Physical Chemistry of Foods

on average, the net force equals zero. This is because of the hard core
repulsion between molecules. As illustrated in Figure 3.1, molecules attain
on average a mutual distance where the net interaction energy is at
minimum, which implies that the net force equals zero. Nevertheless, it costs
energy to move oil molecules from the oil towards the surface, and this is
what occurs when the surface area is enlarged.
Table 10.1 gives examples of the surface tension of liquids and of
interfacial tensions. The values are fairly small for the organic liquids
shown; for these, the attractive energy is predominantly due to van der
Waals forces. For water, gis higher, owing to the extensive hydrogen
bonding between water molecules (Section 3.2). For mercury,gis very high;

TABLE10.1A Some Values of Surface and Interfacial

Tensions—Liquids

Material gagainst aira gagainst water

Water 0 8 C76?

258 C72?

608 C66?

1008 C59?

0.02 molar Na-dodecylate 43 0

Protein solutions & 50 0

Saturated NaCl solution 82 0

Diethyl ether 20 0

Ethanol 22 0

Benzene 29 35

Paraffin oil 30 50

Triacylglycerol oil 35 30

Mercury 486 415

aSaturated with vapour of the material

TABLE10.1B Some Values of Surface and Interfacial Tensions

Between Solids and Liquids (Rough Estimates)

Solid Liquid g

Ice, 0 8 C Water 25

Sucrose Saturated sucrose solution 5

Triacylglycerol crystal Triacylglycerol oil 4

Triacylglycerol crystal Water 31

Approximate data in mN?m
^1 at room temperature, unless mentioned otherwise.