Chemistry Reference
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hydrocarbons and halogenated hydrocarbons; weakly surface-active materials, such
as nitriles, methyl esters, ketones, and aldehydes; more surface-active monohydric
alcohols; and fatty acids. As one might expect, each class has a distinct effect on the
phase transitions in a ternary surfactant system.
For hydrophobic additives, one generally finds that an increase in the relative
concentration of the additive results in an increase in the range over which one
encounters liquid isotropic and middle phases, leading ultimately to a viscous iso-
tropic mesophase. When the additive possesses a slightly polar group (ester, amide,
etc.), the solubilizing capacity of both the liquid isotropic and middle phases may
be increased significantly, resulting in the formation of a lamellar neat phase at high
additive concentrations.
When the additive is a monohydric alcohol, a much more complex system may
be encountered, often with the formation of the lamellar neat phase at water con-
centrations much higher than normal. It is also common to find a second liquid iso-
tropic phase in which the alcohol becomes a secondary solvent. Additional complex
mesophases of indefinite structure may arise, leading ultimately to a reversed mid-
dle phase. In the presence of fatty acids, the phase diagram will resemble that of the
monohydric alcohols except that the liquid isotropic solution in the acid will usually
be found to incorporate more water.
Obviously, the phase behavior of a surfactant is a complex matter that may sig-
nificantly affect its activity in a given application. While the discussion above is
highly abbreviated, it should serve to illustrate again the great importance that
surfactant structure and environment can have in complex (and sometimes simple)
systems.
5.4. SOME CURRENT THEORETICAL ANALYSES OF NOVEL
MESOPHASES
The attractive, simple picture of amphiphilic aggregation structures is, as already
noted, blurred by reality. The classical method for studying the phase behavior of
surfactants has been through the construction of phase diagrams—a delicate and
laborious process that requires care and dedication (read: ''many ready and willing
graduate or undergraduate hands''). Even then, the interpretation and application of
the results to predictive theory requires care, knowledge, intuition, and significant
mathematical ability (and computer power). In practice, it is still not possible to
accurately predict a phase diagram from the molecular composition and structure
of an amphiphile alone. That goal is thus far beyond our reach, in part because we
still do not understand the finer points of such factors as specific ion effects govern-
ing the delicate balance of water-hydrophobe or electrostatic and dispersion force
interactions at the molecular level. Nor do we have a really good grasp of mem-
brane rheology, (e.g., bending elastic moduli and undulation forces) or entropic
contributions that predominate in the self-assembly process. The basic problem
remains the impossibility of deducing a complete theoretical phase diagram
from first principles, even for a simple model amphiphile. The problem lies in
