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identical) and increases monotonically with increasing temperature.
Whereas the interfacial tension �bc decreases
(monotonically) with increasing temperature and vanishes
at Tu, because the two phases (c) and (b) become identical
there. This opposite temperature dependence of �ac and
�bc results in a minimum in their sum, �ac + �bc. In order to
assure the stability of the water/oil interface �ab � �ac + �bc
must hold. Otherwise a thin layer of the middle-phase
would penetrate between the water- and oil-rich excess phase.
Therefore �ab has to pass through a minimum within the
three phase region, i.e. being more precisely at the phase
inversion temperature.
In conclusion, at the PIT the minimum in the water/oil interfacial
tension enables the optimal solubilisation of water and
oil, i.e. with the minimum amount of surfactant �. That this
correlation between phase behaviour and interfacial tensions
holds also for technical applications was shown by the removal
of hexadecane from synthetic tissue, which reaches a
maximum within the three phase region [41,42]. The value
of the water/oil-interfacial tension �ab can be estimated
considering the simple argument from Volmer [43,44] that
thermal energy will lead to dispersion as soon as the interfacial
tension times the area of the object becomes smaller
than kT, i.e. �abr 2 ≈ kT.
2.2 MICROSTRUCTURE
Most of the recent applications of microemulsions depend
on the fact that microemulsions, although macroscopically
homogeneous, are heterogeneous on the sub-microscopically
scale. Topologically ordered interfacial fi lms are formed
by the surfactant molecules which are forced into the microscopic
water/oil interface because of their amphiphilicity.
The nature and properties of these microscopic interfacial
fi lms are essential for the formations of microemulsions
and in particular for the most interesting feature of microemulsions,
i.e. their microstructure. 50 years ago, Winsor
[2] and Schulman [45] suggested that microemulsions
are always spherical exhibit microstructures, and that a
layered, lamellar structure exists as the exception in the
middle phase. In 1976 Scriven [46] put forward the crucial
idea of the bicontinuous structure of the surfactantrich
middle phase, which 10 years later was proven with
the help of NMR self-diffusion measurements [25,26] and
the direct visualization by freeze fracture electron microscopy
(FFEM) [25,27]. Further studies of the micro-structure
by NMR-self diffusion, transmission electron microscopy
(TEM) and scattering techniques (SAXS and SANS) proved
structures droplet- and wormlike micro, sample spanning
networks and bicontinuous structures. Furthermore, liquid
crystalline meso-phases such as the cubic (V), hexagonal
(H) and lamellar phases (L� ) exist and compete with these
complex fl uids. It has been realized that the main parameter
determining the microstructure is the local curvature of the
amphiphilic interfacial fi lm. Thus, controlling the curvature
is the ultimate goal for choosing any desired structure.
Strey has studied the variation of the microstructure as
function of temperature exemplary and in large detail for
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BUNSEN-MAGAZIN · 8. JAHRGANG · 6/2006
the ternary nonionic system H2O – n-octane – C12E5 [30].
He used both freeze fracture electron microscopy (FFEM)
and small angle neutron scattering (SANS) to characterize
the microstructure. Both methods complement each other:
While FFEM yields the shape of the structure, statistical
information about frequently occurring distances in these
micro-structured systems are obtained by SANS.
Figure 2: Temperature dependence of the characteristic length scale �
for the systems H2O – n-octane – C12E5 together with freeze fracture
electron microscopy (FFEM) pictures. The �(T)-curve is shaped and runs
through a maximum at the phase inversion temperature PIT, where the
microstructure is found to be bicontinuous. At temperatures below and
above the three phase region the optimal microstructure consists of
spherical oil-in-water (o/w) and water-in-oil (w/o) droplets, respectively.
Fig. 2 shows the variation of the length scale � of the microstructure
as a function of temperature. The measurements
were are performed at a composition where the structures
are maximally swollen maximal with water, oil or both, i.e.
~
at the emulsifi cation failure boundaries or point �, respectively
[31]. Starting at low temperatures the microstructure
consists of spherical oil-in-water (o/w) droplets.
The FFEM picture [47] was taken at T = 26.1°C (by cooling
microemulsion rapidly (>10000 K/s) to liquid nitrogen
temperature T = -196°C) and yields a mean radius
= � = 12 ± 5 nm, which is in almost quantitative
agreement with the radius determined by SANS. With increasing
temperature, the length scale increases in size
and runs through a maximum at the phase inversion temperature
(T=32.6 °C). The bicontinuity of the structure is
nicely confi rmed by the FFEM-picture [48] which shows
water- and oil-rich domains that are mutually intertwined
in a sponge-like fashion. The decoration of the oil domains
stems from the shadowing of the fractured surfaces with
tantalum (Ta) -tungsten (W). By increasing the temperature
further the length scale of the microstructure decreases.
The FFEM picture [47] taken at T = 36.3°C shows that water
droplets in the continuous oil phase are formed.
The trend observed with increasing temperature from oil-inwater
(o/w) to water-in-oil (w/o) structures via the bicontinuous
structure is due to the gradually change of the
curvature of the amphiphilic fi lm. At low temperatures the
size of the surfactant head group is larger than that of the