Page 169 - Basic Principles of Textile Coloration

158 AUXILIARY CHEMICALS FOR WET PROCESSING AND DYEING

9.2.2 Surface activity and micelle formation

We will first examine how soaps function as detergents and emulsifying agents; the
more complex synthetic surfactants act in an identical manner.

When a small amount of a soap, such as sodium stearate, is added to water in a
beaker, the first step is dissociation into sodium and stearate ions. The latter then
accumulate at the water–air and water–glass boundaries. This is the phenomenon
of surface activity, but why does it occur? We know that the molecules have a
long, unbranched, hydrophobic alkyl chain that does not interact with water
molecules. In fact, the alkyl chain causes the surrounding water molecules to pull
back from it and aggregate together by hydrogen bonding, forming a structured
cage wall with the alkyl chain of the soap inside, minimising its contact with the
water. The soap is soluble because the terminal anionic carboxylate group is
hydrophilic and strongly solvated by polar water molecules. At the boundary
between water and a more hydrophobic surface such as glass, air or oil, the soap
molecules arrange themselves so that the long alkyl chain orients itself into the
hydrophobic surface away from the water, with the carboxylate ion remaining in
the water. A single layer of such oriented molecules occupies the surface between
the two phases.

The available surface between the two phases becomes saturated with soap
molecules at quite low soap concentrations and they then begin to accumulate in
the bulk of the water. This unfavourable situation, in which the hydrophobic alkyl
chains and their surrounding water cages are quite incompatible, is remedied by
the soap molecules accumulating together to form molecular aggregates called
micelles. The solution is colloidal since the micelles are large enough to scatter a
light ray. The hydrophobic alkyl chains come together in the centre of the micelle,
excluding any water molecules. In this way, the alkyl chains interact only with
each other and avoid contact with water molecules. The carboxylate ion ‘heads’ of
each soap molecule are at the outer surface of the micelle oriented into the water,
thus keeping it in solution (Figure 9.3). The micelle may incorporate a number of
sodium ions, but in smaller numbers than the carboxylate ions so that the micelle
is anionic overall.

Surface adsorption and micelle formation are governed by the laws of
thermodynamics. The ordering of the soap molecules at a phase boundary, or in a
micelle, represents the most stable state of these molecules in the solution. When
a soap molecule passes from the aqueous solution into a micelle, the structured
cage of hydrogen-bonded water molecules surrounding the alkyl chain collapses.
Thus, hydrogen bonds will be broken but the water molecules gain considerable