Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 29

2.7 SurFace tenSIon oF lIquIdS
Surface tension of liquids has been extensively analyzed in the literature. Some typi-
cal values of surface tension of different liquids are given in Table 2.1. A brief analy-
sis of these data is given in the following text.
Some comments are needed on these data in order to explain the differences in
surface tension data and molecular structure. The range of γ is found to vary from ca.
20 to over 1000 Nm/m. The surface tension of Hg is high because it is a liquid metal
with a very high boiling point. This indicates that it needs much energy to break the
bonds between Hg atoms to evaporate. Similarly, γ of NaCl as a liquid (at high tem-
perature) is also very high. The same case is found for metals in liquid state.
The other liquids can be considered as under each type, which should help under-
stand the relation between the structure of a molecule and its surface tension.
Alkanes : The magnitude increases by 1.52 mN/m per two CH 2 when the alkyl
chain length increases from 10 to 12 ( n- Decane = 23.83; n- Dodecane = 25.35).

n -Heptane 20.14

n -Hexadecane 27.47

n -Hexane 18.43

n -Octane 21.62

Alcohols: The magnitude of γ changes by 23.7 − 22.1 = 1.6 mN/m per -CH 2 -
group. This is based upon the γ data of ethanol (22.1 mN/m) and propanol (23.7
mN/m). These observations indicate the molecular correlation between bulk forces
and surface forces (surface tension γ) for homologous series of substances.

2.7.1 e f f e c T o f T e m p e r a T u r e a n d p r e S S u r e o n
S u r f a c e T e n S I o n o f l I q u I d S
All natural processes are found to be dependent on the temperature and pressure
effects on any system under consideration. For example, oil reservoirs are generally
found under high temperature (ca. 100°C) and pressure (over 200 atm). Actually,
humans are aware of the great variations in both temperature (sun) and pressure
(earthquakes) with which natural phenomena surround the earth. Even the surface
of the earth itself comprises temperature variation of −50°C to +50°C. On the other
hand, the center mantle of the earth increases in temperature and pressure as one
goes from the surface to the center of the earth (about 5000 km). Surface tension is
related to the internal forces in the liquid (surface), and one must thus expect it to
bear a relationship to internal energy. Further, it is found that surface tension always
decreases with increasing temperature.
Surface tension, γ, is a quantity that can be measured accurately and applied in the
analyses of all kinds of surface phenomena. If a new surface is created, then, in the
case of a liquid, molecules from the bulk phase must move to the surface. The work
required to create the extra surface area, d A , will be given as

dG = γ d A (2.41)