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Electrolytes must be used Electrolytes – inorganic salts such as magnesium sulfate or sodium chloride – must be present in the emulsion because they will stabilise the system through various mechanisms of action. For example, NaCl has been shown to decrease the particle size through electrostatic and steric repulsion among the droplets. CaCl2 has proven to decrease attractive forces between water droplets and MgCl2 can reduce interfacial tension and enhance interfacial film strength.3 Each of these mechanisms may prevent
one or more of the following from happening: l Ostwald ripening, also referred to as disproportionation, is caused by the difference in solubility in emulsion droplets. Smaller droplets are more soluble than larger ones, and with prolonged time, the smaller droplets tend to diffuse in the bulk and are deposited on larger droplets. Therefore, larger droplets eventually grow at the expense of smaller ones. Adding salts will counterbalance the driving force for Ostwald ripening, which is related to the total pressure and pressure in the droplets.3
l Sedimentation is another unstable condition where there is no change to the droplet size, but droplets move to the bottom. The addition of various salts could improve stability by decreasing particle size and reducing the interfacial tension. Salts enable tighter packing of surfactant molecules at the O/W interface.3
l Coalescence is when droplets join, creating larger sized droplets with water separation at the bottom. Adding salts will reduce the attractive forces between water droplets, which will reduce their collision frequency, and thereby prevent droplet coalescence and increase emulsion stability.3
Depending on the emulsifier, the polarity of the oils used must be specific Unlike O/W systems, the polarity of the oils used in the oil phase has an outsized influence on the stability of the emulsion, and the performance of high-polarity compared to low-polarity oils will be significant. This is because of the “like dissolves like” rule. In general, chemicals of similar polarities demonstrate better interaction. If the lipophilic tails of your emulsifier are polar, perhaps having esters or hydroxyl groups in its carbon backbone, for example, then the emulsifier is better suited in an oil phase of medium to high polarity. The opposite holds true as well. If the polarity of the oil phase does not match that of the emulsifier, the emulsion will not be stable.
November 2020 High energy homogenization Low energy homogenization
Figure 2: Energy homogenisation. x Coalescence Figure 3: Interfacial tension.
How to improve the sensoriality of W/O emulsions The choice of raw materials is essential to improve the sensory characteristics of W/O emulsions. The oil phase is obviously the first thing
to look at in order to improve the sensoriality of a W/O emulsion, since it is this that will be felt first when applying the product on the skin. The oil phase can be optimised to reduce the greasy effect by choosing dry esters that have a lighter touch than other types of emollients or oils. Thickeners such as waxes could be used also to reduce this greasy effect, but the induced viscosity increase can modify other sensory parameters such as spreadability. Low melting point waxes should be tested in the first instance in order to minimise some unwanted effects such as breaking or rough feel. Another solution is to add powders, like
starches, silica or cellulose, that improve the skin-feel by reducing the tacky and greasy effects and increasing the soft afterfeel. It is important to find the right combination and dosage, too much powder could lead to an uncomfortable texture that is difficult to spread on the skin.
A W/O emulsifier Gattefossé has applied its strong knowledge in lipid chemistry and polyglycerol esters to the development of Emulium®
Illustro (INCI: Polyglyceryl-6 Polyhydroxystearate (and) Polyglyceryl-6
Polyricinoleate), an innovative W/O emulsifier of 100% natural origin.
A fine selection of raw materials Three carefully selected raw materials were reacted together by esterification. Polyglycerol-6 (PG-6) is a naturally
hydrating ingredient of vegetable origin and is mild to the skin. A polyglycerol-6 was chosen due to the stability brought by the large PG-6 polar headgroup. A special grade containing a very low amount of free glycerol was used to prevent the formation of small glycerol esters that can displace the main emulsifier and cause instability over time. 12-hydroxystearic acid (12-HAS) is a
homopolymer of a C18 fatty acid with a hydroxyl group on carbon 12. The polymer is well known for its dispersing properties. It is a solid ingredient at room temperature. Finally, ricinoleic acid (RA) is a very similar
structure to the 12-HAS, however possesses an unsaturation on the carbon backbone. This unsaturation brings mobility to the chains, as a result this fatty acid is a liquid at room temperature. The association of solid 12-HAS and liquid
RA enabled the creation of a structure that is liquid at room temperature and can be used in a cold process. When formulated, the emulsifier forms a water-in-oil emulsion. The PG-6 polar head can be found in the water phase and the 12-HSA and RA long carbon chains extend into the oil phase, bringing stability to the emulsion by steric repulsion.
PERSONAL CARE ASIA PACIFIC Stable emulsion
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