Liquid-gas and liquid-liquid interfaces 87lowering the c.m.c. Micelles containing more than one surfactant
often form readily with a c.m.c. lower than any of the c.m.c.'s of
the pure constituents.
Organic molecules may influence c.m.c.'s at higher additive
concentrations by virtue of their influence on water structuring.
Sugars are structure-makers and as such cause a lowering of
c.m.c., whereas urea and formamide are structure-breakers and
their addition causes an increase in c.m.c.Structure of micellesMicellar theory has developed in a somewhat uncertain fashion and is
still in many respects open to discussion. Possible micelle structures
include the spherical, laminar and cylindrical arrangements illustrated
schematically in Figure 4.14. Living cells can be considered as
micellar-type arrangements with a vesicular structure.
Typically, micelles tend to be approximately spherical over a fairly
wide range of concentration above the c.m.c., but often there are
marked transitions to larger, non-spherical liquid-crystal structures at
high concentrations. Systems containing spherical micelles tend to
have low viscosities, whereas liquid-crystal phases tend to have high
viscosities. The free energies of transition between micellar phases
tend to be small and, consequently, the phase diagrams for these
systems tend to be quite complicated and sensitive to additives.
Some of the experimental evidence favouring the existence of
spherical, liquid-like micelles is summarised, as follows:
- Critical micelle concentrations depend almost entirely on the
nature of the lyophobic part of the surfactant. If micelle structure
involved some kind of crystal lattice arrangement, the nature of
the lyophilic head group would also be expected to be important. - The micelles of a given surfactant are approximately monodispersed
and their size depends predominantly on the nature of the
lyophobic part of the surfactant molecules. One would expect the
radius of spherical micelles to be slightly less than the length of
the constituent units; otherwise the hydrocarbon chains would be
considerably buckled or the micelle would have either a hole or
ionic groups in the centre. The radii of micelles calculated from
diffusion and light-scattering data support this expectation. For
straight-chain ionic surfactants the number of monomer units per