Chemistry Reference
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character. Similar behavior has been observed for aqueous solutions containing
nonionic surfactants and other water-miscible organic solvents.
The forces leading to micelle formation in polar organic solvents are not well
understood, but they probably lie somewhere between the classical aqueous driving
forces and the more-or-less opposite phenomena operating in nonpolar systems.
There is undoubtedly a spectrum of mechanisms to be explored on theoretical
and practical grounds for the ambitious graduate student or industry intern.
4.7.2. Micelles in Nonpolar Solvents
One reason for the scarcity of information on nonaqueous micelles is the relative
difficulty of obtaining good, reproducible data. In water, micellization can be rela-
tively easily followed using laboratory techniques such as surface tension measure-
ments, conductivity, light scattering, and dye solubilization. In organic media, the
two classic workhorses of surfactant studies, surface tension and conductivity
measurements, are pretty useless. Dye solubilization has been used, but tends to
be difficult to repeat quantitatively. A number of spectroscopic techniques have
also been used with varying degrees of success. However, because the aggregation
process in organic solvents is apparently not a sudden-onset phenomenon as in
water, identification of the exact concentration of surfactant present when the pro-
cess occurs is often subject to a wide range of interpretations. Precise data, there-
fore, are hard to come by.
The forces and changes involved in surfactant aggregation in nonpolar nonaqu-
eous solvents differ considerably from those already discussed for water-based
systems. The orientation of the surfactant relative to the bulk solvent will be the
opposite that in water (hence the term ''reversed'' micelle; see Figure 4.6c). In addi-
tion, the micelle, regardless of the nature of the surfactant, will be un-ionized in
solvents of low dielectric constant, and thus will have no significant electrical pro-
perties relative to the bulk solvent. Electrostatic interactions may, as we shall see,
play an important role in the aggregation process, but in a sense opposite that in
aqueous solution where strong head group repulsion tended to work against micelle
formation.
As pointed out previously, the primary driving force for the formation of micelles
in aqueous solution is the unfavorable entropic effect—also referred to as the
''hydrophobic effect'' in some literature—of ordering water molecules around
the surfactant tail. In nonaqueous solvents, that effect would not be expected to
be important since there would be little energy difference between solvent-solvent,
solvent-tail, and tail-tail interactions. That is the case even if the solvent is
aromatic or halogenated, rather than a simple hydrocarbon. Systems containing
fluorinated materials or silicones are possible exceptions, as indicated by the fact
that the surface tensions of some organic liquids is lowered by such surfactants.
A more significant energetic consequence of nonaqueous micelle formation is the
reduction of unfavorable interactions between the polar or ionic head groups of the
surfactant molecules and the nonpolar solvent molecules. By analogy, such an
effect might be called a ''hydrophilic effect.''
