Chemistry Reference
In-Depth Information
here, emulsions were produced by repeated high-pressure homogenization
involving up to 24 passes through the homogenizer. Particle-size polydispersity
was minimized by continually recreating new surface until as much of the
surfactant as possible had moved to the interface and the surfactant concen-
tration in the bulk was reduced to a minimum. Hydrodynamic stresses within
the homogenization process produce a driving force towards a single-sized
particle. However, this can take some time to achieve because surface area is
proportional to diameter squared and the required energy input is at a minimum
proportional to the new surface area produced. So halving the diameter requires
at least four times the passes needed to get to the starting point!
We have used three different surfactants and have produced nearly identical
size distributions (d
32
B
130 nm) with all three: 2 wt.% Tween 20 (Sigma, UK),
1 wt.% Caflon phc060 (ethyl alcoxylate, Uniqema UK) and 20 wt.% polyglyc-
erol ester PGE L-7D (decaglycyl monolaurate, Sakamoto Pharmaceuticals,
Osaka, Japan). A coarser emulsion (330 nm) was generated using Caflon as
emulsifier by increasing the oil/surfactant ratio by 4 (see Table 1). We have also
studied the effects of varying surfactant concentration and number of passes
through the homogenizer in the case of Caflon. We have used a Shield
homogenizer which is described elsewhere.
20
The emulsions were characterized
in terms of (a) three different small amphiphilic surfactants that are quite
different chemically and (b) two different mean sizes, d
32
¼
130 and 330 nm.
In the case of the surfactants, Tween 20 produces micelles of around 15 nm in
diameter, which can be measured by dynamic light scattering or by ultrasound
scattering. On the other hand, Caflon phc060 micelles could not be detected by
either techniques, and it was chosen because it is a much smaller molecule than
Tween 20 and a purer chemical substance. PGE was chosen to provide a
comparison with the work of Higami et al.
9
The emulsification principle was to pass the emulsion repeatedly through
the homogenizer until the size distribution stopped changing, and to control the
mean size by varying the oil/surfactant ratio. The idea was to reduce the
surfactant level in the continuous phase (solvent) to a minimum value and
continually create new surface until finally a monodisperse emulsion was
produced. In all probability the distributions were much narrower. At least
10 passes through the homogenizer were required in all cases.
In detail the preparation steps were as follows. First, it was ensured that both
the oil phase and the emulsifier were completely melted to avoid the blockage of
the homogenizer and loss of materials. Surfactant was weighed out and made
up with water to 95 g. The mixture was stirred at a high enough temperature
(601C) to ensure that all components were liquid. The oil phase was weighed
(5 or 20 g) and added gradually to the prepared aqueous phase. The mixture
was homogenized at 601C for 30 min, making 10 separate passes into an empty
container while removing the air bubbles after each pass by creaming. The
particle-size distribution was measured using a Malvern Mastersizer 2000 static
light-scattering apparatus (Malvern, UK). Each sample was degassed in an
ultrasonic bath for 10 s, and then loaded into the sound velocity measurement
(UVM) cell, stirring the sample using a magnetic stirrer, and removing air
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