surfactants, the advancing and receding contact angles are often affected to
a different extent; in other words, the hysteresis then depends on surfactant
type and concentration.
Question
Two aqueous detergent solutions, A and B, are compared for their ability to remove
fat (F) from china (C). Suppose that the interfacial tensions (in mN?m^1 ) are as
follows:
A–F¼ 16 B–F¼ 12
A–C¼ 25 B–C¼ 20
C–F¼ 35- Which detergent would be superior?
- Can the data just given be determined? If not, what would you do to find an
answer?
Answer
- By application of Eq. (10.10), we can calculate the contact angley, as measured
in the detergent phase (D). This leads to cosy¼(gFC–gDC)/gFD. For D¼A, it yields
(35 – 25)/16¼0.625 ory¼ 518 ; for D¼B we have (35 – 20)/ 12¼1.25, implying that
y¼0. If the contact angle in the aqueous phase is zero, the detergent will completely
wet the china and thus dislodge the fat from it; consequently, B will be the superior
detergent. An even simpler way to reach this conclusion is by using Eq. (10.12).
How? - No, the data cannot be obtained, because interfacial tensions involving a solid
surface cannot be determined. What can be measured, however, is the contact angle,
and that is a sufficient criterion, as mentioned.
10.6.3 Capillary Displacement
When a porous solid, in which the pores are filled with air, makes contact
with a liquid, this will lead to the liquid displacing the air if the contact angle
as measured in the liquid is acute or zero. An example is given by a sugar
cube brought in contact with tea: the tea is immediately sucked into the
pores between the sugar crystals. A liquid can also be displaced by another,
immiscible, liquid. An example is a plastic fat—i.e., a continuous network of
fat crystals filled with a continuous oil phase—where the oil can be displaced
by an SDS solution of sufficient strength; cf. Figure 10.25. If gravity is
acting and the displacement is upwards, the displacing liquid will move ever