Chemistry Reference
In-Depth Information
various industrial and governmental organizations involved in the related research.
As a result, there are a number of gaps in our understanding of the structural rela-
tionships among surfactant, solvent, and additive. Such information it becomes
more readily available, might facilitate the extension of current knowledge to
new applications.
6.1.2. Surfactant Structure and the Solubilization Process
Earlier chapters introduced some of the wide array of chemical species that exhibit
surfactant properties and are potentially useful in solubilization processes. Just as
molecular structure is important to such surfactant characteristics as the cmc, aggre-
gation number, and micellar shape, it also controls the ability of a surfactant to solu-
bilize a third component. Conversely, the presence of a third component in a
surfactant solution can often affect its aggregation characteristics. It is documented
in a number of reports that the presence of a solubilized additive, even though the
additive has no inherent surface activity, can change the cmc of a surfactant sub-
stantially from that of the pure system. As noted in Chapter 4, the existence of such
an effect means that great care must be exercised in the interpretation of experimen-
tal data on micellization derived from solubilization results.
Whether micelles formed in the presence of a third component are the same as
those formed in its absence is a subject of some controversy. It has been shown that
micellar activity may be induced in surfactant solutions below the ''normal'' cmc in
the presence of small amounts of solubilized additives. In some cases such effects
have been attributed to additive-induced micellization. In others, effects have been
seen at concentrations several orders of magnitude below the cmc, suggesting the
presence in solution of submicellar species possessing some properties of the fully
aggregated system.
Some researchers have suggested that surfactants in dilute solutions undergo a
low level of molecular aggregation at concentrations well below their cmc levels,
during which dimers, tetramers, and other ''premicellar'' aggregates are formed.
That may be especially true for surfactants having unusually large or bulky hydro-
phobic groups, such as the bile acids and tetraalkylammonium halides. Large reac-
tion rate enhancements have been found for such materials when used as phase
transfer catalysts (see discussion below), suggesting that they are acting in a
micelle-like fashion even though normal micelle formation is precluded by their
molecular structure. Some effect due to the formation of tight or solvent-separated
ion pair aggregates is usually invoked to explain the observed catalytic results.
Since the cmc's of most surfactants occur at rather low concentrations, evidence
of premicellar aggregate formation quite often becomes a question of the interpre-
tation of results lying at the limits of sensitivity and accuracy of many experimental
techniques, and, of course, the view of the individual interpreter of those results.
There seems to be little doubt that in nonaqueous solvents, the formation of
dimer and other lower aggregates occurs readily. Fluorescence and electron spin
resonance techniques have also shown the presence of such species in very bulky
surfactant systems in water. However,
there is presently little unambiguous
