Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 111

table 5.2
contact angles, θ , of Water on
different Solid Surfaces (25°c)

Solid θ

Teflon (PTE) 108

Paraffin wax 110

Polyethylene 95

Graphite 86

AgI 70

Polystyrene 65

Glass 30

Mica 10

forces of the liquid (surface tension of liquid). If a small drop of water is placed on a
smooth surface of Teflon or glass (Figure 5.4), one finds that these drops are different
because there are three surface forces (tensions) that, at equilibrium, give rise to a
contact angle θ (Table 5.2).
The relationship Young's equation describes is the interplay of forces (liquid sur-
face tension, solid surface tension, liquid-solid surface tension) at the three-phase
boundary line. It is regarded as if these forces interact along a line. Experimental
data show that this is indeed true. The magnitude of θ is thus only dependent on the
molecules nearest the interface, and is independent of molecules much farther away
from the contact line.
Further, it has been defined that

When θ is less than 90°, the surface is wetting (such as water on glass).
When θ is greater than 90°, then the surface is nonwetting (such as water on
Tel on).

Most important to mention is that, by treating the glass surface using suitable chemi-
cals, the surface can be rendered hydrophobic. This is the same technology used in
many utensils treated with Teflon or similar material.

5.4 meaSurementS oF contact anGleS
The magnitude of the contact angle, θ, between a liquid and solid can be deter-
mined by various methods. The method to be used depends on the system, as well as
the accuracy required. The two most common methods are measurement by direct
microscope and a goniometer by photography. It should be mentioned that the liquid
drop that one generally uses in such measurements is very small, such as 10 to 100
μL. There are two different systems of interest: liquid-solid or liquid 1 -solid-liquid 2 .
In the case of some industrial systems (such as oil recovery), one needs to determine
at high pressures and temperature. In these systems, the value of θ can be measured