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bubbles on the sample surface by a pipette. Temperature was controlled by
immersing the UVM cell in a thermostatic bath (Grant, UK). Sound velocity
measurements had begun once the temperature inside the cell was at 351C.
More details regarding the sound velocity measurement and the solid content
determination can be found elsewhere.
14,15
(The emulsion preparation method
is given in detail here because it was found to be essential to follow this
procedure to obtain repeatable results.)
We define a sample as 'monodisperse' when it was stable to repeated temper-
ature cycling between 0 and 351C, where during each cycle the dispersed phase
was frozen and thawed. We define 'stability' as the precise reproduction of the
sound velocity versus temperature curve from cycle to cycle, without any change
in apparent particle size between the beginning and end of the experiment.
27.3 Crystal Nucleation Theory
The classical theory of crystal nucleation in lipid systems has been presented in
detail elsewhere;
16,17
it will not be repeated here. The concept behind our
experiments was to reduce particle size progressively so that volume homoge-
neous nucleation and volume heterogeneous nucleation were completely
suppressed, and either the particles did not crystallize at all or there was
surfactant-initiated surface heterogeneous nucleation. It is known that the
lauric acid moieties in Tween 20 dissolve into the lipid phase and can nucleate
crystallization in the bulk and there is strong evidence that they do this too in
cocoa butter and mineral oils.
18-20
Choice of surfactants was also governed by a
desire to use different chemical groups (lauric acid in PGE and Tween 20, and a
short methylene chain,
C
10
, in the case of Caflon) to influence crystal
nucleation. In an earlier experiment
19
we also used sodium caseinate.
It has been suggested
21
that critical fluctuations at the particle interface may
play an important role in nucleation. This means that, as particle size is reduced,
volume homogeneous nucleation may once again come to play an important
part in the crystallization. It is important to recognize that, as the particle size
decreases, so the layer of molecules which is in contact with the surface - and
which therefore is influenced by it - contributes an increasing proportion of the
total volume of the dispersed phase (see Figure 1). As particle size decreases, a
reduction in the melting point and the enthalpy change on fusion may also be
expected.
22
There are many potential factors influencing crystal nucleation in particles:
diffusion rates, initial reactant distribution, interdroplet collisions, intermicel-
lar exchange of reactants, chemical reactions, Ostwald ripening, the particle-
size distribution, autocatalysis, surfactant film flexibility and the critical nucleus
size. All of these were accounted for in theoretical modelling work by Tojo and
co-workers.
23
One can easily add the following factors: surfactant concentra-
tion, type and organization; the application of ultrasound; thermal history;
polymorphism (in n-hexadecane, as used in our experiments, a rotator-triclinic
transformation has been reported
9
); solubility of surfactant in the oil; the
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