Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 36

The measured value of γ of CCl 4 is 27 mN/m (Table 2.1). The large difference can be
ascribed to the assumption that Stefan's ratio of 2 was used in this example.

2.7.2.2 other Surface Properties of liquids
Surface waves on liquids: A liquid surface, for example, that of oceans or lakes,
exhibit wave formation when strong winds are blowing over it. It is known that such
waves are created by wind energy being transposed to waves. Hence, humans have
tried to convert wave energy to other useful forms of energy. Both transverse capil-
lary waves and longitudinal waves can deliver information about the elasticity and
viscosity of surfaces, albeit on very different time scales. Rates of adsorption and
desorption can also be deduced. Transverse capillary waves are usually generated
with frequencies between 100 and 300 Hz. The generator is a hydrophobic knife
edge situated on the surface and oscillating vertically, while the usual detector is
a lightweight hydrophobic wire lying on the surface parallel to the generator edge.
The generator and detector are usually close together (15-20 mm) so that reflections
set up a pattern of standing transverse wave. The damping of capillary ripples arises
primarily from the compression and expansion of the surface and the interaction
between the surface film and propagation at the top of the path, and in the reverse
direction at the bottom (Adamson and Gast, 1997).
This leads to compression and expansion of the surface. If a surface film is pres-
ent, compression tends to lower the surface tension, whereas expansion raises it.
Then a Marangoni flow is generated, which opposes the wave motion and dampens it
(Birdi, 2002, 2009). Furthermore, if the material is soluble, the compression-expan-
sion cycle will be accompanied by capillary ripples according to the hydrodynamic
theories that have been described earlier. For longitudinal waves, a barrier (usually
on the trough of a surface film) generate the length of the trough.

2.7.2.3 Interfacial tension of liquid 1 -liquid 2
It is a well-known adage that oil and water do not mix. However, it will be shown
that, by changing the interfacial forces at the oil-water boundary, one can indeed
disperse oil in water (or vice versa). At the oil-water interface there exists interfacial
tension (IFT), which can be measured by some of the methods mentioned earlier
(e.g., by drop weight, pendant drop, or Wilhelmy plate).
The interfacial tension γ AB between two liquids with surface tension γ A and γ B
is of interest in such systems as emulsions and wetting (Adamson and Gast, 1997;
Chattoraj and Birdi, 1984; Somasundaran, 2006). An empirical relation was sug-
gested (Antonow's rule), by which one can predict the surface tension γ AB :

γ AB = |γ A(B) − γ B(A) | (2.52)

The prediction of γ AB from this rule is approximate but found to be useful in a
large number of systems (such as alkanes:water), with some exceptions (such as
water:butanol) (Table 2.3). For example,