Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 39

3 Surfactants (Soaps

and Detergents) and
Physicochemical
Properties

3.1 IntroductIon
As known from experience, any physical property of a liquid will change when
a substance (called solute ) is dissolved in it. Of course, the change may be small
or large, depending on the concentration of the solute and other parameters.
Accordingly, the magnitude of surface tension of a liquid will change when a
solute is dissolved in it. This is to be expected from physicochemical consider-
ations. It also becomes apparent that, if one could manipulate the surface ten-
sion of water, then many application areas would be affected drastically. In this
chapter, some important surface-active substances having such properties will
be described. These are specific substances that are used to change the surface
tension of water in order to apply this characteristic to some useful purpose in
everyday life.
The magnitude of surface tension change will depend on the concentration and
the solute added. In some cases, the surface tension (γ) of the solution (such as NaCl)
increases. The change in γ may be small (per mole added) (as in the case of inorganic
salts) or large (as in the case of such molecules as ethanol or other soap-like mol-
ecules) with the addition of solute (equal gram per liter):

Inorganic salt: Minor change (increase) in γ
Ethanol or similar: Small change (decrease) in γ
Soap or similar: Large change (decrease) in γ

The following are typical surface tension data of different solutions:

Surface tension(mN/m) 72 50 40 30 22

Surfactant (C 12 H 25 SO 4 Na) 0 0.0008 0.003 0.008 —

Ethanol 0 10 20 40 100

This shows that, to reduce the value of γ of water from 72 to 30 mN/m, one would
need to add 0.005 moles of sodium dodecyl sulfate (SDS) or 40% ethanol. Of course,

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