Chemistry Reference
In-Depth Information
a given cell structure. There must be some feedback mechanism through which the
organism ''knows'' what material characteristics are needed under a given set of
conditions so that it can be provided when and where called for.
Other aspects of the interactions of lipids and bilayer structures in biological sys-
tems can be understood in the context of molecular geometry, association phenom-
ena, and general interfacial interactions. Unfortunately, those topics are too broad to
be included here. It will be interesting to see how future research in molecular biol-
ogy is able to incorporate the fundamentals of surface and colloid science into a
better understanding of the function of membranes, cells, and entire organisms.
5.7. MICROEMULSIONS
As indicated above, the status of systems commonly called ''microemulsions''
among surface and colloid chemists is somewhat uncertain, despite very extensive
more recent investigations and discussions. Various experimental approaches have
been used in an attempt to ascertain all the details of their thermodynamic and
structural characteristics. As a result, new theories of the formation and stability
of these interesting but quite complex systems are appearing. Hand in hand with
understanding have come ideas for new potentially useful applications. Although
a great deal is known about microemulsions, there is much more to be learned
about the requirements for their preparation and the relationships among the che-
cs
mical structure of the oil phase, the composition of the aqueous phase, and the
structures of the surfactant and the cosurfactant, where needed. As new data
become available, it is clear that any discussion of structure-property relationships
between surfactants and microemulsions becomes just as rapidly outdated. Several
excellent top and reviews that address the theoretical and practical aspects of
microemulsion theory and practice are cited in the Bibliography. The following dis-
cussion is limited to the presentation of comparisons among microemulsions and
related systems (e.g., swollen micelles and macroemulsions) and some general rela-
tionships that have been developed between surfactant structures and microemul-
sion formation.
The distinction between microemulsions and conventional emulsions is fairly
clear. Although emulsions may be kinetically stable for long periods of time,
they must all, in the end, suffer the same fate: phase separation to attain a minimum
in interfacial free energy. The actions of surfactants, polymers, and other stabilizing
aids may shift the rate of droplet coalescence to extremely long times through
decreased kinetic rate constants, but the thermodynamic driving force to mini-
mize interfacial area of contact between immiscible phases remains unchanged.
Microemulsions, on the other hand, appear to be thermodynamically stable compo-
sitions with essentially infinite lifetimes, assuming no change in such factors as
composition, temperature, and pressure.
In addition to the thermodynamic distinction usually drawn between macro- and
microemulsions, the two classes of colloids differ in several other more tangible
characteristics,
including the size of droplets
formed and the mechanical
