Chemistry Reference
In-Depth Information
cmc, however, the simplicity of the relationship between electrolyte concentration,
cmc, and solubilization seems to disappear.
A study of the solubilization of decanol in solutions of sodium octanoate showed
that at low surfactant concentrations the solubilization of the additive increased
rapidly after the cmc was exceeded, and continued to do so for some time as the
concentration of sodium chloride was increased. At higher surfactant concentra-
tions, however, it was found that there was an initial increase in decanol incorpora-
tion, which reached a maximum and then began to decrease as the salt level
continued to increase. When the octanoate concentration well exceeded the cmc,
the addition of salt resulted in an immediate decrease in the ability of the system
to incorporate the additive. Such complex interactions have been attributed to
alterations in the thermodynamics of mixed micelle formation for the decanol and
carboxylate salt. Similar results may be seen in systems where the increased elec-
trolyte content produces a change in the character of the micellar system: a sphere-
to-rod micellar transformation or the development of a mesophase, for example.
Variations in results for the effect of added electrolytes on solubilization by ionic
surfactants might also be related to the nature of the additive and its potential loca-
tion in the micelle. For nonpolar additives or those lying deep in the palisades layer
of the micelle, it seems reasonable to expect that the increased volume of the micel-
lar core produced by electrolytes would lead to a greater capacity for solubilization.
For more polar materials, which would be expected to be incorporated less deeply
in the micelle, added electrolyte results in a closer packing of ionic head groups,
which could reduce the available space for solubilized molecules. Changes in
micelle shape, from spheres to rods, for example, would also cause less surface
volume to be available in the palisades layer as a result of closer packing of the
head groups or, put another way, less curvature in the layer.
In the case of nonionic surfactants, the effects of added electrolytes seem to par-
allel their effects on the micellization process. When such addition produces an
increase in micellar aggregation number, an increase in solubilizing capacity for
hydrocarbon additives is also found. The results for the solubilization of polar mate-
rials is, again, less clear-cut. A similar trend is generally found for cationic and
amphoteric surfactants.
6.1.7. Miscellaneous Factors Affecting Solubilization
Other factors that can affect the ability of a particular surfactant system to solubi-
lize materials include pH and pressure. The effects of such factors, however, have
not been as extensively reported in the literature as the factors discussed above, and
they are often very specific to each surfactant system. Obviously, surfactants that
show special sensitivity to pH such as the carboxylate salts will also be expected
to exhibit significant changes in solubilization with changes in that factor. In addi-
tion, changes in pH can affect the nature of the additive itself, producing dramatic
changes in its interactions with the micelle, including the locus of solubilization.
Such effects can be especially important in many applications of solubilization,
especially in the pharmaceutical field.
