Chemistry Reference
In-Depth Information
are somewhat less important, although they cannot be ignored. As in human society,
history can play an important role in the characteristics of all solid surfaces.
10.3. ADSORPTION AT THE SOLID-LIQUID INTERFACE
Because of their inherently high surface energies, most newly formed solid sur-
faces, except those of very low energy polymers or waxes, will exhibit a strong ten-
dency to adsorb almost any available material, including gases normally considered
to be ''inert'' such as nitrogen, helium, and argon. The adsorption of such materials
has been extensively studied since around 1905 and has given rise to a broad quan-
titative understanding of the nature of solid surfaces and their adsorption character-
istics. Such adsorption, however, differs significantly from that of surfactants and
other amphiphiles, and the details are left for the adventurous reader to pursue.
The adsorption of surface-active materials onto a solid surface from solution is
an important process in many situations, including those in which we may want to
remove unwanted materials from a system (detergency), change the wetting char-
acteristics of a surface (waterproofing), control the triboelectric properties of a sur-
face (static control), or stabilize a finely divided solid system in a liquid where
stability may otherwise be absent (dispersion stabilization). In these and many
other related applications of surfactants or amphiphilic materials, the ability of
the surface-active molecule to situate itself at the solid-liquid interface and produce
the desired effect is controlled by the chemical natures of the components of the
system: the solid, the surfactant, and the solvent. The following discussions sum-
marize some of the factors related to chemical structures that significantly affect the
mechanisms of surfactant adsorption and the orientation with which adsorption
occurs.
10.3.1. Adsorption Isotherms
The experimental evaluation of the adsorption from solution of amphiphilic mate-
rials at the solid-fluid interface usually involves the measurement of changes in the
concentration of the adsorbed material in the contacting solution, for S/L interfaces,
or the amount actually adsorbed onto the solid for S/V systems. The usual method
for evaluating the adsorption mechanism is through the adsorption isotherm. The
important factors to be considered are (1) the nature of the interaction between
the adsorbate (the amphiphile) and the adsorbent (the solid); (2) the rate of adsorp-
tion; (3) the shape of the adsorption isotherm and the significance of plateaus in it;
(4) the extent of adsorption (i.e., monolayer or multilayer formation); (5) the inter-
action of solvent, if present, with the solid surface; (6) the geometric orientation of
the adsorbed molecules at the interface; and (7) the effect of environmental factors
such as temperature, solvent composition, and pH on the adsorption process and
equilibrium.
Early classifications of adsorption isotherms at solid-vapor interfaces were
found to fit one of five basic shapes (Figure 10.4) at temperatures below the critical
temperature (T
c
) of the adsorbate. Type I isotherms were originally interpreted to
