Skoulios and ^uillon (54) propose that critical phenomena may account for the stability of micro
emulsions in some regions of their occurrence on the phase diagram. They consider that tricritical or other polycritical points can occur in systems of water, oil and emulsifier. In the neighbourhood of such a point the state of the system is governed by the competition between two order parameters. For microemulsions the authors refer to these as micellar ordering and osmotic demixing. There are local fluctuations between micellar and emulsion structures and if the rate of emulsion
coalescence is slow and the rate of micelle formation rapid, under the influence of these order parameters then
the system can behave as though it is stable, whilst constantly renewing itself. It is possible that this dynamic stability may exert an influence over an
extended region of the phase diagram.
It has been suggested (50,55) that metastable states may occur in parts of the rnicroemulsion regions shown on phase diagrams, in the vicinity of phase separation areas. Such states would be analogous to supercooled liquids, where nucleation can take an increasingly long time as the critical point is reached.
Rosoff and Giniger (50) have studied systems con taining water, sodium dodecyl sulphate and pentanol with
1,4.8. Cont1d .
hexadecane and benzene. In samples containing hexa decane it was found that it was possible to mix two solutions, giving the required final composition, which did not clear on vigorous shaking but only on mild
sonication. Heating such a "microemulsion" caused
turbitity but on cooling the system cleared again shovr- ing reversibility. They concluded that m etastable
states were involved since the expected transition did not always take place. They suggested that a liquid crystalline phase might be involved, producing a kinetic barrier to the formation of the W/0 micelles which is not overcome by ordinary mixing.
Ahmad, Shinoda and Friberg (56) found certain "solutions" which appeared to be microemulsions in the sodium dodecyl sulphate pentanol, water, benzene system. These "solutions" occurred near the boundary of a
lamellar liquid crystalline phase. Separation was achieved by ultracentrifiguration at 30,000g for lhr. The bulk birefringence of these "solutions" is not mentioned.
1-5 PHYSICAL PROPERTIES AND PHYSICAL METHODS OF INVESTIGATION
1.5,1
LIGHT SCATTERINGMicroemulsions are normally translucent, scattering light in the Tyndall range, appearing bluish by re
flected light and yellowish when the transmitted light is observed. They may also be transparent or turbid depending on the precise definition of microemulsion which is assumed.
Light scattering depends on:
1. The particle size: as this decreases systems go
from turbid to translucent to clear.
2 . the difference in refractive index of the con
tinuous and disperse phases
3 . the droplet size distribution: the scattering
by a few large droplets may . mask that due to many smaller ones
4. particle shape
Droplet size distribution is not usually important in microemulsion systems since these systems are not likely to exhibit a high degree of polydispersity.
Where the droplets or aggregates are extremely uniform in size, colours other than blue and yellow can be seen when the sample is viewed from different angles to the incident beam of light (37)• The spectral colours are purer for a higher degree of monodispersity.
1.5-1. Cont1d.
Light scattering can be used to determine particle size and also to give information about particle shape. Ballaro et al (58) used it to show that particles in their four component system (using a surfactant plus a cosurfactant) were spherical in the microemulsion and
emulsion regions but rods and possibly lamellar in the intervening liquid crystalline regions. The micro
emulsion droplet size changed on traversing the regions; for both types of micro emulsion it increased on
approaching the liquid crystalline phase. 1 .5.2. VISCOSITY
When dispersed aggregates are other than spherical they offer more resistance to flow than when they are spherical, so that the viscosity of the system is
greater. In systems in which microemulsions occur liquid crystals often occur also. These exhibit a high
viscosity; a large increase in viscosity as composition or temperature is changed is usually one indication of the formation of a liquid crystalline phase. Falco et al
(59 ) measured viscosities in the system potassium oleate, hexanol, water and hexadecane as a function of the water to oil phase volume ratio. They found low viscosity W/0 and 0/W microemulsions separated by a high viscosity
liquid crystalline region. The viscosity of the F/0 microemulsion measured as a function of increasing water content indicated spherical aggregates which then changed to cylindrical aggregates as the liquid crystalline phase
1.5.2. Cont’d.
was approached. It would appear from their evidence that formation of cylindrical aggregates precedes the actual formation of the liquid crystalline phase.
Kumar and Balasubramanian (60) used viscosity measurements in their studies of Triton X-100,
alcohol-water-cyclohexane systems (section 1.5.6). They found systems of spherical reverse micelles exhibiting Newtonian viscosity. For a surfactant: alcohol ratio of 4:1 on adding water to the initial reverse micellar system they obtained an optically anisotropic phase in which there was a considerable rise in the relative viscosity as the water content was increased and which exhibited thi xotropy. The viscosity increased still further on adding more water to obtain macroemulsions.
Lalanne et al (52) studied the viscosity in the system sodium dodecyl sulphate, n-butanol, water and toluene and considered their results in conjunction with
ntnr spin-lattice relaxation time (T., ) measurements (Fig.1.8). They found evidence for spherical micelles of 0/W and
also of W/0 ; these regions (£,H) exhibited low vis
cosity but a high ’’microviscosity” (their interpretation of I j ) for the disperse phase and a low viscosity for the continuous phase. Between these regions, region I
OH
Ttlvenc
Fig,l,8*The sodium dodecyl sulphate - n-hutanol -water -toluene system with lines of isoviscosity, and the maximum viscosities in zones I and L indicated, (from ref, 52)*
1.5-2. Cont1d .
exhibited a low "microviscosity" for both oil and water but the highest macroscopic viscosity. They suggested that in this region there was a progressive phase in version from water continuous to oil-continuous. Their suggestion for the structure, admittedly styelised and hence "not satisfactory”, was of networks of micelles of oil and water overlapping and separated by sur factant. Region L close to the surfactant corner of the phase diagram showed a high "microviscosity" for both oil and water and a high macroscopic viscosity. They suggested that this was compatible with water in a "pseudo oil” consisting mainly of surfactant and co surfactant with a small amount of oil.
Region J was intermediate between regions I and L in its properties.
1.5-3- ULTRA c e n t r i f u g a t i o n m e a s u r e m e n t s
The ultracentrifuge has been used to investigate microemulsions. Smith et al (6l) used it to look at
samples from the ternary system 2-propanol, water and hexane. Microemulsion samples showed some sedimentation after ultracentrifugation but returned to homogenity on standing. Other samples which were not regarded as microemulsions showed no sedimentation. They also found some samples which were unstable to the high centrifugal forces and classified these are metastable micro
emulsions •
1.5.3. Cont'd.
Hwan et al (62) used ultracentrifugation to investigate phase continuity and drop size in micro emulsions containing synthetic petroleum sulphonate surfactants, an alcohol cosurfactant, sodium chloride brine and an oil.
Dvolaitzky et al (
63
) used ultracentrifugation inconj-.unction with neutron scattering in their studies of water in cyclohexane microemulsions produced using sodium dodecyl sulphate and 1-pentanol. The volume fraction of the dispersed phase was very low (1-4%) wThich is much lower than in many microemulsion systems.