Chemistry Reference
In-Depth Information
size.
4,5
There is no doubt that smaller oil droplets can be produced;
6,7
however,
careful examination of the evidence shows that it is doubtful if any of these is an
'emulsion' in the sense of containing distinct, particulate, pure oil particles.
A further difficulty in interpreting evidence in the literature is the lack of
essential information on particle sizing techniques, such as the dilution method
used in the case of light scattering. Additionally the size characterization
methods most frequently used (static and dynamic light scattering) apply only
to dilute systems and can give misleading results. This is particularly true with
regard to polydisperse systems whose size covers the effective range of the two
techniques. A particular issue is the destabilization of the system by Ostwald
ripening due to a few large droplets undetectable by dynamic light scattering:
the large droplets grow at the expense of the small ones and then cream/
sediment out of the system, never to be detected, or their existence in the
detected size distribution is rejected as an artefact. Another issue is the disparity
between differential scanning calorimetry (DSC), ultrasound and density
determinations. For example, the DSC data of Montenegro et al.
8,9
are in
striking contrast to the solid content determination from ultrasound, density
and X-ray diffraction measurements by other workers, perhaps because of the
use of the highly hydrophobic perfluorohexane, which was added to prevent
Ostwald ripening of the emulsion, indicating that this nominal 218-nm emul-
sion was in fact far from monodisperse (size distribution not supplied). The
suppression of Ostwald ripening by incorporation of hydrophobic additives is
described by Lindfors et al.,
11
but the additive was oil soluble, and therefore
Montenegro et al.
8
were no longer studying pure n-hexadecane, but rather a
24:1 mixture of oil + perfluorohexane. There are also many disparities between
different sets of DSC data,
7-10
a testament to the huge impact that surfactants
and other non-oil components can have on crystallization processes.
We are interested in crystallization in 'nano-emulsions' because it offers a
route to large-scale production of small solid particles whose size, shape and
size distribution may be extremely well controlled by the properties of the initial
emulsion. Once a monodisperse emulsion can be produced and crystallized,
then perhaps we shall be able to produce a monodisperse sample of solid
particles. All that is necessary then is to reduce the aqueous phase volume by
osmotic dehydration, and large quantities of a concentrated nano-sized disper-
sion may be produced. For example, n-hexadecane may then be a useful and
low-cost source of hydrophobic solid particles, for later use as an emulsion
stabilizer
12
or foam stabilizer.
13
An additional reason for wanting to work with
monodisperse samples is that a high degree of emulsion stability can be
achieved with just three components in the system - pure oil, pure surfactant
and pure water.
27.2 Materials and Methods
Nano-emulsions may be produced by high-pressure homogenization, ultrasoni-
cation and, even under some circumstances, gentle stirring. In the work reported
Search WWH ::
Custom Search