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More importantly, coalescence was expected to lead to the growth of disper-
sion droplets, which was not observed in the measurement of ISAsome size
by DLS; on the contrary, ISAsome size remained practically constant. These
experiments were performed with a three-dimensional DLS setup in order to
suppress multiple scattering in the turbid dispersions. Another important
observation regarding the stability of ISAsomes to coalescence was that there
was no signifi cant infl uence of the concentration of the dispersed phase
(hydrophobic material containing lipid or lipid
oil) on the transfer rates, as
shown in Figure 6.8c. The transport of oil and/or MLO among the ISAsomes
is presumed to be supported by surfactant (F-127) micelles. The linear increase
in rate constant with increasing concentration of added F-127 (Fig. 6.8d) is
direct evidence for this fact.
The mechanism of material transfer in ISAsome systems appears to be very
similar to Ostwald ripening (Kabalnov, 2001; Taylor, 2003). However, due to
comparable sizes of individual ISAsomes and relatively low surface tensions,
the entropy of mixing becomes the main driving force. This is a characteristic
driving force for so-called compositional ripening and is usually two to three
orders of magnitude larger than that typically observed for Ostwald ripening
(Taisne et al., 1996).
The question that now arises is: What is being transferred, oil or MLO, or
both? Mixtures of two different systems—one with no oil (i.e., plain cubo-
somes;
+
0) —
were used (Salonen et al., 2010a). The resulting phases thus have the average
composition of an EME (
δ
=
100) and the other with no MLO (i.e., alkane emulsion;
δ
=
50). The kinetics of formation of the EME were
measured with the established protocol discussed above. However, in this case,
it was more important to monitor (using DLS) the changes in respective
droplet size. This was done for two different sets of experiments as follows:
In the fi rst set of experiments, the radius of initial cubosomes was kept
constant at
δ
=
110 nm, while the initial emulsion-droplet radius of the different
oils was varied. The radius of the resulting EMEs (after the transfer process)
was plotted against the droplet radius of the initial oil emulsions, as shown in
Figure 6.9 a.
In the second set of experiments, the size of the initial cubosome was varied,
while the initial emulsion droplet size was kept constant for the different
alkanes (110 nm for decane and 140 nm for tetradecane and octadecane). The
radius of the resulting EMEs was plotted against cubosome radius, as shown
in Figure 6.9b.
The rates of increase in the radius of EME droplets (after equilibration)
for three different oils were obtained from a linear fi t to the data, as shown in
Figures 6.9a and 6.9b. These rates were plotted as a function of alkane chain
length, as shown in Figure 6.10, which sheds light on the transfer mechanism.
In the case of shorter C-chain oils, such as decane, the transfer of oil to cubo-
somes was found to be much faster than the transfer of monoglycerides to the
emulsion droplets, but for less soluble oils the trend was reversed. These rates
are equal at the crossover point (see Fig. 6.10) where the monoglyceride (18
∼
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