56 NATURALS
demonstrated by its ability to create stable emulsions in either cold- or hot-processes (Fig 2). While the polyglycerol-based emulsifier is added in the oil phase, the water phase is added in the oil phase (already containing the emulsifier) at the same temperature of the oil phase. The cold-process feature of the polyglycerol- based emulsifier becomes interesting especially when formulating with heat- sensitive oils or extracts. The superior efficacy of the polyglycerol- based emulsifier was demonstrated in comparison with other polyglycerol-based emulsifiers (Fig 3). In the conditions tested, only the emulsion made with the new polyglycerol-based emulsifier could remain stable for a period of one month at 50°C. It can also be observed that the new polyglycerol-based emulsifier produces a particularly homogenous emulsion. Those results simultaneously demonstrate the self- emulsifying properties of the new polyglycerol-based emulsifier. Similar comparative tests were achieved but with emulsions made ‘cold-process’ at room temperature. In the conditions tested, polyglyceryl-3 oleate, polyglyceryl-2 isostearate and polyglyceryl-2 dipolyhydroxystearate did not succeed to create stable emulsions (Fig 4A, B and D, respectively). While the emulsion made with diisostearoyl polyglyceryl-3 dimer dilinoleate could remain stable after 1 month at 50°C, it separated after 3 months at room temperature (Fig 4C). Polydisperse water
Emulsifier Polyglyceryl-3 Oleate Polyglyceryl-2 Isostearate 5%
Diisostearoyl Polyglyceryl-3 Dimer Dilinoleate
PolyAquolTM -OS2 * 10% Sweet Almond Oil and 15% Paraffinum Liquidum 10% 15% Unstable ✓Stable Cold Process PolyAquolTM -OS2 Hot Process PolyAquolTM -OS2 80°C 25°C
80°C 25°C
Figure 2: The polyglycerol-based emulsifier is added in the oil at either cold temperature (around 25°C) or at high temperature (around 80°C). The water phase is then added under homogenization at the same temperature of the oil phase.
droplets observed in the latter emulsion would eventually coalesce leading to aqueous phase separation. The emulsion made with the new polyglycerol-based emulsifier remained fully stable for one month at 50°C and for over 12 months at room temperature (Fig 4E). The micro-structure of the emulsion is uniform and does not reveal any water droplet polydispersity.
The capacity of an emulsifier to contain a significant water phase (70% or more) in the
Polar oil* Apolar oil* Stability
(1 month at 50ºC) Unstable Unstable
continuous oil phase of a W/O system is key to emulsion stability. As observed in the previous experiments water droplet coalescence and water phase separation are phenomena that will lead to W/O emulsion instability and coarsening. Water droplet coalescence is primarily induced by van der Waals forces favouring the attraction and interaction between neighbouring water droplets.1
Eventually, droplets will merge
and coalescence occur. The coaction between water and oil at the interface of those two immiscible phases will predict the stability of the emulsion. While the emulsifier unquestionably represents the cornerstone of a stable emulsion other factors are known to contribute to the robustness of the system. Particularly for W/O emulsions, the presence of electrolytes complements the action of an effective emulsifier to confer stability.2 Van der Waals forces are electrostatic in
nature and thus depend upon the polarisability (electronic environment or cloud) of individual particles (water droplets) and the surrounding medium (oil phase).3
It is known that the polarisability of
a substance will affect how light (an electromagnetic wave) is bent when passing through it. Therefore, the refractive index can be used to measure the polarisability of a liquid.4
In the case of a Optical microscopy analysis (600X)
Figure 3: Comparison of PolyAquol-OS2 with other polyglycerol-based emulsifiers. Emulsions were made with 5% of emulsifiers added individually in a total 30% oil phase (10% sweet almond oil; 15% liquid paraffin) mixed with 70% water phase including 1% MgSO4
. Emulsions were made at 80°C.
Stability was visually assessed (phase homogeneity or separation) after 1 month at 50°C. Micrographs were taken with a Motic microscope BA3110E equipped with a LED light source and a Moticam 5 (5.0 MegaPixel) camera. Magnification 600X.
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W/O emulsion, such attractive forces will thus be determined by the difference between the polarisability of the water droplets and that of the continuous oil phase. The refractive index of pure water is around 1.33 while that of vegetable oil is around 1.47. This difference between the polarisability of the water phase and that of the oil phase would favour interaction between ‘alike’ water droplets. Indeed,
June 2018
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