Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 42

between polar and non-polar regions, allowing enhanced wetting. Therefore, sur-
factants play a vital role in interface science.
The addition of surfactant can transform a non-wetting aqueous solution into a
wetting solution, even on hydrophobic substrates. The term wetting agent is applied
to any substance that increases the ability of water or an aqueous solution to wet a
solid surface. Altering the wetting ability of water is an important property shown
to some degree by all surfactants, although the extent to which they exhibit this phe-
nomenon varies greatly. Surfactants adsorb at relevant interfaces lowering the total
free energy of the system. It is often said that surfactants favour expansion of the
interface by lowering the surface tension of the liquid. Additionally, micellisation
takes place as an alternative mechanism for decreasing the interfacial energy of a
surfactant solution by arranging molecules into energetically favourable aggregates.
Here we introduce CMC (the c ritical m icelle c oncentration) as the main parameter
of a surfactant solution. Below CMC, surfactant molecules are freely solubilised in
the solution; above CMC, spontaneous aggregation takes place. Structure of surfac-
tant aggregates mostly depends on the nature and type of the surfactant, but also
on the properties of the solvent in which the surfactant is dispersed. Israelachvili
[2] showed that geometrical factors dictate the aggregate structure and introduced
the surfactant parameter, N S , (packing ratio) as the most convenient parameter for
prediction of the aggregate structure:

V
la o ,

N S = (3)

where V is the volume of the hydrophobic part (the tail), a o is the area of the sur-
factant head group and l is the length of the tail. Estimation of these geometrical
parameters may be challenging, since the area of the surfactant head and espe-
cially the length of the tail are influenced by the type of interactions with the
solvent molecules, surfactant concentration and other environmental factors [25].
Very small values of N S indicate spherical or ellipsoidal micelles; for values of N S
close to unity, vesicles and bilayer structures are expected. When N S is above 1,
inverted micelles are formed [26].
Pure water does not spread spontaneously over hydrophobic surfaces. On hy-
drophobic substrates, normally characterised by low surface energy, non-wetting
occurs with the finite contact angle, θ eq > 90 ◦ . However, surfactant solutions can
spread spontaneously on hydrophobic substrates and the spreading rate depends on
the concentration of the surfactant. In the very beginning of the spreading process,
when the surfactant solution is for the first time in contact with the hydrophobic
surface, the contact angle, θ , is bigger than 90 ◦ and the liquid cannot spread. Trans-
fer of surfactant molecules from the liquid onto all three interfaces may then take
place. Surfactant adsorption occurs at (i) the inner solid-liquid interface, which
results in a decrease of the solid-liquid interfacial tension, γ sl , (ii) the liquid-gas
interface, which results in a decrease of the liquid-air interfacial tension, γ lg ,and
(iii) transfer from the drop onto the solid-gas interface just in front of the drop.