Studies in the physical state of adsorbed dyestuffs

Rahman, Syed M. K (1961) Studies in the physical state of adsorbed dyestuffs. PhD thesis, University of Glasgow.

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The main purpose of the research was to gain a better understanding of the state of dyes when adsorbed by a variety of organic and inorganic substrates, with the view to providing a solid experimental basis for any new quantitative treatment of the mechanism of dyeing, and of solution adsorption in general. Broadly the work is classified into three main sections, dealing respectively with adsorption at the surface of anodic alumina film and of alumina powder; aggregation in adsorption at organic surfaces and by solution in water, and the relation between the state of aggregation of dyes and their light fastness. The adsorption of organic dyes from solution by the two forms of alumina is studied to find out whether the dyes are adsorbed as associated particles. The specific surface area values of the alumina calculated from the adsorption isotherms of various dyes are compared, with the value obtained by phenol adsorption, and it is concluded that a proportion of dye is adsorbed as micelles. A unique effect has been observed in the adsorption of dyes by anodic alumina: the amount adsorbed varies greatly with the volume of solution used, in apparent contradiction of thermodynamic principles. This is discussed, and an explanation given. Spectrophotometric tests have also been made to determine whether dyes adsorbed in solid films of regenerated cellulose and gelatin used as models, of cellulosic and protein fibres respectively, are mono-disperse or heterodisperse. The method used was a study of "Beer's law curves" (optical density against concentration) for a large number of dyes at high dye concentration, with and without disaggregating agents. The absorption spectra in the visible region of a number of dyes have been examined when in solution and adsorbed in transparent films, and the presence of the x and y bands in nearly all cases has been demonstrated and used to detect aggregation and disaggregation effects. The application of Beer's law is extended to the relationship between light reflectance and concentration of colour in a mass of coloured particles and fibres. The theoretical reason for Beer's law failure in transmission and reflection systems is discussed and it is shown that a more generally true relation is obtained by substituting for c, the colour concentration factor in the Beer's law formula, the empirical factor cl/l+acx, where a and x are constants (x<1), probably functions of the refractive index difference between the dye and surrounding medium, and x is a simple fraction. From an examination of absorption spectra and fading rate curves of many types of dye of high light fastness it was found that micelles are present in the substrate, and this probably accounts for the high light-fastness. The effects of various treatments used commercially to improve the light-fastness properties of dyes, have been studied and it is shown that aggregation of the dye is the probable cause of the improvement. It is shown that on irradiation the aggregates often break up, probably because of the heat of the lamp. This phenomenon, which can be minimised by keeping the irradiated pattern cool, actually has an opposite effect to fading, i.e. it tends to increase the apparent depth of colour. Change in the surface activity of dyes, produced by changes in the size and form of their bydrophobic residues and the position of the ionic groups is shown to affect light fastness by virtue of an influence upon the physical form of the adsorbed dye, A statistical investigation of the relation between the molecular shape of dyes and their light-fastness shows that there is a tendency for fastness to rise with fall in axial ratio, i.e. the more nearly square is the dye molecule the higher is its light fastness, Next the effect of fluorescent brightening agents on the light fastness of dyed cellulose is examined and it is shown that these agents do not usefully alter light fastness. Some work proceeding is described in which the light fastness of disperse dyes adsorbed in hydrophobic substrates, especially polyesters, is shown to depend partly upon the state of aggregation of the dye, but mainly upon the low moisture content of the substrate. This low moisture content has the effect of raising the light fastness as the crystallinity of the substrate increases. This is the reverse of the normal rule for hydrophilic substrates.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: C H Giles
Keywords: Physical chemistry
Date of Award: 1961
Depositing User: Enlighten Team
Unique ID: glathesis:1961-73491
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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