Page 17 - Adsorptionbypowders & poroussolids muyace

CHAPTER 1. INTRODUCTION 3

appreciate the role of the solid surface. He proposed a general mathematical relation
for the isotherm, which we now refer to as the Freundlich adsorption equation.
In 1909 McBain reported that the uptake of hydrogen by carbon appeared to occur
in two stages: a rapid process of adsorption appeared to be followed by a slow
process of absorption into the interior of the solid. McBain coined the term sorption
to cover both phenomena. In recent years it has been found convenient to use 'sorp-
tion' when it is not possible to make a clear distinction between the stages of uptake,
and also to use it to denote the penetration of molecules into very narrow pores
(Barrer, 1978).
During the early years of the twentieth century, various quantitative investigations
of gas adsorption were undertaken. The most important advances in the theoretical
interpretation of gas adsorption data were made by Zsigmondy, Polanyi and
Langmuir: their ideas set the scene for much of the research undertaken over the past
80 years.
In 191 1 Zsigmondy pointed out that the condensation of a vapour can occur in very
narrow pores at pressures well below the normal vapour pressure of the bulk liquid.
This explanation was given for the large uptake of water vapour by silica gel and was
based on an extension of a concept originally put forward by Thomson (Lord Kelvin)
in 187 1. It is now generally accepted that capillary condensation does play an impor-
tant role in the physisorption by porous solids, but that the original theory of
Zsigmondy cannot be applied to pores of molecular dimensions.
The theory proposed by Polanyi in 1914 was developed from an older idea of long-
range attractive forcesemanating from the solid surface. The adsorbed layer was pic-
tured as a thick compressed film of decreasing density with increase in distance from
the surface. The original 'potential theory' did not give an equation for the adsorption
isotherm, but instead provided a means of establishing a 'characteristic curve' -
relating adsorption potential to amount adsorbed - for a given system. In spite of its
initial appeal, it soon became apparent that the principles underlying the potential
theory were not consistent with the emerging treatment of intermolecular forces.
However, more recently the concept of a characteristic curve has been modified and
adopted by Dubinin and his co-workers in their theory of micropore filling.
The year 1916 brought a radical change in the approach to surface science. In that
year the first of Langmuir's monumental papers appeared (1916, 1917, 1918). Lord
Rayleigh's earlier conclusion that certain films of polar oils on water were one
molecule thick had not received the attention it deserved and Langmuir's great con-
tribution was to bring together all the available evidence to support the unifying
concept of the monomolecular layer (the monolayer). He proposed that adsorption on
both liquid and solid surfaces normally involved the formation of a monomolecular
layer. In retrospect it is not surprising that the advent of the Langmuir theory
produced a renaissance in surface science.
Langmuir's work on gas adsorption and insoluble monolayers prepared the way
for more progress to be made in the interpretation of adsorption from solution data.
In the light of the Langmuir theory, it seemed logical to suppose that the plateau of a
solute isotherm represented monolayer completion and that the monolayer capacity
could be derived by application of the Langmuir equation.