A modo de cierre

The effect of the oil phase volume (i.e. the percentage of oil in the emulsion) on the

stability of O/W emulsions adjusted to pH 2 was investigated. Emulsions were prepared

at three various oil/water ratios: 10/90, 20/80 and 30/70 with a silica particles

concentration constant at 2%. The emulsion stability was assessed by measuring the

Chapter 4. Silica Particle Dispersions and Stability of O/W Pickering emulsions

Figure 4-8: Effect of oil phase volume on the stability of O/W emulsion adjusted to pH 2 prepared with 2% Aerosil 200 – Average size D(4,3) as a function of O/W ratio and time.

As can be seen in Figure 4-8, for an oil/water ratio of 20/80 and above, the average

emulsion droplet size does not change after a month. For a ratio of 10/90, droplet size

was around 6.5 m after emulsification and around 11 m after 1 month, which represents a significant change. In first instance, emulsion stability seems to depend on

the O/W ratio. However, no oil released was observed at the top of emulsion after a

month. The change in the droplet size at low oil ratio may be explained by considering

the packing of oil droplets in the cream layer.

By decreasing the oil phase volume, the number of oil droplets formed during

emulsification is likely to decrease, which results in lowering the “naked” oil surface.

As a consequence, all the particles are not adsorbed at the interface, and some remain

“free” in the system. These particles are likely to form a network with the silica particles adsorbed at the O/W interface at quiescent conditions, as discussed in section 4.3.1.

Chapter 4. Silica Particle Dispersions and Stability of O/W Pickering emulsions

Over a month, uncharged silica particles (“free” and adsorbed) would aggregate. These silica flocs may affect the measurement of emulsion droplet size by light scattering,

which can explain the difference between the emulsion droplet size measured after

emulsification and after a month.

The O/W ratio also has an effect on the emulsion droplet size. As can be seen in Figure

4-8, the droplet size increases by increasing the concentration of oil in the system. As

mentioned earlier, the droplet size is determined by the balance between the break-up

and re-coalescence events taking place during the process. During emulsification, oil

droplets are formed and dispersed into the bulk phase. Increasing the O/W ratio results

in reducing the distance between oil droplets in the bulk phase. This induces higher re-

coalescence of oil droplets at higher O/W ratio. As the re-coalescence during

emulsification increases by increasing the oil concentration, the final emulsion droplets

size also tends to increase.

Optical observations revealed that the O/W ratio has an effect on the emulsion creaming

(see Figure 4-9). For ratios of 20/80 and 30/70, creaming occurs quickly after

emulsification and the aqueous phase is perfectly clear. The cream volume fraction

(after 1 month) of 30/70 emulsion (77%) is slightly higher than the cream volume

Chapter 4. Silica Particle Dispersions and Stability of O/W Pickering emulsions

Figure 4-9: Effect of the oil concentration on the O/W emulsions prepared at pH 2 with 2 wt/wt% Aerosil 200 – after 1 month.

At 10/90 ratio, phase separation between cream and aqueous phase also occurs, but

instead of creaming, the emulsion droplets tend to sediment, as can be seen in Figure 4-

9. The fact that emulsion droplets sediment at low ratio (10/90) can be explained by

considering the emulsion droplet density. The calculation of the droplet density (Table

4-3) is based on the hypothesis that silica particles form a uniform monolayer at the

droplet interface and all the particles have the same diameter. The volume weighted

diameter D(4,3) is taken as the average diameter of emulsion droplets. As can be seen in

Table 4-3, the average density of emulsion droplets prepared at an O/W ratio of 10/90 is

slightly higher than the water density (1057 kg/m3 > 998 kg/m3), while the average

density of emulsion droplets prepared at higher ratio (20/80 or 30/70) is higher than the

water density. This explains why emulsion droplets tend to sediment when made at an

Chapter 4. Silica Particle Dispersions and Stability of O/W Pickering emulsions

Table 4-3: Calculation of the emulsion droplet average density at 3 ratios, assuming that the silica particles form a uniform monolayer at the droplet interface.

Oil and Particles Properties

3 200 200 2000 / 0.15 A A kg m D m     3 915 / oil kg m   O/W Ratio 10/90 20/80 30/70 Emulsion droplet diameter (Ddroplet) (m) 6.5 14.8 22.3

Oil diameter (Doil) (m)

200

oil droplet A

D D D 6.2 14.5 22.0

Mass of silica particle layer (mA200) (kg)

3 3 200 200 4₃ ( ) A A droplet oil m     D D 3.04110 -13 1.61810-12 3.69910-12

Mass of oil in the droplet (moil) (kg)

3 4

3

oil oil oil

m     D 9.11510

-13

1.16610-11 4.07210-11

Volume of droplet (Vdroplet) (m3)

3 4 3 droplet droplet V    D 1.15010 -15 1.35810-14 4.64510-14

Droplet density (droplet) (kg/m3)

200 droplet A oil droplet droplet droplet m m m V V     1057 978 956