Chemistry Reference
In-Depth Information
molecular orientations may serve to reduce the total free energy of the solution, or
for the formation of molecular aggregates with their hydrophobic portions directed
toward the interior of the micelle. Micellization, therefore, is an alternative mecha-
nism to adsorption for the reduction of solution free energy by the minimization of
the distortion of the structure of the bulk water. Although the removal of the hydro-
phobe from the water environment results in a decrease in energy, the adsorbed or
aggregated hydrophobe may experience a loss of freedom (decrease in entropy) that
would thermodynamically reduce the attractiveness of the process. In this case, the
water wins out, overall.
It must also be remembered that the surfactant molecule also possesses a hydro-
philic group, the interaction of which with water may decrease the free energy of
the system. The partial removal of that group from the solution through adsorption
or micelle formation can result in an increase in free energy. Additionally, the
hydrophilic group may possess an electrostatic charge so that the process of adsorp-
tion or micellization can introduce electrostatic repulsions, which act to inhibit
the removal of the molecule from solution. The situation, then, becomes a tug of
war between the opposing free-energy considerations. The occurrence of micelliz-
ation in a given surfactant system, and the concentration at which micelle forma-
tion occurs, will therefore be determined by the relative balance of the forces
favoring and retarding the molecular aggregation process. Since the magnitudes
of the opposing forces are determined by the chemical compositions of the solute
molecules, where all other aspects (temperature, pressure, solvent, etc.) are held
constant, it is the chemical constitution of the surface-active species that ulti-
mately controls events. It should be possible, then, to make reasonable generaliza-
tions about
the micellization characteristics of surfactants and their chemical
structures.
Using the Hartley concept as a starting model, modern studies using techniques
unimagined by the earlier workers have produced more detailed pictures of the sub-
microscopic nature of micelles. Micelles, of course, are not static species. They are
very dynamic in that there is a constant, rapid interchange of molecules between the
aggregates and the solution phase. It is also reasonable to assume that surfactant
molecules do not pack into a micelle in such an orderly manner as to produce a
smooth, uniform surface structure. If one could photograph a micelle with ultra-
high-speed film, freezing the motion of the molecules, the picture would almost
certainly show an irregular molecular cluster more closely resembling a cocklebur
than a golf ball.
The simplicity of the Hartley micelle has left it open to criticism, since it fails to
adequately explain many experimentally observed phenomena. Models of micelles
have been suggested that appear to differ substantially from those of Hartley. Par-
ticularly significant differences are a much greater degree of penetration of water
into the micelle interior and a relatively smaller interior or core radius. Some of
those models appear to better explain some of the solubilization data for hydropho-
bic additives and the measured or inferred microviscosities of micellar interiors.
They have also been used to better explain some results in micelle-catalyzed
reactions. The various models of micelles are, of course, just that—models that
