Crystallization in Monodisperse Emulsions with Particles in Size Range 20–200 nm - Food Colloids: Self-Assembly and Material Science - page 411

other and with the other emulsions. First, in the case of the coarser emulsion
(d 32 ¼ 330 nm), crystallization begins at the melting point (Figure 8). Secondly,
the onset of melting is sudden rather than gradual. Finally, the finer emulsion
(d 32 ¼ 123 nm) shows some sort of transition on cooling, just above the melting
point, which is associated with a similar but mirror-image transition during the
melting (Figure 7).
27.5 Conclusions
Much more work is needed to clarify in detail the processes observed during
our experiments. In particular, it seems desirable to carry out X-ray scattering
alongside the sound velocity measurements to elucidate the different structures
at the surface and within the emulsion particles. We have strong evidence, at
least in the case of the Caflon-stabilized emulsions, that the emulsifier is itself
nucleating crystallization. In the case of the PGE emulsion, perhaps both
nucleation and crystal growth suppression occur. Also the PGE appears to
penetrate much more deeply into the bulk of the particle than does either of the
two other surfactants. In the case of Tween 20, penetration is little more than
one monolayer, as is probably also the case with Caflon.
It is clear from this study that classical nucleation theory requires revision to
account for the complex behaviour seen in nano-emulsions. Perhaps this is
related to our failure to produce a monodisperse emulsion with mean particle
size below 100 nm using standard high-pressure homogenization. One route to
smaller sizes is via the so-called 'low-energy' emulsification methods, 27 in which
the reduction of surface energy is likely to have a profound impact on the
crystallization process. An alternative route could be via oil-in-water micro-
emulsions.
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