Chemistry Reference
In-Depth Information
preparation of primary emulsions will often break the primary emulsion, resulting
in loss of phase identity.
Multiple emulsions have reportedly been prepared conveniently by the phase
inversion technique mentioned earlier; however, such systems have generally
been found to have limited stability. It generally requires a very judicious choice
of surfactant or surfactant combinations to produce a multiple emulsion system
that has useful characteristics of formation and stability. A general procedure for
the preparation of a W/O/W multiple emulsion may involve the formation of a pri-
mary emulsion of water in oil using a surfactant suitable for the stabilization of
such W/O systems. Generally, that will involve the use of an oil-soluble surfactant
with a low HLB (2-8). The primary emulsion will then be emulsified in a second
aqueous solution containing a second surfactant system appropriate for the stabili-
zation of the secondary O/Wemulsion (HLB 6-16). As noted above, because of the
possible instability of the primary emulsion, great care must be taken in the choice
of the secondary dispersion method. Excessive mechanical agitation such as in
high-speed mixers and sonication could result in gross coalescence of the primary
emulsion and the production of essentially ''empty'' oil droplets. The evaluation of
the yield of filled secondary emulsion drops, therefore, is very important in asses-
sing the value of different preparation methods and surfactant combinations.
The nature of the droplets in a multiple emulsion will depend on the size and
stability of the primary emulsion. A system of classification has been proposed
dividing W/O/W multiple emulsions into three classes according to the nature of
the oil-phase droplets (Figure 9.16). Type A systems are characterized as having
one large internal drop essentially encapsulated by the oil phase. Type B systems
contain several small, well-separated internal drops, and systems of type C contain
many small internal drops in close proximity. It is understood that any given system
will in all probability contain all three classes of drops, but one will be found to
dominate, depending primarily on the surfactant system employed.
Figure 9.16.
Multiple emulsion classification based on droplet characteristics in the primary
emulsion.
