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Anionic emulsifier
When do you choose to use an anionic emulsifier? Whats the benefits of this type of emulsifier?
Also, lately I’ve been doing a lot of research about the importance of the ionic charges on the stabilization of cosmetics emulsions, but unfortunately there’s not much information/studies about it. There’s any recomendations of studies about the importance of charges on emulsion stability?
Thank you!
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5 Replies
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Stability and characterisation of Emulsions in the presence of colloidal particles and surfactants .
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Roman Pichot -
In my experience, I’ve found that anionic emulsifiers have somewhat of a more substantive feel on the skin, not necessarily greasy, almost like a veil. Anionic emulsifiers would not be compatible with cationic materials so use in combination with other anionics or non-ionics.
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Anionics are more ubiquitous, easier to incorporate co-emulsifiers and other ingredients, better to stabilize colloidal dispersions, and are more practical for most HBA applications. The good ‘ol TEA/Stearic acid system served us so well for so long until….California.
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There are many publications with regard to surface charge and emulsion stability, search for the so called zeta potential: If you really want to use surface charge as a marker for stability, then you’d have to use equipment which measures said potential (for example a Zetasizer -> charge determination to estimate stability) or do so indirectly via particle size and size distribution (with for example a Lumisizer -> size/stability determination as an indicator for zeta potential).Since most particles tend to have a slightly negative zeta potential, adding an anionic surfactant more easily results in a highly negative potential and hence it’s more easily to get a higher stability. On the other hand, adding a cationic surfactant will not as easily give a positive enough potential for increased stability. In addition to that, anionic surfactants often have a ‘hard’ charge = concentrated on the surface = higher (negative) value of zeta potential whilst cationics are generally ‘softer’ = diffuse charge = lower (positive) zeta potential. This obviously favours higher stability in the former case.Furthermore, charge is just one of several effects which may be used to increase emulsion stability and it only works well with micelles and somewhat with cubic and some hexagonal phases (though these may not adequately be measured with a Zetasizer) but not with lamellar or inverse phases whilst several other effects do work there too.Bottom line is, depending on the emulsion type, imparting a charge to the outer phase of lipid particles is highly correlated with increased stability and is a simple physical effect or galenic law. As a rule of thumbs, an o/w emulsion with a zeta potential of greater than + or - 60 is regarded as very stable. A lower zeta potential (nearer to zero) requires additional measures such as addition of PEGs (steric hindrance), smaller droplet size, or a water gellant to keep oil droplets from fusing or at least reduce speed of fusion. Another approach is to switch emulsion type for example to a lamellar or ‘liquid crystal’ phase. -
Although I don’t like unnecessary restrictions and prefer conventional (read scary synthetics) materials, I thinks TEA stearate should stay in 90’s (for leave in applications). It’s still heavily used by lush and aesthetics isn’t there. It’s easier to make an emulsion using good old Arlacel 165, and add polymeric emulsifier (such as Aristoflex AVC). Most of polymeric emulsifiers are anionic and as such improve stability and make the product very aesthetically pleasing.
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