Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 61

water (w) plus SDS( s ) (2 g/L):
ssswssssssswsssssssssssssswsssswssssssssssws
sssswssswwwwswwwsssswwwwwwwwwww
sssswssswwwwswwwsssswwwwwwwwwww
This shows that the surface tension of pure water of 72 mN/m decreases to 30 mN/m
by the addition of 2 g/L of SDS. Thus, the surface of the SDS solution is mostly a
monolayer of SDS plus some bound water. The ratio of water:SDS in the system is
roughly as follows:
In the bulk phase: 55 mol water: 8 mmol SDS
At the surface: roughly 100 mol SDS: 1 mol water
This description is in accord with the decrease in γ of the system.
Investigations have shown that, if one carefully sucked a small amount of the
surface solution of a surfactant, then one can estimate the magnitude of Γ. The con-
centration of the surface-active substance was found to be 8 μmol/mL. The concen-
tration in the bulk phase was 4 μmol/L. The data show that the surface excess is 8
μmol/mL − 4 μmol/mL = 4 μmol/mL. Further, this indicates that, when there is 8
μmol/L in the bulk of the solution, the SDS molecules completely cover the surface.
The consequence of this is that, at a concentration higher than 8 μmol/L, no more
adsorption at the interface of SDS takes place. Thus, γ remains constant (almost).
This means that the surface is completely covered with SDS molecules. The area-
per-molecule data (as found to be 50 Å 2 ) indicates that the SDS molecules are ori-
ented with the SO4− groups pointing toward the water phase, while the alkyl chains
are oriented away from the water phase.
This means if one, through foam bubbles, collected the foam continuously, then
more and more surface-active substances will be removed. Such a method of bubble
foam separation has been used to purify wastewater from surface-active substances.
It is especially useful when very minute amounts of surface-active substances (dyes
in the printing industry; pollutants in wastewater). The method is economical to use
and is free of any chemicals or filters. In fact, if the pollutant is very expensive or
poisonous, then this method can have many advantages over the other methods.
It is useful to consider the amount of SDS in a bubble of radius 1 cm. Assuming
that there is almost no water in the bilayer of the bubble, the surface area of the
bubble can be used to estimate the amount of SDS. We can use the following data:
Radius of bubble = 1 cm
Surface area = (4 Π 1 2 ) 2 = 25 cm 2 = 25 10 16 Å 2
Area per SDS molecule = 55 Å 2
Number of SDS molecules per bubble = 0.5 10 16 molecules
Amount of SDS per bubble = 0.5 10 16 /6 10 23 g
= 0.01 μg SDS
It can thus be seen that it would require 100 million bubbles to remove 1 g of
SDS from the solution. Since bubbles can be easily produced at very fast rates (ca.