Page 132 - Adsorption by Powders and Porous Solids

ADSORPTION BY POWDERS AND POROUS SOL1nS

Adsorption isotherms expressed in reduced surface excess
amounts ............................................... 146
5.3.2. Quantitative expression of the energies involved in adsorption
from solution .......................................... .lq8
Definitions of energies or enthalpies of adsorption from solution .. .lq8
Definition of displacement enthalpies (and energies) ........... .lqg
Definition of the enthalpies (and energies) of mixing ........... .lqg
5.3.3. Basic experimental methods for the study of adsorption from
solution ................................................ 150
Methods for determining the amounts adsorbed ............... .150
Methods for determining adsorption energies ................. ,153
5.3.4. Applications of adsorption from solution ..................... .I57
Assessment of surface area and pore size .................... .I57
Adsorption (and displacement) mechanisms .................. .I57

5.1. Introduction

Adsorption at the liquid-solid interface is of great importance in industry and every.
day life (e.g, in detergency, adhesion, lubrication, flotation of minerals, water treat-
ment, oil recovery, and in pigment and particle technology). Adsorption from solutior,
measurements have been used for many years for the determination of the surf=
area of certain industrial materials. Immersion microcalorimetry has also been
applied for the characterization of such materials as clays and activated carbons. The
application of the energetics of immersion is based on the observation by Pouillet in
1822 that the immersion of an insoluble solid in a liquid is a measurable exothermic
phenomenon. To gain an understanding of liquid-solid adsorption phenomena, it is
not enough to know the surface area and porosity of the adsorbent. In addition, it is
necessary to know how the solid behaves in the liquid medium.
The comparison with adsorption at the gas-solid interface is further complicated
by the fact that some adsorbents cannot be outgassed without an irreversible
change in their texture. Also, changes in texture may occur when the adsorbent is
immersed in a pure liquid or a solution. For these reasons, it is necessary to utilize
special methods which provide direct information on the particular liquid-solid
interactions.
In this chapter, our aim is to give an introductory account of the methodology and
underlying thermodynamic principles of adsorption at the liquid-solid interface.
We are mainly, but not exclusively, concerned with the characterization of the
liquid-solid interface. In this context, there are two relevant topics:

(a) the energetics of immersion of solids in liquids;
(b) isothermal adsorption from solutions.
Many attempts have been made to employ immersion calorimetry and solution
adsorption measurements for the determination of the surface area of porous and non-
porous materials (see Gregg and Sing, 1967), but in our view insufficient attention has