Physical Chemistry of Foods

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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


  1. Which detergent would be superior?

  2. Can the data just given be determined? If not, what would you do to find an
    answer?


Answer


  1. 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?

  2. 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

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