Studies in adsorption of organic solutes at inorganic surfaces

Stewart, C. E (1957) Studies in adsorption of organic solutes at inorganic surfaces. PhD thesis, University of Glasgow.

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The adsorption of organic compounds from solution by two forms of alumina, the anodic oxide film on aluminium and acidified chromatographic alumina powder, has been studied. The anodic oxide film used was produced by anodising in chromic acid which yields a highly porous film of almost entirely pure, amorphous or very finely crystalline delta-alumina. The alumina powder used was acidified by washing with hydrochloric acid. The adsorption of organic solutes on both substrates from dry organic solvents is found to occur only when the solute molecule contains a hydrogen atom free to form an intermolecular bond. Thus hydroxy- and amino-compounds are adsorbed, presumably by the formation of hydrogen-bonds with the substrate. In the case of the anodic film the adsorption of amino-compounds is prevented by certain solvents whereas adsorption on alumina powder is unaffected by the solvent. This suggests that in the case of the oxide film the N H O bond formed in adsorption has lower free energy than the O...H...O bond. Adsorption involving hydrogen attached to carbon was also found, e.g. compounds containing acetoxy- and aldehyde groups were adsorbed by the alumina powder. In the case of the oxide film, the results of the experimental work are complicated by suspected decomposition of these solutes. The part played by the azo-linkage in adsorption on both substrates was examined. Azobenzene is not adsorbed from dry organic solvents on either substrate, but is adsorbed in presence of water. This seems to be a result of water acting as a hydrogen-bonding cross-linking agent between the solute and the substrate. In view of the recommendation found in the literature concerning the use of compounds capable of forming lakes with aluminium for colouring anodic oxide films, the adsorption behaviour of two such compounds, viz. alizarin and Solway Blue B, was studied. The irreversible nature of their adsorption suggests that very stable chelate complexes are formed with the aluminium of the oxide film. The anodic film was found to undergo slight dissolution in aqueous solutions. Owing to the cone-shaped pores of the film, this leads to an increase in the surface area available for adsorption which is a function of the weight of film dissolved. Thus it is important to carry out all adsorption experiments at a constant solution; film weight ratio, otherwise alterations in the available surface area: film weight ratio, due to dissolution, would lead to inconsistences in the amount of solute adsorbed per unit weight of film. Both the thermodynamic and kinetic aspects of the adsorption of sulphonated azo dyes by both substrates was studied. In the case of the oxide film adsorption of these dyes is accompanied by a measureable evolution of heat indicating that a chemical reaction, probably salt formation, takes place between the sulphonate groups and the aluminium. Analysis of isotherm data suggests that the adsorbed anions form a monolayer in which they are oriented in such a way that all their sulphonate groups are attached to the substrate. There is no measureable heat of adsorption in the case of the acidified alumina powder. The operative mechanism here appears to be one of ion exchange involving the dye anions and chloride ions taken up by the substrate during the pretreatment with hydrochloric acid. The adsorbed dye anions form a monolayer and are oriented in the same manner as those adsorbed on the oxide film. The affinity of sulphonated azo dyes for both substrates, determined from isotherm data, is the sum of the affinities of the sulphonate groups in the molecule, each contributing to the total affinity an amount characteristic of its position in the molecule. The aromatic residue or any free hydroxy-groups in the molecule do not appear to contribute to the total affinity. The apparent heat of adsorption and the apparent entropy of adsorption were determined for several sulphonated azo dyes on the oxide film. Both generally increase with increasing sulphonation of the dye anion. The apparent entropies of adsorption for dyes adsorbed on alumina powder are comparable to those for similar dyes on the oxide film. The kinetics of the adsorption of sulphonated azo dyes on both substrates were studied. In the case of adsorption on the anodic film the rate of adsorption is controlled, in the earliest stages, by the building up of a layer of dye anions on the outer surfaces of the film followed by diffusion into the pores of the film. Determination of the activation energy of the diffusion process shows that its value is lowest for these dyes most easily removed from solution. The rate of adsorption of sulphonated aao dyes on alumina powder is controlled, in almost all the cases studied, by diffusion through a liquid film surrounding the particles of alumina. In the earliest stages, however, the rate may be controlled by diffusion inside the particles of the substrate. The activation energies of adsorption were determined but do not provide further information as to the form of the kinetics.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: C H Giles
Keywords: Organic chemistry
Date of Award: 1957
Depositing User: Enlighten Team
Unique ID: glathesis:1957-72524
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jun 2019 11:06
Last Modified: 11 Jun 2019 11:06

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