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Liquid-gas and liquid-liquid interfaces 79

Classification of surfactants

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The hydrophilic part of the most effective soluble surfactants (e.g.

soaps,"synthetic detergents and dyestuffs) is often an ionic group.

Ions have a strong affinity for water owing to their electrostatic

attraction to the water dipoles and are capable of pulling fairly long

hydrocarbon chains into solution with them; for example, palmitic

acid, which is virtually un-ionised, is insoluble in water, whereas

sodium palmitate, which is almost completely ionised, is soluble

(especially above its Krafft temperature - see page 93).

It is possible to have non-ionic hydrophilic groups which also

exhibit a strong affinity for water; for example, the monomer units in

a poly (ethylene oxide) chain each show a modest affinity for water

and the sum effect of several of these units in the polymer chain is an

overall strong affinity for water.

Surfactants are classified as anipnic, cationic, non-ionic or ampho-

lytic according to the charge carried by the surface-active part of the

molecule. Some common examples are given in Table 4.2. In

addition, surfactants are often named in relation to their technological

application; hence names such as detergent, wetting agent, emulsifier

and dispersant,

Anionics are the most widely used surfactants on account of cost

and performance. Cationics are expensive, but their germicidal

action makes them useful for some applications. An advantage

enjoyed by non-ionics is that the lengths of both hydrophilic and

hydrophobic groups can be varied.

Rate of adsorption

The formation of an adsorbed surface layer is not an instantaneous
process but is governed by the rate of diffusion of the surfactant
through the solution to the interface. It might take several seconds
for a surfactant solution to attain its equilibrium surface tension,
especially if the solution is dilute and the solute molecules are large
and unsymmetrical. Much slower ageing effects have been reported,
but these are now known to be due to traces of impurities. The time
factor in adsorption can be demonstrated by measuring the surface
tensions of freshly formed surfaces by a dynamic method; for
example, the surface tensions of sodium oleate solutions measured by