Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 42

5.0

80

NaCl

70

Butyric Acid

0 0.5

Mole/Liter

FIGure 3.1 Change in surface tension of water as a function of added solutes (inorganic
salts such as NaCl, surface-active agents such as butyric acid).

80

40

0 200

Mmol/Liter

FIGure 3.2 Surface tension plot of n -butanol solutions.

The surface tension of water increases from 72 to 73 mN/m when 1 M NaCl is
added. On the other hand, the magnitude of surface tension decreases from 72 to 39
mN/m when only 0.008 M (0.008 M × 288 = 2.3 g/L) SDS (mol. wt. = 288) is dis-
solved. It thus becomes obvious, that in all those systems in which surface tension
plays an important role, the additives will become significant. The data for n -butanol
solutions in water are shown to decrease from 72 mN/m (pure water) to 50 mN/M in
200 mmol/L (Figure 3.2).
The γ of water changes slowly as compared to detergent solutions. Methanol-
water mixtures gave the following γ data (at 20°C):

%w Methanol 0 10 25 50 80 90 100

72 59 46 35 27 25 22.7

γ