Chemistry Reference
In-Depth Information
Unlike the situation for aqueous micelles, in which interactions among the
hydrophobic tails contribute little to the overall free energy of micelle formation,
ionic, dipolar, or hydrogen bonding interactions between head groups in reversed
micelles may be the primary driving forces driving micelle formation. In the face
of factors favoring aggregation, there seem to be few obvious factors opposing the
formation of nonaqueous micelles, such as head group repulsion. The possible
exception is unfavorable entropy losses as a result of fewer degrees of freedom
for monomers in the micelle relative to those free in solution.
Of the many possible reasons for the relative scarcity of experimental data on
nonaqueous micelles versus the aqueous variety, one of the most important findings
is the failure of the easy and straightforward techniques applicable in water to work
in most nonaqueous situations. Particularly important are the measurements of con-
ductivity and surface tension. The ionization of charge-carrying species in solvents
of low dielectric constant is, of course, difficult at best, and very high potentials are
required to perform electrochemical measurements in such systems. In addition,
since most surfactants possess hydrocarbon tails, their adsorption at the solution-
air interface, if it occurs, will be such that the polar head group will be directed
outward, a situation that could actually result in an increase in measured surface
tension. Materials that can produce a lowering of the surface tension of organic
solvents, namely, fluorocarbons and silicones, usually do so in a smooth decrease
over a few mN/m, so that a phenomenon such as a cmc cannot be readily defined.
Unlike aqueous surfactant solutions in which micellar size and shape may vary
considerably, small spherical micelles appear to be the most favored, especially
when the reduction of solvent-polar group interactions is important. In much the
same way as in water-based systems, geometric considerations often play an impor-
tant role in determining micelle size and shape. Many materials that commonly
form nonaqueous micellar solutions possess large, bulky hydrocarbon tails with a
cross-sectional area significantly greater than that of the polar head group. Typical
examples of such materials are sodium di-2-ethylhexylsulfosuccinate and sodium
dinonylnaphthalene sulfonate:
C
8
H
17
OOCCH
2
CH
ð
SO
3
Na
þ
Þ
COOC
8
H
17
ð
C
9
H
19
Þ
2
C
10
H
5
SO
3
Na
þ
Since unambiguous experimental data are much less available on micelle formation
in nonaqueous solvents, it is far more difficult to identify trends and draw
conclusions concerning the relationships between chemical structures and critical
micelle concentrations and aggregation numbers. Some compilations of such data
are given in Tables 4.21 and 4.22. Because of the difficulties of obtaining precise
data and the limited number of systems available, the numbers cited should be
taken as approximate values that can change significantly if the conditions vary.
For example, in hydrocarbon solvents, the nature of the polar head group is extre-
mely important in the aggregation process. It has generally been found that ionic
surfactants form larger nonaqueous micelles than do nonionic ones, with anionic
sulfates surpassing the cationic ammonium salts. The aggregation number for
