Chemistry Reference
In-Depth Information
constant above the cmc so that the amount of surfactant in the micelles could be
quantified by subtracting the cmc from the total amount present. It is now known
that such an assumption is not strictly valid. Substantially the same results have
been found in other studies with different surfactants and additives, however. It
was found, for example, that the amount of dimethylaminoazobenzene incorporated
by a series of potassium carboxylate soaps appeared to be almost linearly related to
the carbon number of the surfactant.
Branching of the hydrocarbon chain of the surfactant usually results in a
decrease in the solubilizing power of the system relative to that of the analogous
straight-chain material. That is presumably due to geometric and packing con-
straints, which limit the ability of the micellar core to accommodate the added
bulk of the solubilized molecules. The addition of ethylenic unsaturation and aro-
matic groups also tends to decrease the maximum amount of additive that can be
fitted into the core packing arrangement.
In the case of nonionic surfactants, the amount of aliphatic hydrocarbon that can
be solubilized generally increases as the length of the hydrophobic tail increases
and decreases as that of the POE chain increases. Those results parallel changes
in the cmc's and aggregation numbers of the respective materials. Divalent salts
of alkyl sulfates quite often exhibit a greater solubilizing capacity than do the cor-
responding monovalent salts for materials included in the micellar core. That result
has been related to the increase in volume of the micellar core of the divalent salts.
The relative solubilizing power of the different types of surfactant with a given
hydrophobic tail usually follows the order nonionics
anionic. The
rationale for such a result is usually related to the supposed looser packing of the
surfactant molecules in the micelles of the nonionic materials, making available
more space for the incorporation of additive molecules.
If one considers additives with a more polar character, which might reasonably
reside in or near the palisades layer, fewer generalizations such as those above can
be made. The complex interactions among the various components of the system—
the surfactant head groups, water (or other solvent) molecules, the exposed micellar
core, and the polar group of the additive—appear to be too specifically sensitive to
allow for an easy trend analysis.
It was usually found that compounds such as methylisobutyl ether and n-octyl
alcohol were better solubilized in 0.1 N sodium oleate than in potassium laurate at
the same concentration and temperature, contrary to the results for hydrocarbon
materials solubilized in the micellar core of the same systems. Octylamine, on
the other hand, was incorporated into each to an equal extent. It was also found
that the degree of solubilization of 1-o-tolyl-azo-2-naphthylamine and related mate-
rials in micelles of sodium dodecylpolyoxyethylene sulfates
cationics
>
>
OC
2
H
4
Þ
x
OSO
3
Na
þ
C
12
H
25
ð
where x varied from 1 to 10, increased as the value of x increased. The same mate-
rial showed no change in solubilization with increase in the POE chain length in the
analogous unsulfated nonionic surfactant over the range of x
¼
6-20. That result
