Chemistry Reference
In-Depth Information
Figure 5.11. A schematic representation of oil-in-water and water-in-oil (O/W and W/O)
microemulsions including cosurfactants.
and the relative quantities of each in the system. As mentioned above, most micro-
emulsions, especially those employing an ionic surfactant, will require the addition
of a cosurfactant to attain the required interfacial properties to allow for the spon-
taneous dispersion of one phase in the other. Nonionic surfactants, on the other
hand, quite often form microemulsions without the addition of another component.
5.7.1.1. Ionic Surfactant Systems
Using the schematic picture presented in Figure 5.11, it is possible to qualitatively
analyze the relationship between surfactant and cosurfactant structures and the
most probable microemulsion system formed for ionic surfactants. If the cosurfac-
tant : surfactant ratio is less than 3, the surfactant head groups can approximate a
close-packed monolayer for type 1 curvature and the formation of a O/W system
is favored. If the ratio is three or greater, close packing of the head groups is not
possible and W/O systems are predicted. Longer-chain cosolvents will tend to favor
W/O systems, all things being equal, while electrical double layer effects will tend
to push for O/W systems.
Although the thermodynamic theory of microemulsions still has some way to go
to be more or less complete, a number of generalizations can be made regarding
ionic surfactant microemulsions:
1. A cosurfactant is always required to form W/O microemulsions using ionic
surfactants, not only to lower the interfacial tension
s
but also to reduce head
group crowding due to the type 2 curvature.
