Chemistry Reference
In-Depth Information
discussed in Section 5.7. Other structural factors, such as the charge on the surfac-
tant head group, can significantly affect the locus of solubilization. Materials con-
taining aromatic rings, for example, may be solubilized in or near the core of
anionic systems, but in the palisades layer of cationic micelles because of polariza-
tion interactions between the aromatic ring and the cationic head group.
In addition to the solubilization of additives in the micellar core and the core-
palisades boundary region, they may also be found entirely in the palisades region
(Figure 6.1c) and on the micellar surface (Figure 6.1d). The nature of the polar head
group of nonionic surfactants, especially the polyoxyethylene derivatives, is such
that a relatively large fraction of the micelle volume corresponds to the palisades
region. Because of the bulky nature of the POE chain and its attendant solvent
molecules, it has been suggested that the hydrophilic chain is arranged in an
approximate spiral from the micellar core outward into the solution. As a result,
areas of the palisades near the core will be sterically crowded with the POE chains,
with relatively little room left for waters of hydration or casual water molecules. As
the distance from the core increases, the palisades layer becomes more hydrophilic,
acquiring more characteristics of an aqueous solution. The net effect of such a
situation is that, deep within the palisades layer, the chemical environment may
resemble that of a polyether, so that materials soluble in such solvents will be pre-
ferentially located in that region.
Even though chemical structures may dictate the preferred location for the addi-
tive, solubilized systems are dynamic, as are the parent micelles, and the location of
specific molecules may change over time. It will always be important to remember,
then, that while a given region of the micelle may be preferred by an additive on
chemical grounds, there is no guarantee that all phenomena related to the system
(e.g., catalysis) will be associated with that region.
In surfactant/nonpolar solvent systems where the orientation of the micelle is
reversed, the polar interactions of the head groups provide not only a driving
force for the aggregation process but also an opportune location for the solubiliza-
tion of polar additives. Water is, of course, one of the most important potential polar
additives to nonaqueous systems, and it is located primarily in the polar core. The
nature of such solubilized water is not fixed, however. It has been shown, for exam-
ple, that in the system benzene/(sodium di-2-ethylhexylsulfosuccinate)/water, the
initial water added is tightly associated with the ionic head group of the surfactant
(as waters of hydration), while subsequent additions appear to have the character of
free bulk water. Other polar additives such as carboxylic acids, which may have
some solubility in the organic phase, are probably associated with the micelle in
a manner analogous to that for similar materials in aqueous systems.
The effects of solubilized additives on the micellar properties of nonaqueous sur-
factant systems vary according to the structures of the components. Since such
changes are often greater than those found in aqueous solutions, however, care
must be exercised in evaluating the effects of even small additions on the aggrega-
tion characteristics of surfactants in nonaqueous solvents. Because of the industrial
importance of nonaqueous surfactant systems as cutting oils, lubricants, and corro-
sion inhibitors, a great deal of knowledge about such systems is closely held by the
