Chemistry Reference
In-Depth Information
easily found in systems of monomeric materials. In addition, the presence of poly-
meric materials in the system can retard processes such as creaming by increasing
the viscosity of the continuous phase in addition to reducing the rate of droplet
encounters, which could lead to flocculation or coalescence.
A second class of effective emulsifying agents commonly encountered consists
of the finely divided solid particles. It has been known for some time that particles
of colloidal dimensions (e.g.,
1 mm in diameter) that are wetted by both aqueous
and organic liquids can form stabilizing films and produce both O/W and W/O
emulsions with significant stability. Emulsion stabilization by solid particles relies
on the specific location of the particles at the interface to produce a strong, rigid
barrier that prevents or inhibits the coalescence of drops. If the solid has a native
electrical charge, it may also impart a degree of electrostatic repulsion that enhances
the overall stabilizing power of the system.
There are three keys to the use of particulate solids as emulsion stabilizers: par-
ticle size, the state of stabilizer particle dispersion, and the relative wettability of the
particles by each liquid component of the emulsion system. The stabilizer particles
must be small compared to the emulsion droplet and in a state of incipient floccula-
tion; that is, the particle dispersion must be near the limit of stability so that their
location at the interface will result in some attractive particle-particle interaction to
give strength to the system.
For the third condition, the solid must exhibit a significant contact angle at the
three-phase (oil-water-solid) contact line, as measured conventionally through
the aqueous phase. For maximum efficiency, it is usually found that the stabilizer
should be preferentially wetted by the continuous phase. If the solid particles are
too strongly wetted by either of the two liquid phases, the optimum stabilizing
action will not be attained. It is usually necessary, therefore, to closely control
such factors by controlling the system pH or by adding materials that adsorb
onto the particles and impart the required surface characteristics. For example, if
a mineral particle is used, but it is wetted too well by the aqueous phase, it can
be treated with amphiphilic materials that will partially or completely coat the par-
ticle surface and thereby modify its interactions with water and/or the oil phase. A
similar effect may, in some cases, be achieved by modifying the surface charge of
the particles through specific-ion adsorption, pH changes, adsorption, and other
variables. More details on those processes are given in Chapter 10.
The last major class of emulsifiers and stabilizers is that of the monomeric sur-
factants that adsorb at interfaces and produce electrical, mechanical, and steric bar-
riers to drop coalescence, in addition to their role in lowering the interfacial free
energy between the dispersed and continuous phases. Since these materials are
the central concern of this work, they are addressed in detail below.
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9.2.1. Lifetimes of Typical Emulsions
Any discussion of the stability of emulsions must be concerned not only with
the mechanism of stabilization but also with the timeframe of the stability require-
ments and the conditions of preparation. The rates of degradation of emulsions
