Chemistry Reference
In-Depth Information
Figure 3.4.
Dividing surfaces at fluid interfaces: (a) ideal interface; (b) diffuse (''realistic'')
interfacial region.
often several, molecular distances. To quantitatively treat the phenomena related to
surface activity, especially adsorption phenomena, it is necessary to mathematically
define the location of the dividing line or surface at which the change is assumed to
occur.
For convenience, it is usually assumed that a dividing surface can be defined as
shown in Figure 3.4a, where an ideal plane lies between phases 1 and 2. Such an
ideal model is unrealistic, however, especially in the event of adsorption at the
interface. Such an adsorbed interfacial film not only will possess a finite thickness
related to the size of the adsorbed molecule but may also alter the nature of mole-
cules of phases 1 and 2 located near the interface and result in an interfacial region
in which the composition changes more or less continuously over a considerable
distance (Figure 3.4b).
At this point it may be useful to reiterate that the general term ''interface'' refers
to the above mentioned boundary between any two phases. In common usage, how-
ever, the term ''surface'' is used with reference to systems in which one phase is a
gas, such as in the ''surface tension'' of a liquid. Throughout the subsequent discus-
sions, the use of ''interfacial'' will imply applicability to multiple types of boundary
region, while ''surface'' will apply only to S/V and L/V systems.
3.2.1. A Thermodynamic Picture of Adsorption
In the consideration of adsorption processes, two aspects must be addressed: (1) the
effect of the adsorbed species on the final equilibrium interfacial energy of the
system and (2) the kinetics of the adsorption process. For the most part, the discus-
sions to follow are concerned only with equilibrium conditions, and dynamic pro-
cesses are not addressed. For many surfactant applications, such a restriction will
not result in significant limitations to the validity of the concepts involved. Obvious
exceptions would be processes involving dynamic interfaces (i.e., coating opera-
tions), polymer adsorption, and other kinetically ''slow'' processes.
