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Title: Studies in monolayer absorption
Author: MacEwan, Thomas H.
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1959
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The main aims of this work are to interpret mechanisms of adsorption of dyes, etc. by fibres, especially protein, nylon, and other hydrophobic fibres, e.g. cellulose acetate, from the results of (i) monolayer experiments with model compounds, and (ii) a systematic study and classification of the forms of adsorption isotherms. (i) Experiments in Monolayers: The action of aqueous tannic acid solutions on monolayers of stearyl alcohol, N-methylstearamide, cetyl acetate, cetylaniline and cellulose triacetate has been studied. A mixed film, and other techniques, have been used to study the interaction between cellulose triacetate and a non-ionic disperse dye. (The dye alone was found to be capable of forming a condensed film on water). The results, in conjunction with earlier work, are used to interpret the mechanism of adsorption of solutes by cellulose acetates. The molecular area and compressibility data of the force/area curves show that in the presence of water the ketone group has much weaker hydrogen-bonding power than the acetate group, and the films of ketones are expanded much less by all the solutes than the acetate films are. Highly ionised compounds, including sulphonated dyes, cause considerable expansion of the cellulose triacetate films, even though they have no affinity for this material in bulk, whereas, the films appear to be unaffected by a highly substantive nonionic 'disperse' dye. Several hypotheses are suggested to account for these facts. Further support for these arguments is set out in an Appendix, based on the work of earlier investigators. The effect of aromatic sulphonates, including some dyes, upon casein monolayers spread on acid or on buffer solutions near the isoelectric point has been studied. Large dye molecules with a non-ionic proton-donor group at each end have an effect on the film similar to that of tannic acid. Surface-active mono-basic dye molecules with weak hydrogen-bonding groups penetrate the film and at high pressures increase its solubility. It is suggested that the latter effect may be due to the adsorption of a layer of dimerised dye molecules below the film. Small sulphonate molecules and disulphonates with weak hydrogen-bonding centres (e.g. anthraquinone-disulphonates) are probably adsorbed beneath the film. The results are used as a basis for the hypothesis that the affinity of monobasic anions for protein fibres arises from their own mutual attraction, which assists them in forming a monolayer or a layer of micelles adsorbed on the fibre, rather than from specific anion-fibre attraction. 2:1-Unsulphonated azo-dye metal complexes and other solutes have been applied to monolayers of edestine and methoxymethylnylon. The dyes are highly aggregated in the cold and their effects on the films are explained as the result of adsorption in the form of aggregates. It is shown that the aggregate, if it is an ordered one, very probably has a structure with "fins" of protruding aromatic nuclei, between which chain molecules in the film engage, by non-polar, and in some cases polar, forces. A complex without pendant polar groups markedly increases film rigidity; proton-acceptor groups in a complex solubilise the films; and proton-donor groups cross-link and stabilise them. The mechanism of dyeing of nylon and protein fibres with these complexes is considered to be adsorption (probably of micelles), (a) in water-accessible regions by (i) ion exchange, (ii) van der Waals forces, and (iii) (if suitable groups are available) hydrogen bond donation by the dye to backbone -CONH- groups; and (b) in water-inaccessible regions by hydrogen-bonding. Process (b) ("solid solution") gives linear portions of adsorption isotherms. (ii) Adsorption Isotherms. Part II of this work is a study of the general factors responsible for determining the form of isotherms for adsorption of a solute by a solid from solutions, illustrated by examples from recent experimental work. The isotherms for this type of adsorption are divided into four main classes, characterised by the initial portion being convex (S-type) or concave (L-type) to the solution-concentration axis, linear (Ln-type), or coincident with the substrate-concentration axis (HA-type), The 'L'-type curve is the most common and usually represents adsorption of a high affinity-solute; the others are encountered only in more particular systems. The 'S'-type curve may perhaps represent high affinity of the solvent for the solid substrate and low affinity of the solute, but often it appears to indicate that the adsorbed solute molecule (or ion) is oriented perpendicular to the solid surface. The 'Ln' type curve occurs when the solvent has low swelling power for the substrate; and the 'HA' type only in rare cases where the solid has very high, and the solvent very low, affinity for the solid. The present evidence suggests that this isotherm, when observed in adsorption of ionised solutes from water, probably always represents adsorption of ionic micelles.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available