Chemistry Reference
In-Depth Information
In conclusion, it appears that the characteristics of the oil-water interfacial film
that enhance the kinetic properties or metastability of emulsions in the presence of
monomeric surfactant species can be related to their function in the context of
Gibbs-Marangoni effects and the formation of specific interfacial complexes lead-
ing to enhanced stability. By damping out local variations in interfacial tension and
distortions in the intervening interstitial film, surfactants help maintain a uniform
lamellar thinning process between approaching droplets, ''healing'' local areas of
weakness that could lead to droplet coalescence, and increasing the stability of the
system. Current results suggest that the role of interfacial viscosity, in the absence
of polymeric species, is minimal.
9.6. AMPHIPHILE MESOPHASES AND EMULSION STABILITY
The mechanical strength, elasticity, and rheological properties of the interfacial film
stabilizing an emulsion obviously have a significant impact on the overall stability
of the system. Chapters 4 and 5 introduced the concept of liquid crystal and meso-
phase formation in surfactant systems, usually in the context of increases in the con-
centration of surface-active material in solution. As indicated, such phases possess a
degree of order that produces substantial changes in the properties of the system
relative to those of the molecular or simple micellar solutions, including a higher
degree of rigidity, larger structural units, and less fluctuation in composition. In
view of the mechanisms of emulsion stabilization discussed above, such phases,
if present at the O/W interface, might be expected to impart an added degree of
stability to systems in which they are present (Figure 9.12). In a practical sense,
liquid crystals or other mesophases may be compared to the mixed interfacial com-
plexes discussed in the preceding section, only producing a more complex interfa-
cial situation on a larger scale. The presence of liquid crystals in the region of high
drop curvature (the plateau border region mentioned in Chapter 8 and shown in,
Figure 8.4) will also help overcome the Laplace pressure differential.
The presence of liquid crystals or other such structures at or near the oil-water
interface has been shown to produce improvements in the stability of numerous
emulsions, although the exact mechanism of their action is still subject to some
question. Even in the absence of complete understanding (as is often the case for
surfactant-related topics), the usefulness of such structures at O/W interfaces has
been demonstrated in practical applications.
By analogy with monomolecular films at liquid-air interfaces, surfactants at the
liquid-liquid interface will normally form monolayers with various molecular
packing densities ranging from relatively loosely packed arrangements normally
associated with fluid phases (gases and liquids) to the close-packed solid phase.
Classically, amphiphilic adsorption at interfaces has been roughly classified in
terms of film types related to normal states of matter (Figure 9.13):
1. Liquid-expanded (L1), in which the adsorbed molecules may assume a variety
of orientations with regard to the dividing surface ranging from perpendicular
