Triggerable Release from Polymer Structured Oils 115!

A P5 dodecane o/w emulsion (Φ

oil

= 0.70) containing the hydrophobic dye

Lumogen® Rosa 285 (0.1 wt. %) in the oil phase was produced, the laser diffraction

analysis of which is shown in Figure 4.15. The data were in good agreement with

previously produced emulsions (D(4,3) = 8.13 µm, span = 1.22) and was used in the

formation of a multi-layer EE (Figure 4.16).

Figure 4.15. Laser diffraction size distribution curve result of a P5 stabilised dodecane o/w

emulsion containing Lumogen® Rosa 285 (0.1 wt. %) in the oil phase.

When creating PSOs comprising two layers of EE, it was noticed that, if left

overnight, guest molecules could diffuse between layers, suggesting that PSOs could

potentially provide a controllable release mechanism for guest molecules. It is

thought that dehydration in an EE system containing hydrophobes may result in

triggered release due to the removal of the interstitial water layers. In order assess

hydrophobe diffusion, a layered PSO was allowed to dehydrate for 24 hours (Figure

4.16). The top layer was a P5 dodecane o/w emulsion containing hydrophobic dye

(Lumagen® Rosa 285, 0.1 wt. %) and the bottom layer was the same emulsion

containing no guest molecules. After 24 hours, the bottom layer had become orange

(Figure 4.16(b)), indicating some dye diffusion between layers had occurred.

Figure 4.16. Multi-layered monolith made up of a hydrophobic dye containing layer (top) and a

standard EE (bottom) (a) after production at t = 0 and (b) a PSO after 24 hours dehydration.

The bottom layer appears to contain Lumogen® Rosa 285. Scale bars represent 2.5 mm.

0"

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0.1"

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In contrast to Figure 4.16, if a layered monolith was kept hydrated for long periods

of time, for example by submersion in an acidic solution, no diffusion was observed.

(Figure 4.17).

Figure 4.17. Multi-layered monolith made up of a hydrophobic dye containing layer (top) and a

standard EE (bottom) (a) after production at t = 0 and (b) the same structure at t = 24 hours.

Sample was kept hydrated by submersion in acid solution. Scale bars represent 2.5 mm.

When water was removed from interstitial sites upon dehydration, it may become

possible for the hydrophobic dyes to diffuse though the polymer-polymer barrier into

another oil droplet, shown schematically in Figure 4.18.

Figure 4.18. Schematic of dehydration-triggered diffusion in a monodisperse o/w emulsion

system. Removal of water from interstitial sites allows diffusion of hydrophobes.

Before dehydration, the dye would also have to move through the continuous water

phase, meaning diffusion from the oil droplet was much more unfavourable. The

removal of most of this interstitial water means that not only will a larger surface

area of droplets be in contact with other droplets, but also that most of the space

between droplets will only consist of a polymer-polymer barrier. The loss of these

water barriers means that hydrophobes should now diffuse between droplets more

easily. This hypothesis was measured quantitatively by monitoring the dye release

kinetics of Oil Blue from a P5 dodecane EE into a surrounding dodecane reservoir

and comparing it to release from an analogous PSO using UV-Vis spectroscopy

(Figure 4.20). The laser diffraction size distribution curve for this Oil Blue

containing emulsion (Φ

oil

= 0.71) is shown in Figure 4.19, and is in good agreement

with previously produced emulsions (D

(4,3)

= 8.25 µm and span = 1.21).

Figure 4.19. Laser diffraction size distribution curve obtained for a P5 dodecane o/w emulsion

containing Oil Blue (0.1 wt. %) in the dispersed phase.

In contrast to biphasic EEs, which displayed no dye release over 3 hours, significant

release was observed from the single-phase PSOs. The interstitial water barrier

clearly inhibits hydrophobe inter-droplet diffusion throughout the structure, but no

interstitial sites are present at the surface of the structure to prevent dehydration from

the outer layer of droplets. Since no Oil Blue released into the surrounding dodecane

oil phase was measured, some water must be present on the surface of the monolith,

perhaps bound to the hydrophilic components of the polymeric surfactant. This water

layer appears to be sufficient to prevent the release of Oil Blue into a continuous

dodecane phase. Error bars in Figure 4.20 were calculated using the standard

deviation of three repeats of each measurement.

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Figure 4.20. Release curves of Oil Blue dye from a pre-loaded dodecane EE (open squares) and

a dodecane PSO prepared with P5 at a concentration of 2 w/v % (closed triangles) and 1 w/v %

(closed circles).

As can be seen in Figure 4.20, the release of Oil Blue from the dodecane PSO could

be accelerated by reducing the concentration of P5

used to stabilise the initial

emulsion droplets.This result confirms that the diffusion process is dependent on the

‘thickness’ of the polymer network at the oil-oil interface. Any excess polymer in the

water phase would also be deposited at the oil-oil interface upon dehydration.

Therefore, with increased amount of P5, these droplet boundaries may become

slightly thicker if more excess polymer is present in the water phase, potentially

leading to lower rate of diffusion of hydrophobes from droplets. Therefore, release

from these emulsion-based polymer-structured oils can be tuned by controlling the

degree of hydration, with further refinement permitted by simply altering the

concentration of polymer at the droplet interface.

Digital images of both P5 dodecane EEs and PSOs containing dye were

captured before and after they were placed into a dodecane continuous phase (Figure

4.21). The EE did not appear to have released any dye into the surrounding dodecane

reservoir after 2 hours (Figure 4.21(b)), in good agreement with dye release

measured by UV-Vis spectroscopy in Figure 4.20. In contrast, the PSO had clearly

released dye into the continuous oil phase after 2 hours (Figure 4.21(d)), again in

good agreement with the previous work. A PSO containing dye could also be

rehydrated to form an EE before addition to the dodecane reservoir and, dependent

on full rehydration, would also keep the hydrophobe encapsulated due to the return

of water to the EEs interstitial sites.

Figure 4.21. Digital images of (a) a dodecane EE containing oil blue, (b) the EE in dodecane with

no dye release visible after 2 hours. (c) A dodecane PSO containing Oil Blue, (d) The PSO in

dodecane after 2 hours. The surrounding dodecane appears to contain Oil Blue that has diffused

from the PSO. Scale bars represent 2.5 mm.

In document Engineered emulsions, polymer structured oils and responsive polymer nanoparticles via polymer design and emulsion templating (Page 126-131)