the sequence of solvents: water < methanol, ethanol, and metals: Cd < Bi. The electrosorption valency has been calculated. It was found that on the Cd(0001) plane the electrosorption valency of I⁻ anions has a constant value that is similar for methanol and ethanol and also for these solvents on Bi single crystal planes.     

电极/溶液界面单分子吸附层的统计力学处理 I. 非水溶液中汞电极的内层微分电容

本文提出电极/溶液界面溶剂化层偶极取向分布模型, 应用统计力学方法及热力学平衡条件导出普遍化的单层吸附等温方程, 其电解质溶液的溶剂组成可以是纯态的或混合物(多组份)的. 文中分别以甲酰胺、碳酸亚乙酯和甲醇等三种纯溶剂的汞/溶液界面为例, 采用曲线拟合计算内层微分电容随表面电荷变化关系。预计本模型处理对汞/水溶液或汞/(混合溶剂)溶液界面仍可适用。     

Adsorption parameters of cyclobutane carboxylic acid at the interface of a mercury electrode with aqueous solutions of 0.4 M Na2SO4 + 0.1 M H2SO4

Based on the results of the regression analysis of differential capacitance curves of a mercury electrode in solutions of cyclobutane carboxylic acid (CBCA) at 25, 50, and 75°C, the adsorption parameters that enter into both the Frumkin isotherm and the model of two parallel capacitors are estimated. The free energies of adsorption found from the analysis of the temperature dependence of these parameters are incorrect due to the partial blowing out of CBCA during the deaeration of the solutions. The corresponding adsorption parameters are corrected under the assumption that the hydrophobic effect associated with the increase in the liquid-water entropy at the removal from it of CBCA molecules is the same as for n-valeric acid. The physical meaning of the squeezing-out effect is revised.