Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 108

table 5.1
calculation of Spreading coefficients, Sa/b,
for air-Water Interfaces (20°c; a = air; w =
water; o = oil)
oil

γ w/a − γ o/a − γ o/w = Sa/b conclusion

n -C 16 H 34 72.8 − 30.0 − 52.1 = −0.3 Will not spread

n -Octane 72.8 − 21.8 − 50.8 = +0.2 Will just spread

n -Octanol 72.8 − 27.5 − 8.5 = +36.8 Will spread

Another example is that the addition of surfactants (detergents) to a fluid dra-
matically affects its wetting and spreading properties. Many technologies utilize
surfactants for control of wetting properties (Birdi, 1997). The ability of surfactant
molecules to control wetting arises from their self-assembly at the liquid-vapor,
liquid-liquid, solid-liquid, and solid-air interfaces and the resulting changes in the
interfacial energies (Birdi, 1997). These interfacial self-assemblies exhibit rich struc-
tural detail and variation. The molecular structure of the self-assemblies, and the
effects of these structures on wetting or other phenomena, remain topics of extensive
scientific and technological interest.
In the case of oil spills on the seas, these considerations become very important.
The treatment of such pollutant systems requires knowledge of the state of the oil. The
thickness of the oil layer will be dependent on the spreading characteristics. The effect
on ecology (such as birds and plants) will depend on the spreading characteristics.
Young's equation at liquid 1 -solid-liquid 2 has been investigated in various sys-
tems where it has been found that the liquid 1 -solid-liquid 2 surface tensions meet at
a given contact angle. For example, the contact angle of water drop on Teflon is 50 o
in octane (Chattoraj and Birdi, 1984; see Figure 5.3).
In the water-Teflon-octane system, the contact angle, θ, is related to the different
surface tensions as follows:

γ s-octane = γ water-s + γ octane-water cos(θ) (5.6)

cos(θ) = (γ s-octane − γ water-s )/γ octane-water (5.7)

GL

Oil Water

CA

GS GLS

Solid

FIGure 5.3 Contact angle at water-Teflon-octane interface.