Complications. Below are some factors of importance:
Particle shape. Calculation of the interaction energy works well for
perfect spheres, i.e., for most O–W emulsions. Although equations
for a number of simple particle geometries are to be found in the
literature, application to real nonspherical particles often involves
several difficulties. For instance, the shape may be irregular and
variable, and the particles can encounter each other in a number of
orientations.
Particle size. The deficiency of the theory was already mentioned.
Currently, a number of refinements of the DLVO theory have been
proposed to solve this problem, but agreement has not yet been
attained.
For real particles, which are often inhomogeneous, the value of the
Hamaker constant may be uncertain.
Likewise, real particles may not have a hard and smooth surface, and
this may interfere with the equality of surface and zeta potential (the
position of the slipping plane not coinciding with the outside of a
Stern layer).
Several other interaction forces may act. These are for the most part
discussed in the rest of this Chapter.Question 1
Consider emulsions made of 2%triglyceride oil in 0.1%surfactant solutions that
have an ionic strength of 5 millimolar and a pH of 7. The surfactants are (a) sodium
dodecyl sulfate; (b) glycerol monolaurate; and (c)b-lactoglobulin. Moreover, to part
of each of the emulsions 0.5%NaCl is added, and to another part sufficient HCl to
bring the pH down to 5. Nine emulsions are thus obtained; typical droplet sizes are
4 mm. Which of the emulsions are expected to be stable to aggregation and which are
not? For some of them you can predict this without making calculations (which
ones?), for others you have to make calculations. You may assume that the Hamaker
constant equals 5? 10 ^21 J and that the surface potential equals40 mV for (a) and
15 mV for (c) at pH 7. What calculations would you perform? Why are the
concentrations of oil and of surfactant given?
Question 2
In Section 11.2.2, drainage of liquid from a foam is discussed. In a foam with flat
lamellae between the bubbles, drainage from a lamella to the adjacent Plateau
borders is caused by a difference in Laplace pressure, which equalsrgH, according to
Eq. (11.1), whereHis the height above the bottom of the foam. For an aqueous foam
atH¼1 cm, the pressure difference would thus be about 100 Pa. Assuming that the