Physical Chemistry of Foods

film does not rupture, drainage will go on until the net colloidal disjoining pressure in
the film equals the mentioned Laplace pressure difference. Can you calculate at what
film thickness this will be the case if repulsion is caused by an adsorbed ionic
surfactant? We assume that the Hamaker constant of the film material will be
30? 10
^21 J (value for water for not very smallh), the surface potential
32 mV, and
the ionic strength 0.01 molar.

Answer

The DLVO theory would be appropriate for the present case. However, we need
pressure, not interaction energy per unit area. The (repulsive) disjoining pressure can
be derived from

pdisj¼


dVp

dh

Using Eq. (12.2) for the van der Waals attraction and Eq. (12.8) for the electrostatic
repulsion between flat plates, we arrive after differentiation at

pdisj¼


A

6 ph^3

þ 1 : 4? 10 
^9 c^20 k^2 expð
khÞ

Using Eq. (12.6) forkand inserting the values given above, the disjoining pressure
can be calculated as a function ofh. The result ispdisj& 16? 105 (e
h/3
h
^3 ), in Pa for
hin nm. The result equals 100 Pa ath&27 nm. Actually, the van der Waals attraction
will have been overestimated for such a distance due to retardation occurring. Hence,
the equilibrium film thickness would be 30 nm or a little more.
Further question: What would be the effect of a higher ionic strength, e.g., 0.2
molar, on the relation betweenpdisjandh?

12.3 ROLE OF POLYMERS

Polymers can strongly affect colloid stability. Many polymers can adsorb
onto particles and then cause steric interaction (Section 12.3.1), which is
often repulsive and thereby stabilizing, although attractive interaction can
also occur. If polymer molecules adsorb on two particles at the same time,
they cause bridging (Section 12.3.2), hence aggregation. Polymers in
solution can also cause aggregation via depletion interaction (Section
12.3.3), or they can stabilize a dispersion by immobilizing the particles in a
gel network.
Chapter 6 gives basic aspects about polymers, and Chapter 7 about
proteins, the polymers often used in stabilizing food dispersions. Adsorption
of surface active polymers is discussed in Section 10.3.2.