Chemical and Biomolecular Engineering - Theses
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ItemEffects of shear on the stability and the release properties of water-in-oil-in-water (W/O/W) emulsions( 2014)Water-oil-water (W/O/W) emulsions are of great interest with many potential applications in different fields such as pharmaceutical products, cosmetics and food. W/O/W emulsions offer the possibility of reduction of the fat content, encapsulation of active species (e.g., vitamins, minerals) and controlled release of those active species. However, the inherent thermodynamic instability of W/O/W emulsions restricts their applications. Furthermore, emulsions are subjected to environmental stresses such as shear forces during their fabrication and use. An excessive shear force may lead to a breakdown of the primary emulsion, causing a decrease in the encapsulation efficiency of the W/O/W emulsion. The objective of this study was to investigate the shear stability of W/O/W emulsions as a function of parameters such as interfacial properties, viscosity and phase volume fractions. Mechanisms controlling the release of the internal aqueous phase during shear were identified and direct methods to measure the release of the internal aqueous phase were developed. W/O/W emulsions are fabricated with a two-step homogenisation method followed by an osmotic swelling process, i.e., a process of water transport from the external aqueous phase to the internal aqueous phase that swells the internal aqueous droplets. This method allows the fabrication of W/O/W emulsions containing different phase volume fractions while maintaining a low and constant amount of oil phase. In this research, the W/O/W emulsions were prepared using sunflower oil (polyglycerol polyricinoleate [PGPR]) as the emulsifier for the primary water-oil (W/O) emulsions and gum arabic as the emulsifier for the secondary W/O/W emulsions. In order to obtain different rheological properties of the W/O/W emulsions, different thickeners – xanthan gum, locust bean gum and hydroxyethyl cellulose (HEC) – were added into the external aqueous phase. Initially, experiments were carried out to determine the formulation space for the primary W/O emulsions. Stable primary W/O emulsions can be produced using as low as 1.00 wt% PGPR when D-glucose is contained in the internal aqueous phase. The presence of D-glucose decreases the chemical potential of water efficiently and thus induces an osmotic pressure opposing the Laplace pressure. As a result, it reduces the rate of Ostwald ripening in W/O emulsions. These simple emulsions were then processed to fabricate W/O/W emulsions. The concentration of PGPR had a significant effect on the stability of the W/O/W emulsions. As the concentration of PGPR was increased from 1.00 to 3.00 wt%, the overall zero viscosity of the W/O/W emulsions also increased. This indicates that zero or insignificant release of the internal aqueous phase during the fabrication process. In addition to its stabilising role at the inner (W/O) interface, PGPR was able to move to the second interface and was co-adsorbed with gum arabic to form interfacial complexes to provide an enhanced viscoelastic barrier against droplet coalescence and the loss of encapsulated materials from the internal aqueous phase. The shear stability of the W/O/W emulsions was tested by subjecting them to shear stress sweep using a stress-controlled rheometer. The shear stability was investigated, first as a function of globule volume fractions. Initially, the shear experiments were conducted on W/O/W emulsions containing 3 wt% PGPR and xanthan gum (XG) as a thickener in the external aqueous phase. No release of the internal aqueous phase or breakup of the globules was observed for all systems. The enhanced stability of the W/O/W emulsions against shear was attributed to a sufficient amount of PGPR to stabilise both the inner and the outer interfaces, thereby increasing the stability of the globules against deformation and breakup during the shear treatment. The shear thinning behaviour of the XG may contribute to the shear stability of W/O/W emulsions. As shear force is applied, the viscosity of the W/O/W emulsions containing XG decreases drastically. This leads to an insufficient shear stress to overcome the Laplace pressure of the globules, thus making it difficult to deform and break up the globules. In order to further study the shear stability of the W/O/W emulsions as a function of globule volume fractions and to investigate the mechanisms leading to the globule breakup and release of the internal aqueous phase, the thickener XG was replaced by locust bean gum (LBG) and the concentration of the PGPR was decreased from 3.00 to 1.00 wt%. Results from the shear experiment indicated that the globules did not undergo fragmentation; however, the release of the internal aqueous phase was observed. This indicates that the release of the internal aqueous phase occurs mainly through coalescence, which is due to the thinning of the thin liquid films separating the internal aqueous droplets and the globules’ surface, which eventually ruptures. The results also indicate that globule volume fraction, ϕg, does not significantly influence the release of the internal aqueous phase when the internal droplet volume fraction is constant. However, the release of the internal aqueous phase increases as a function of the internal droplet volume fraction, ϕi, and the size of the internal aqueous droplets. In the final set of experiments, the shear stability of the W/O/W emulsions was tested as a function of the internal aqueous droplet volume fractions. The role of globule breakup on the release of the internal aqueous phase was also studied. Hydroxyethyl cellulose (HEC) was used to replace LBG as the thickener due to solubility issues associate with LBG, especially at a high concentration. The presence of the internal aqueous droplets was shown to alter the rheological properties of the globules. As the internal droplet volume fraction increases, the viscoelastic properties of the globule also increase. As a result, higher shear stress is required to deform and fragment the globules. The shear experiments on the W/O/W emulsions as a function of the internal droplet volume fraction were initially conducted at low thickener concentration. The results showed that the viscous stress was insufficient to fragment the globules. However, the release of the internal aqueous phase was detected for the most concentrated globules, showing that breakup is not a necessary condition for release to occur under shear. In the second part of the study, the concentration of the thickener was increased to induce globule fragmentation. The results showed that the extent of fragmentation decreases with an increase in the internal droplet volume fraction; however, the extent of release was significantly higher for emulsions with an initial droplet volume fraction above the random close packing fraction. It should be noted that the extent of release is still higher than that of the W/O/W emulsions whose globules did not undergo globule breakup. Therefore, it can be deduced that although globule breakup may lead to the release of the internal aqueous phase, the effect is less significant than the release of the internal aqueous phase through coalescence. Thus, depending on the application, the behaviour of W/O/W emulsions under shear may be controlled by varying the formulation parameters of the concentration of emulsifiers, the viscosity and the phase volume fractions of the W/O/W emulsions.