Sarangapani, S. (1980) Phenomena at electrode-solution interface: adsorption of phenols at mercury/aqueous solution interface. PhD thesis, Madurai-Kamaraj University.

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Abstract

A brief survey of the structural and thermodynamic aspects of ionic double layer theory is made. The recent advances in the study of adsorption of organic compounds at mercury-solution interfaces are discussed. Frumkin’s theory of the adsorption of organic compound is presented and the recent developments in the field by Frumkin and co-workers are highlighted. Butler’s theory of organic adsorption, the electrostatic model (proposed by Devanathan), Hansen’s mode, the molecular approach to adsorption (proposed by Bockris and co-workers) and the generalized surface layer model (proposed by Frumkin and Damaskin) are also reviewed. The congruence problem of adsorption isotherms has been presented and the implications of pseudo and simultaneous congruence are discussed at length. The adsorption of phenol, pyrocatechol (O-dihydroxybenzene) and phloroguchinol (sym. Trihydroxybenzene) were studied using capacity and interfacial tension measurements. An aqueous solution of 0.5 M sodium per chlorate was used as supporting electrolyte in all the experiments. Differential capacitance measurements were carried out using an a.c. bridge and DME at 400 Hz. In addition, the adsorption of phloroglucinol was investigated using a HMDE at a frequency of 30 Hz. Interfacial tension measurements of pyrocatechol were carried out using a Lippmann capillary electrometer. A maximum bullble pressure electrometer was also constructed and used for acquiring interfacial tension data for phloroglucinol. The results were processed using an IBM 370/155 operating system. Separate programmes were written to process differential capacity and interfacial tension data. Both constant charge and constant potential analysis were carried out. The programmes were so designed to yield all the necessary derived data required for the interpretation of the adsorption of organic compounds. Isotherm fits were also carried out using these programmes. Phloroglucinol exhibited a peculiar behavior at the interface. Where as the mono and di hydroxyl substituted benzenes behaved as normal aromatic compounds, the symmetric hydroxy benzene alone showed a break in the capacitance-potential curves; this behavior resembled the one exhibited by Camphor. Adamentanol and Borneol with undergo two dimensional condensation. However, the fine structure of the capacitance curves of phloroglucinol was different from the C-E curves of the above mentioned compounds. Phloroglucinol showed well-defined desorption peaks at all concentrations showed well-defined desorption peaks at all concentrations (except the lowest) and the break occurred after the peak; the capacitance curves for different concentrations fused together at the adsorption region, but the capacitance was much higher (13 F/cm2) than that observed for a condensed monolayer. For the isotherm analysis, Frumkin, FVT (free volume theory) and Florry isotherms were chosen, Tests of congruence were made and conclusions drawn on the basis of two different tests. Isotherm fit could not be carried out for the capacitance data of phloroglucinol since as ‘all or none’ adsorption behavior was exhibited. Hence isotherm fit was attempted using electrocapillary data. Strangely enougn a potential dependent negative ‘a’ value (indicative of repulsion) was obtained even though the E-C curves showed a condensation type behavior. FVT isotherm fit to the same data (and to other two substances too) yielded and almost potential independent ‘a’ value. The results (both at constant charge and constant potential) of isotherm fits were discussed in the light of these observations. The M-2 vs plots at constant charge density were linear in the case of pyocatechol and phloroglucinol. But, phenol exhibited a distinct break at positive surface charge densities. This result is analysed along with the corresponding. E vs plot at constance E for any possible reorientation of the adsorbate. The field dependence of free energies of adsorption and the interaction constants for all the three energies of adsorption and the interaction constants for all the three compounds are compared and a general discussion is given relating on the structural aspects. A model for the structure of the double layer in presence of phloroglucinol has been advanced based on the research obtained by us and the available crystallographic data of phloroglucinol and its dehydrate

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Adsorption of phenols
Subjects:	Electrodics and Electrocatalysis
Divisions:	UNSPECIFIED
Depositing User:	Dr. N Meyyappan
Date Deposited:	13 Jun 2012 05:34
Last Modified:	13 Jun 2012 05:34
URI:	http://cecri.csircentral.net/id/eprint/2831

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