Physical Chemistry of Foods

implies that the interfacial tension gASequals the sum of gLS and the

projection ofgALon the solid surface. This leads to theYoung equation,

gAS¼gLSþgALcosy ð 10 : 10 Þ

given here for the three phases (A, L, S) in the situation depicted. The

characteristic parameter is the contact angley, which is conventionally

taken in the densest fluid.

For the system solid paraffin, water and air, which is depicted in

Figure 10.24c, a value ofy& 1068 is measured.gAW¼72 mN?m
^1 , but the

other interfacial tensions cannot be measured. However, 72 6 cos 106¼


20, which value must equal the differencegLS
gAS; this 20 mN?m
^1 is

the same value as the difference between the corresponding values for liquid

paraffin against water and air, as given in Table 10.1. However, the equality

is not perfect, and for other systems the discrepancy may be greater.

Different substances give, of course, different contact angles andy

increases when going from Figure 10.24a to b to c. In case a it is said that the

solid is preferentially wetted by the liquid, in case c by the air. It is also

possible that (gAS–gLS)/gAL>1, which would imply cosy>1, which is of

FIGURE10.24 Contact angles (y) of liquids at an A–S surface (upper row) and at
an A–W surface (lower row). The pictures of the drops are cross sections through the
largest diameter. A is air, L is liquid, O is oil, S is solid, W is water (or an aqueous
solution). Numbers are interfacial tensions (g)inmN?m
^1 .Psis spreading pressure.
The scale varies among the panels.