Liquid-gas and liquid-liquid interfaces 85
McBain pointed out that this seemingly anomalous behaviour
could be explained in terms of organised aggregates, or micelles, of
the surfactant ions in which the lipophilic hydrocarbon chains are
orientated towards the interior of the micelle, leaving the hydrophilic
groups in contact with the aqueous medium. The concentration
above which micelle formation becomes appreciable is termed the
critical micelle concentration (c.m.c.).
Micellisation is, therefore, an alternative mechanism to adsorption
by which the interfacial energy of a surfactant solution might
decrease.
When one considers the energetics of micellisation in terms of the
hydrocarbon chains of the surfactant molecules, the following factors
are among those which must be taken into account:- The intermolecular attractions between the hydrocarbon chains in
the interior of the micelle represent an energetically favourable
situation; but it is not one which is significantly more favourable
than that which results from the alternative hydrocarbon-water
attraction in the case of single dissolved surfactant molecules.
Comparison of the surface tension of a typical hydrocarbon oil
with the dispersion component of the surface tension of water (as
discussed on page 67) illustrates this point. - Micellisation permits strong water-water interaction (hydrogen
bonding) which would otherwise be prevented if the surfactant
was in solution as single molecules wedged between the solvent
water molecules. This is a most important factor in micelle
formation and also of course, in any adsorption process at an
aqueous interface. It is often referred to as the hydrophobic
effect^,
Experimental study of micellesCritical micelle concentrations can be determined by measuring any
micelle-influenced physical property as a function of surfactant
concentration. In practice, surface tension, electrical conductivity
and dye solubilisation measurements (see Figure 4.13 and page 90)
are the most popular. The choice of physical property will slightly
influence the measured c.m.c., as will the procedure adopted to
determine the point of discontinuity.
Information concerning the sizes and shapes of micelles can be