Chemistry Reference
In-Depth Information
Figure 2.11.
Basic behavior of surfactants: surface adsorption, micelle formation, and critical
micelle concentration.
with water. Above the critical micelle concentration (CMC), the available interfaces are
saturated with the surfactant monomers causing the remaining monomers to self-
assemble into macromolecular structures to reduce the interaction of the hydrophobes
with water. This process is illustrated in Figure 2.11. These structures include micelles
(which may be spherical or cylindrical) and vesicles (which are basically spherical
bilayers). Micelles can generally be thought of as having a hydrophilic shell surrounding
a hydrophobic core. The CMC is a function of the surfactant structure and is commonly
used as a measure of its ef
cient surfactant.
In many cases, it is possible for hydrophobic materials to be incorporated or
ciency. A lower CMC indicates a more ef
“
in the interior of micelles. This key property of surfactants in solution
is another reason for their incorporation in pharmaceutical formulations. Solubilization
occurs when one forms thermodynamically stable isotropic solutions of normally
insoluble materials in a solvent (usually water) through the addition of surfactants.
The structures that form may be thought of as swollen micelles, although it is somewhat
ambiguous where the transition between swollen micelles or micro/macroemulsions
occurs. Generally, solubilization occurs at a high ratio of surfactant to hydrophobic
solute and is smaller than what is seen with amorphous forms. The ability of a particular
surfactant to solubilize a given solute is highly dependent upon the structural character-
istics of both moieties and the micelles formed. The apparent increase in solubility has
been used by itself to improve the bioavailability of poorly soluble drugs and is also
involved in increasing the dissolution rate of poorly soluble drugs by increasing transport
rates from the solid surface. An example of a surfactant-enhanced bioavailability is
shown by El Eini et al., where the solubility of a model drug, dexamethasone, increased
linearly with added nonionic surfactant [37].
solubilized
”
2.2.3 Applications of Surfactants
In pharmaceutical applications, surfactants are used as emulsi
ers, wetting agents,
dispersants, foaming agents, and penetrating agents. The types of surfactants used in
