Chemistry Reference
In-Depth Information
where C
2
is the molar concentration of component 2 before adsorption, C
2
is its
molar concentration at adsorption equilibrium,
C
2
C
2
ΒΌ
C
2
, and V is the
volume of the liquid phase in liters.
For surfactant systems, the concentration of adsorbed material can be calculated
from the known amount of material present before adsorption and that present in
solution after adsorption equilibrium has been reached. Awide variety of analytical
methods for determining the solution concentration of surfactants are available and
almost all have been used at one time or other. The utility of a specific method will
depend ultimately on the exact nature of the system involved and the resources
available to the investigator.
10.4.1. Adsorption and the Nature of the Adsorbent Surface
As indicated above, the nature of the solid surface involved in the adsorption
process is a major factor affecting the manner and extent of surfactant adsorption.
At this point it might be useful to briefly discuss some of the specific effects of
solid surface characteristics on the mode of adsorption, as well as to clarify the
meaning of ''efficiency'' and ''effectiveness'' as applied in the context of surfactant
adsorption.
In the current context, ''efficiency'' is intended to define the equilibrium concen-
tration of surfactant, C
0
, in the liquid phase required to produce a given level of
adsorption under defined conditions of temperature and other parameters. The
''effectiveness'' of adsorption refers to the amount (concentration) of surfactant
actually adsorbed on the solid surface at surface saturation. The effectiveness of
adsorption of a given surfactant can, of course, be determined directly from the
adsorption isotherm.
When one considers the possible nature of an adsorbent surface, three principal
groups readily come to mind: (1) surfaces that are essentially nonpolar and hydro-
phobic, such as polyethylene, polyproplyene, and Teflon; (2) those that are polar but
do not possess discrete surface charges such as polyesters and such natural fibers as
cotton; and (3) those that possess strongly charged surface sites. Each of these sur-
face types is discussed, beginning with what is probably the simplest, the nonpolar,
hydrophobic surface.
10.4.2. Nonpolar, Hydrophobic Surfaces
As already mentioned, adsorption of surfactants onto nonpolar surfaces is by dis-
persion force interactions. From aqueous solution, it is obvious that the orientation
of the adsorbed molecules may be such that the hydrophobic groups are associated
with the solid surface with the hydrophilic group directed toward the aqueous
phase. In the early stages of adsorption it is likely that the hydrophobe will be
lying (approximately) on the surface much like trains or Ls (Figure 10.11a,b). As
the degree of adsorption increases, however, the molecules will gradually be
oriented more perpendicular to the surface until, at saturation, an approximately
close-packed assembly results (Figure 10.11c).
