The above W/O emulsions were added to 0.5% NaCN solution to form the sequential W/O/W emulsion. The properties of these W/O/W emulsions were studied to further determine the effect of PGPR concentration in the oil phase.
4.3.2.1. Droplet size distribution
The droplet size distribution of W/O/W emulsions prepared with varying range of concentrations of PGPR (between 1 and 8% w/v) and NaCN (0.5% w/v) in the external aqueous phase displayed a bimodal distribution (Fig. 4. 3). Generally, the size of emulsion droplets could be classified into two ranges: 1: diameter < 1.23 µm and 2: diameter > 1.23 µm and < 50 µm. PGPR concentration (between 1 and 8% w/v) had no significant effect on the droplet size distributions when 0.5% (w/v) NaCN was used in the external aqueous phase (p < 0.05).
Chapter 4: Effect of PGPR and macromolecular materials on the formation and stability of W/O/W emulsion 0 1 2 3 4 5 6 7 8 9 10 0 1 10 100 Particle Diameter (µµµµm) V o lu m e ( % )
Fig. 4.3. Droplet size distributions of W/O/W emulsions prepared with distilled water in the internal aqueous phase, 0.5% (w/v) NaCN in the external phase and 1% (), 2% () and 8% () (w/v) PGPR in the oil phase. Emulsions were prepared using high-pressure homogenizer.
It appears that PGPR has an effect on the mean droplet size of W/O emulsions, but it does not have a significant effect on the droplet size distribution of W/O/W emulsions measured by the MasterSizer technique. As the oil droplets in W/O/W emulsion are significantly larger than water droplets inside them, changes to the oil droplets caused by entrapped water droplets migrating from inside the oil droplet to the aqueous continuous phase, disruption of the water droplets and coalescence inside the oil droplet may not cause detectable changes on the whole population of oil droplets as detected by the MasterSizer.
4.3.2.2. EE of W/O/W emulsion
The EE values of poly R- 478 dye (initially and after 4 weeks storage at 5 or 20 °C) of W/O/W emulsions prepared with different concentrations of PGPR (between 1 and 4% w/v) and NaCN (0.5% w/v) in the external phase are presented in Fig 4.4. The
Chapter 4: Effect of PGPR and macromolecular materials on the formation and stability of W/O/W emulsion
initial EE value was greatly influenced by the PGPR concentration in the oil phase. An EE value of > 90% could be achieved at a concentration of 2% PGPR (w/v) in the oil phase. At PGPR concentrations of 4% (w/v), EEs of almost 100% were achieved. PGPR was not tested at higher concentrations as the results indicated that 4% (w/v) PGPR was sufficient to successfully encapsulate virtually all the poly R-478 dye. Reducing PGPR concentration from 4 to 0.5% (w/v) caused a significant decrease in EE values (p<0.05). For example, at 0.5% (w/v) PGPR concentration, an EE value of 38.3% was obtained, indicating that greater than 60% of the aqueous phase of the original primary emulsion was exposed to, and mixed with, the external aqueous phase during the second emulsification stage.
Although W/O emulsion prepared with 0.5% (w/v) PGPR do not show phase separation, the sequential W/O/W emulsions had very low EE. The intense mechanical stresses that W/O emulsions went through during the second homogenisation step have the potential for disrupting the W/O emulsion droplets (Surh, Vladisavljevic, Mun and McClements, 2007). PGPR at low concentrations (0.5% w/v) was not sufficient to resist the rupture force during the homogenization of the sequential W/O/W emulsions thereby significantly reducing the EE value of W/O/W emulsions. The results provide evidence that concentration of PGPR is critical to the EE and stability of W/O/W emulsions. The primary contribution of PGPR might be to allow the stabilisation of a stable interfacial area thereby resulting in reduced water droplet size during the formation of the primary W/O emulsion and hence a reduced rate of coalescence. In addition the increase in viscosity of the W/O emulsions as PGPR concentration increased could also have resulted in increased resistance to the effects of shearing during the secondary homogenisation stage and subsequently reduced the rate of coalescence of the water droplets.
Chapter 4: Effect of PGPR and macromolecular materials on the formation and stability of W/O/W emulsion 0 20 40 60 80 100 0.5 1 2 4
Concentration of PGPR in oil phase (%)
E n c a p s u la ti o n e ff ic ie n c y ( % )
Fig. 4.4. Effect of different concentrations of PGPR in the oil phase on the encapsulation efficiencies of W/O/W emulsions. Blank bar: freshly prepared W/O/W emulsions; Dotted bar: same emulsions after storage for 4 weeks at 20
°°°°C. External aqueous phase contained 0.5% (w/v) NaCN. Measurements were performed at least twice on duplicate emulsions, error bars represent standard deviation. Emulsions were prepared in non-buffered system.
In addition, the EE of W/O/W emulsion decreased over time as a function of PGPR concentration (Fig. 4. 4.). At low PGPR concentrations (0.5 and 1%, w/v), storage of these emulsions for 4 weeks at 20 °C resulted in a significant (p <0.05) decrease in EE values compared to the initial EE values. In contrast, the EE of W/O/W emulsion prepared with 2 and 4% (w/v) PGPR showed no significant change after 4 weeks storage at 20 °C, the result indicating that the dye remained entrapped within the internal aqueous phase during the storage period. Poly R-478 dye is a large molecular weight material (Mw 50,000 Da), it is unlikely that it could transport through the oil phase to the external aqueous phase. The loss of dye in emulsions with lower PGPR concentration is probably through the rupture or expelling of internal water droplets. This will be discussed in depth in Chapter 6.:Characteristics of W/O/W emulsions containing hydrophilic encapsulants in the internal aqueous phase.
Chapter 4: Effect of PGPR and macromolecular materials on the formation and stability of W/O/W emulsion