Usman, M. (1969) Electrical double layer adsorption at metal-methanol interfaces. PhD thesis, Aligarh Muslim University.

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Abstract

This thesis consists of three parts, the first, theoretical, the second, experimental and the third, discussion of the experimental results and their significance. The theoretical part begins with a critical discussion of the significance of various potentials, electrostatic and electrochemical, used in describing the structure of the electrical double layer. The thermodynamics of interface is then outlined and the concept of surface-exess used by Gibba is examined in detail. The adsorption isotherm applicable to interface when both charged and uncharged species are present is derived using the principle of electrochemical equilibrium as originated by Gibba and developed by suggenheim. The thermodynamics of electro capillary are then examined with reference to mercury/methonolic solution interface which is proposed to be studied. The basic electro capillary equation applicable to this system is derived in a form in which only chemical potentials of electrically neutral combinations occur. Special cases of this equation, applicable to the determination of components of charge at the mercury/methanol interface are deduced with emphasis on the importance of considering, the reference electrode as an integral part of the system. The thermodynamics of non-polarisable systems is also briefly discussed and the possible influence of curvature on electro capillary is examined. Methods of evaluating components of charge by electrical and thermodynamic methods are examined for their suitability. The theory of the electrical double layer in aqueous solutions in presence of specific adsorption is discussed critically from the point of view of the ideal ionic double layer in aqueous solutions. Departure from ideal behavior is discussed in terms of specific adsorption of anions and cations from the salvation-desolvation angle. Structure of the electrical double layer in aqueous solutions in presence of specific adsorption is discussed critically from the viewpoint of fine structure of the compact layer, specific adsorption isotherms and the discreteness of charge-effector Esin and Markov effect. The isotherm derived by Levins, et al. is examined in detail and shown to be mathematically inconsistent and to be contradictory to the experimentally established facts of specific adsorption. The phenomenon of “humps” is examined with respect to the possible role of solvent. The importance of the study of non-aqueous systems as such and in understanding the nature of humps outlined. The methanol system is recommended as a suitable non-aqueous system for systematic study with the view to elucidating the structure of electrical double layer. In the experimental part, the technique of differential capacity measurements on solid metals in contact with anhydrous ionic solution in methanol is described in detail. The ultrapurification techniques, essential for static surfaces, are also detailed. A capillary electrometer and associated measuring equipments, suitable for thermodynamic study of the same system is also described. The results are reported for the following systems. The numerical integration of differential capacity curves in order to obtain both qM and on a printing calculator (Friden 1217) is explained. Graphical differentiation of curves is also described. In the discussion part, the importance of the reference electrode in the same medium and reversible to a common ion is emphasized with illustrative examples from the literature. The experimental capacity data obtained in this work is compared with the data published by other workers in respect of accuracy and reproducibility. The main features of the differential capacity curves on mercury are discussed with reference to potential span, capacity range, concentration effects and ionic character. The diffuse layer parameters for the methanol systems have been tabulated as functions of charge and potential. Concept of solvent capacity is discussed for methanolic solutions and an absolute calculation of its magnitude attempted. The result is found to be 10% lower than the value deduced from the experimental data. It is shown that this is due to under estimating the dielectric constant by 10%. The concept of solvent capacity requires demonstration that its value is independent of the nature of the metal too. The capacity data reported here for three metals has shown that provided the correct surface roughness is used, the minimum capacity point gives the solvent capacity, subject to the usual correction for the diffuse layer. The concept of solvent capacity in methanol is established for the first time, its value being 11 F/cm2, in contrast to a value of 17 F/cm2 in case of water. The thermodynamic data for mercury has been processed to obtain components of charge, namely surface, excess of ammonium and halide ions. Comparison of data with aqueous solution shows practically the same behaviour as in aqueous solutions with bromide being more adsorbed than chloride. Using the theory of the structure of electrical double layer, these components of charge have been further classified into specifically adsorbed charge and the diffuse layer components. The results are presented for three values of the charge for both chloride and bromide ions. The nature of specific adsorption in methanolic solution is examined with the data obtained and with the aid of Devanathan’s model of electrical double layer. It is found that the calculation of components of charge using the model method is seriously handicapped by the inadequate range on the charge scale available in methanol. This natural limitation restricts the application of specific adsorption isotherm to regions where the charge is too small for discrete-ion-effects to be observed

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Electrical double layer adsorption
Subjects:	Electrodics and Electrocatalysis
Divisions:	UNSPECIFIED
Depositing User:	Dr. N Meyyappan
Date Deposited:	13 Jun 2012 04:58
Last Modified:	13 Jun 2012 04:58
URI:	http://cecri.csircentral.net/id/eprint/2824

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