Chemistry Reference
In-Depth Information
was attributed to the compensating effects of an increased potential for solubiliza-
tion due to the increase in the POE chain, and a decrease due to a smaller aggrega-
tion number. The solubilization of Yellow OB dye in both the sulfated and
unsulfated POE surfactants was greater than that in the corresponding sodium
alkyl sulfate.
The addition of a second polar group to a surfactant molecule can either increase
or decrease its solubilizing power, depending on the nature of the additive. Studies
comparing the solubilizing capacity of the monoesters of maleic acid
ROOCCH
Na
þ
CHCOO
with those of the disodium salts of the equivalent monoesters of sulfosuccinic acid
SO
3
Na
þ
Þ
COO
Na
þ
ROOCCH
2
CH
ð
found that the introduction of the second ionic group decreased the solubilizing
capacity for nonpolar additives such as n-octane, while that for n-octanol was
increased. According to the preceding discussion, that result can be explained by
the fact that the introduction of the sulfonate group decreases the aggregation num-
ber of the micelle, thus limiting its capacity to include the hydrocarbon in the core.
At the same time, the bulky ionic groups increased the steric requirements at the
micelle surface, increasing the relative volume of the palisades layer available to
the more polar n-octanol molecules.
The solubilizing power of amphoteric surfactants has not been as widely studied,
or at least as widely reported, as that for the simpler ionic and nonionic materials.
However, the available data indicate a solubilizing capacity range somewhere
between the extremes; the exact results possibly are more sensitive to the nature
of the additive than are those for the other classes of surfactants.
6.1.3. Solubilization and the Nature of the Additive
The quantity of a substance that can be solubilized in surfactant micelles will
depend on many factors, some of which have already been discussed. From the
standpoint of the additive itself, such factors as molecular size and shape, polarity,
branching, and the electronegativity of constituent atoms have all been found to be
of some significance, depending on the exact system. One of the most extensively
explored factors relating the chemical structure of the additive to its solubilization
has centered around the relationship between the molar volume of the additive and
the maximum amount of material that can be incorporated in a given surfactant
solution. Investigations into the solubilization of the hydrocarbons hexane, heptane,
and octane, and the aromatics benzene, toluene, ethylbenzene, propylbenzene, and
butylbenzene, showed that there existed an inverse relationship between the mole-
cular volume of the additive and the amount of material solubilized. A similar study
of polycyclic aromatics in sodium laurate revealed similar results. In each study,
linear relationships between the logarithm of the volume of the material solubilized
