Chemistry Reference
In-Depth Information
The presence of an ionic surfactant in mixture with a nonionic usually results in
an increase in the cloud point of the nonionic component. In fact, the mixture may
not show a cloud point, or the transition may occur over a broader temperature
range, indicating that the ionic component is forming mixed micelles with the non-
ionic surfactant, thereby increasing its ''solubility'' at higher temperatures. As a
result, it is possible to formulate mixtures of ionic and nonionic surfactants for
use at temperatures and under solvent conditions (electrolyte, etc.) in which neither
component alone would be effective.
As we shall see in subsequent chapters, many mixtures of surfactants, especially
ionic with nonionic, exhibit surface properties significantly better than do those
obtained with either component alone. Such synergistic effects greatly improve
many technological applications in areas such as emulsion formulations, emulsion
polymerization, surface tension reduction, coating operations, personal care and
cosmetics products, pharmaceuticals, and petroleum recovery, to name only a few.
The use of mixed surfactant systems should always be considered as a method
for obtaining the optimal performance for any practical surfactant application.
A more unique and less extensively researched class of mixed surfactant systems
is that in which the two components are of opposite charge, that is, a mixture of a
cationic and an anionic surfactant. In aqueous solvents such mixtures will often
result in precipitation of stoichiometric amounts of the two materials due to ion
pairing of the two surface-active components. A very careful combination of the
two ionic classes can produce interesting results in terms of surface tension lower-
ing (effectiveness) due to the formation of close ion pairs in the surface monolayer.
In nonaqueous solvents, on the other hand, interesting and useful results may be
obtained since the ion-paired combination may be significantly soluble in the
organic solvent while still retaining useful properties in terms of aggregation and
adsorption. Some such combinations have shown promise as phase transfer cata-
lysts in which one or both components act as ''mules'' to shuttle reactants and pro-
ducts between aqueous and organic phases.
As fluorinated surfactants become more widely used throughout industry,
regulatory constraints permitting, there often arise needs for a mixture of hydrocar-
bon and fluorocarbon materials to meet system performance requirements. For
example, fluorocarbon surfactants are excellent at lowering the surface tension of
aqueous systems at very low concentrations, but are normally of little use for
forming or stabilizing emulsions. In a system requiring a low surface tension
and emulsion stabilization it may be convenient to use both classes of materials.
In such instances it must be determined whether the two types of surfactant
will form mixed micelles, or whether two different types of homogeneous micelles
will result. The presence of two distinct micellar types in a single solution, with
all the accompanying characteristic differences, poses many interesting theo-
retical and practical questions. One might expect that given the known im-
miscibility of heavily fluorinated materials with hydrocarbons, comicellization
would not be the rule. Although the experimental data are limited, there seems
to be good evidence that the formation of two micellar species does occur in
many instances.
