Studies in the mechanisms of adsorption from solution

Nakhwa, Shantaram Narayan (1962) Studies in the mechanisms of adsorption from solution. PhD thesis, University of Glasgow.

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Abstract

This thesis describes a series of investigations in the thermodynamics of solution absorption, designed to throw light on the mechanisms of the process; these investigations include qualitative and quantitative studies of the specific hydrogen-bond forces responsible for some kinds of adsorption. Solution adsorption isotherms are divided into four main classes according to the slope of the initial portion of the curve to the concentration axis, and thereafter into subgroups. The four classes are: S Curves. The initial portion is convex to the concentration axis. They represent high affinity of the solvent for the solid substrate and low affinity of the solute; the surface becomes increasingly attractive with coverage; this is the case when molecules attract each other strongly so that they tend to orient vertically on the surface. 1 Curves. The normal or "Langmuir" isotherms, the initial portion is concave to the concentration axis. .These curves usually indicate that the adsorbed molecules lie flat on the surface, but when competition from the solvent is weak (e.g. polar solutes on polar substrates from non-polar solvents) and the intermolecular attraction is high, even vertical orientation can produce these curves. H Curves. ("High affinity"). (These commence at a positive value on the "concentration in solid" axis.) The solute has very high and the solvent low, affinity for the substrate. They are often given by solutes adsorbed as ionic micelles and by high affinity ions exchanging with low affinity ions. 0 Curves ("constant partition"). These are linear curves, given by solutes which penetrate into the solid more readily than does the solvent. The sub-groups of these classes are those in which there is evident: (1) only an initial portion of the isotherm is obtained. (2) the first plateau indicating the completion of the first layer. (3) a plateau, followed by a further rising portion, caused by either adsorption on the top of the first layer or by further penetration of the solute into the accessible regions, or by reorientation of the molecules from the flat to the vertical position. (4) a second plateau, caused by completion of the processes given under (3) (mx) a maximum, followed by a decrease in adsorption, perhaps due to predominance of solute-solute attraction over solute-substrate attraction. The measurement of specific surface area of solids has hitherto been the principal use of solution adsorption isotherms. P-Nitrophenol is one of the best solutes for this purpose because, it (i) is highly polar and therefore attaches to the whole surface of polar solids; (ii) has a hydrophobic benzene nucleus which enables it to be adsorbed by non-polar solids; (iii) has a small planar molecule and is often adsorbed as a vertically oriented close packed monolayer; (iv) is not highly surface active and therefore does not form three dimensional micelles at the solid surface; (v) is coloured and can be easily analysed; (vi) is soluble in water, but is also soluble in non-polar solvents so that it can be used with water-soluble solids. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Thermodynamics
Date of Award: 1962
Depositing User: Enlighten Team
Unique ID: glathesis:1962-72901
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jun 2019 11:06
Last Modified: 11 Jun 2019 11:06
URI: http://theses.gla.ac.uk/id/eprint/72901

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