small temperature range all droplets still devoid of crystals then (partially)
crystallize. For triglyceride oils, this especially occurs if the surfactant used
has a long and saturated paraffin chain and these chains are closely packed
in the interface. For tristearate, surface nucleation temperatures of 1–20K
aboveThomhave been observed, according to surfactant type.
A curve as steep as shown in the figure is, however, not very common.
It needs a pure surfactant giving a fully packed interfacial layer, and it
works best at temperatures below the chain crystallization temperature (see
Section 10.3.1). Most surfactants are mixtures (or are at least impure), or the
concentration is too small to obtain a plateau value of the surface load. In
such cases, the interfacial layer may contain patches that are catalytic for
nucleation. This then means that the droplets contain catalytic impurities,
but their number would be proportional to droplet surface area rather than
volume. In the simplest case, Eq. (14.14) is to be modified by replacingvNcat
byNScatpd 532 =4, where the superscript S indicates that the number is per unit
area. The curves obtained then are rather similar to those for volume
nucleation, and it needs painstaking experiments to determine which of the
two mechanisms prevails.
Seed Crystals. Crystals inside droplets may occasionally stick out
of the surface over several nanometers. If such a drop encounters another
one by Brownian motion (see Section 13.2.1), the protruding crystal may
occasionally pierce the surface of that droplet. If the latter is still fully liquid,
the crystal may act as a seed and induce crystallization. This has been
observed to occur in emulsions of hexadecane in water, where part of the
drops were solid and part liquid (undercooled). It is a slow process, for
instance taking two weeks for completion. It has been calculated that about
one in 10^7 or 10^8 encounters was effective in such a case.
It may well be that the same mechanism can act in crystallizing
triglyceride emulsion droplets, but the author is unaware of clear evidence.
Question 1
An emulsion of milk fat in a Na–caseinate solution is made and the average droplet
size is determined atd 32 ¼1.8mm. The emulsion is cooled to 15 8 C and kept at that
temperature. What will roughly be the value ofymaxreached, and how much time
will it take to obtain values ofy¼0.1 and 0.9? If the milk fat contains no catalytic
impurities, to what temperature should the emulsion then be cooled to obtain the
same cooling times?