Chemistry Reference
In-Depth Information
sensitive to the characteristics of the source and the nature of the nutrients
employed in the fermentation. The complexity of their chemical structures also
make biosurfactants hard to study at the level of basic interfacial chemistry.
7.3. POLYMERIC SURFACTANTS AT INTERFACES: STRUCTURE
AND METHODOLOGY
This section will introduce some of the basic factors controlling the behavior
and function of polymeric surfactants in solution and in colloidal systems. Poly-
meric surfactants, in principle, at least, can perform all the same functions of
''normal'' surfactants; the main differences between the two are the wide variation
in molecular mass between the two classes of materials, the higher-order structural
conformations found for polymeric materials in solution, and the energetic and
kinetic consequences of those conformations and changes to them imparted by
changes in the complete functional system.
Certain classes of polymeric surfactants are found to associate in solution to give
micelles, while others seem to prefer the solitary life of the lone, coiled polymer
chain. Polymeric surfactant micelles can apparently perform most of the functions
of normal micelles, including the solubilization of materials such as drugs, or they
can associate into higher-level structures to give large changes in viscosity, which
are useful for stabilizing colloidal dispersions. Although a great deal of information
is not readily available, it may be assumed that at least some polymeric systems
can form the equivalents of the mesophases found for normal surfactants. For a
typical A-B block copolymer, a single-chain micelle may be pictured as shown
in Figure 7.3, where the A units (heavy line) are hydrophobic and the B units
(light lines) are hydrophilic. A similar picture can be used for protein micelles or
globules where the different portions of the chain are sufficiently different in terms
of their hydrophobic/hydrophilic character.
Figure 7.3.
A schematic model of an A-B block copolymer micelle.
