K.G. Baikerikar

Electrosorption of some aliphatic compounds at the mercury—solution interface

Abstract The electrochemical adsorption of four aliphatic compounds, i.e., n -butanol, isopentanol, n -pentanoic acid, and n -hexanoic acid has been investigated by employing a capillary electrometer of greatly improved sensitivity. As observed by previous workers from this laboratory, for each adsorbate, π versus In α curves at different polarizations were found to be superimposable by abscissa translation implying thereby that the adsorbate-adsorbate interactions in the surface layer are independent of the electrical nature of the interface and that the surface charge density varies linearly with surface excess at fixed polarization. The improved precision attained in the measurements of interfacial tension permitted construction of well-documented ln(π/α) versus π curves; such plots do not appear to have been published before. These curves resemble parabolas and for a given adsorbate, the family of these curves obtained at different polarizations were superimposable by ordinate translation. The Frumkin equation was used to fit the data for each absorbate. Three further methods that can be used to determine the parameters of the Frumkin equations are discussed. These are based on the analysis of In (π/α)-π, In α-θ, and π-In α data. The free energies of adsorption referred to specified standard states have been calculated on the basis of the Frumkin equation. The model evolved in the previous work correlating the standard free energies of adsorption with polarizabilities of the adsorbate molecules has been successfully tested.

Frequency dependence of capacitance at the desorption potentials of organic molecules

Abstract A theory on kinetics of adsorption is presented which assumes that diffusion is the sole rate determining step in adsorption. The impedance of the electrical double layer is modelled as a capacitor and resistor in series and equations have been derived relating capacity to frequency. The theory has been tested using capacity data obtained at anodic desorption peak maxima of two neutral organic adsorbates, and found to agree reasonably with experimental results.

An electrocapillary study to test the applicability of the Flory-Huggins isotherm☆

Abstract The Flory-Huggins isotherm with interaction parameter has been tested using accurate electrocapillary data for four aliphatic compounds at the mercury-electrolytic solution interface. When surface coverages derived by differentiation of surface pressure-activity data were tested in the Flory-Huggins isotherm and when all parameters were freely optimized, the equation fit the data very well in all cases, but the size ratio parameter thus obtained was about 0.5. When parameters of the Flory-Huggins isotherm were fixed by limiting properties of surface pressure-activity data directly (with no differentiations performed), the size parameter was about 1.0. The size parameter expected physically is about 3.0. The Flory-Huggins isotherm with realistic size parameters did not fit the data as well as the simpler Frumkin equation in any of the cases considered. The physical basis underlying both models is weak for interaction parameters as large as encountered in aqueous-organic solute systems.

Adsorption of benzene and toluene on mercury

The electrochemical adsorption of benzene and toluene on a mercury electrode was investigated from different aqueous background electrolyte solutions. The experimental techniques employed were capillary electrometer and impedance bridge. The interfacial tension data obtained from perchloric acid solutions were employed for fitting to the Frumkin equations and parameters of adsorption were obtained. The anomalous behavior of capacity at anodic polarization appears to be associated with specific interaction between anion adsorption and organic solute adsorption; anodic capacity peaks are absent in the presence of perchlorate and iodide ions but not in the presence of sulfate and chloride ions.

Electrosorption of 2-butanol at constant chemical potential of base electrolyte and in the moderate bulk concentration region☆

Abstract The adsorption of 2-butanol on a polarized mercury electrode was investigated at constant activity of the base electrolyte using low and moderate bulk concentrations of the adsorbate. Capillary electrometer and impedance bridge measurements were used as experimental techniques. Both interfacial tension and capacity data were employed for fitting to the Frumkin equations. Results paralleled closely those obtained in earlier adsorption studies of aliphatic compounds employing a base electrolyte solution of fixed concentration.

A comparison of double-layer capacity obtained by numerical differentiation of interfacial tension data with that measured dierectly

Abstract A polynomial fitting technique was employed to smooth the equilibrium interfacial tension data obtained for six different aqueous electrolyte solutions using a high-precision capillary electrometer. Analytical double differentiation of the polynomial fit yielded capacity values which reproduced the general shape of the directly determined capacity curves, and the two sets of data agreed well except at extreme polarizations.

Investigation of the Hg/HCl aq. system by capillary electrometers based on maximum bubble pressure method and by classical capillary electrometer: Comparison of interfacial tension and double-layer capacity data

Abstract The Hg/HCl aq. system was studied using capillary electrometers based on the maximum bubble pressure principle (that employed either an inverted capillary or a vertical capillary) and by the classical capillary electrometer. All three versions of the capillary electrometer yielded interfacial tension data of high precision, except that the modified capillary electrometer with a vertical capillary and the classical electrometer gave data of slightly lower quality over a narrow range of extreme negative polarization for 0.01 M HCl solution. Based on the electrocapillary and double-layer capacity measurements carried out on identical HCl solutions (0.01 M–1 M), it is shown that there exists a very good correspondence between the measured and derived quantities as compared to the agreement previously observed by Bockris and co-workers (who employed a classical capillary electrometer in their interfacial tension measurements).

SURFACE EQUATIONS OF STATE IN ADSORPTION FROM SOLUTION

Evaluation of a surface equation of state generally involves an evaluation of its ability to represent raw experimental data, or information derived from such data, after the parameters of the equation have been adjusted to optimize this representation, followed by an evaluation of the physical reasonableness of the optimizing parameters in terms of the physical model on which the equation of state is based. This process is analyzed critically for the adsorption of polar organic compounds at the mercury-electrolytic solution interface using high precision electrocapillary data as test data and the Frumkin and Flory-Huggins equations as test equations. In one approach, π vs ln a data were represented by a set of hyperbolae with coefficients chosen to give best least squares representations of data, and adsorption isotherms obtained by analytical differentiation; parameters for the isotherms were selected to give best least squares fits to these data. Parametrizations depending on high surface pressure limiting tangents to π vs ln a plots, intercept and slope of low surface pressure ln (π/a) vs π plots, were also investigated. Parameter sets obtained by different methods of parametrization agreed only moderately well, reflecting sensitivities to different portions of the experimental data. When all Flory–Huggins parameters were freely adjusted, best fits resulted when the water co-area was taken as larger than the organic compound co-area, a physically unrealistic result.