less well known; assumingA¼ 4? 10 ^21 J, we arrive atpdisj&210 Pa, i.e., much
smaller than for air–water–air.
Question 2
When a solution of 1 or 2%tristearate in triglyceride oil, made at about 70 8 C, is
cooled to room temperature, small tristearate crystals form, and these tend to
aggregate rapidly. Assuming the crystals to be spheres of about 0.1mm diameter,
what would be the attractive free energy between them? Is this sufficient to cause
aggregation? It is also observed that the addition of a little glycerol monolaurate to
the system tends to cause disaggregation. How could that be explained?
Answer
As mentioned above, the interaction can be calculated by Eq. (12.1), assumingAto
equal 0.5 timeskBTandh, 0.4 nm. SinceRis 50 nm, the result would be5.2kBT.
This is sufficient to cause aggregation, although not high enough to prevent
occasional disaggregation. Addition of glycerol monolaurate presumably leads to its
adsorption onto the crystal surfaces (it will not in itself crystallize at room
temperature). This would increase the closest possible distance between approaching
crystals. Assuming thathwould then be at least 2 nm, the resulting value forjVvdWj
can become at most 1kBT, insufficient to cause much aggregation.
12.2.2 Electrostatic Repulsion
Any aqueous surface or interface carries an electric charge. The surface of
pure water is negatively charged, due to a preferential adsorption of OH
ions. Also in salt solutions, there is a certain preference for some ions,
generally anions, to be located at the interface. In many food systems, ionic
surfactants (including proteins) are adsorbed at an aqueous interface,
thereby giving rise to a considerable charge. Of course, the charge may
depend greatly on pH.
Electrostatic Potential. The presence of an excess charge at the
interface causes it to have anelectrostatic surface potentialc 0. This may
cause repulsion between two charged surfaces that are close together. In
Section 3.1, Coulombic electrostatic interaction was mentioned [Eq. (3.1)],
but the repulsion between charged surfaces separated by an aqueous phase
cannot be considered Coulombic. This is because the charge is shielded by
ions, as was discussed in Section 2.3.2; see Figure 2.10. Counterions (ions of