The secret life of the emulsifier.
These days, one could be forgiven for thinking that creating an emulsion is child’s play. We have available at our fingertips tens, if not hundreds of self-emulsifying pre-blends that require no fiddly HLB calculations or secondary stabilisers. Well, that’s the theory anyway. You see, when you start making skincare formulations the chances are you will mostly be working on relatively simple oil-in-water concepts with stable, predictable actives and a simple vegetable-oil derived dispersed phases. Indeed, it is even possible to make some pretty exciting anti-ageing, brightening or cleansing formulations without hitting any hurdles all of which add to the general feeling of accomplishment for the freshly graduated chemist. But it doesn’t take long for that initial bravado and ‘beginners luck’ to fade and when it does it pays to have a few tricks up your sleeve.
I want to forget for a moment that we have these self-emulsifying blends and concentrate on working from first principals, not so we can do lots of math and get lost on somewhat superfluous detail but so we can design our creams from the ground up, so that we can make anything and so that it has at least a fighting chance of working out for us!
So let’s start by asking ourselves a few questions, what do you need to consider when selecting an emulsifier?
The way I see it there are four main categories that deserve consideration:
- Marketing/ Product Positioning.
- Physical Character
As Cosmetic Chemistry is an applied science I feel it is appropriate to start with the marketing as this is the main reason for us embarking on this formulation work after all – to sell something.
Emulsifier Free Claim.
Like every other aspect of cosmetic formulating trends in emulsifier selection abound. Right now we have an emerging niche of ‘emulsifier free’ creams designed around the philosophy that the emulsifier, being a surface-active ingredient may be a source of irritation for very sensitive skins and that avoiding the use of an emulsifier may make the product more suitable for this demographic. Now this may or may not work out to be true but nevertheless the concept is of interest as there is some truth in the idea that surface-active ingredients (of which emulsifiers are a part) can contribute to a products irritation potential.
So if you don’t use an emulsifier to hold the oil and water together, what do you use?
Small amounts of oil can be held in suspension with the use of thickeners/ stabilisers such as carbomer and even, to a lesser degree xanthan and sclerotium gums. These aren’t emulsifiers and neither are they forming an emulsion, more of a suspension but, if the oil phase is light enough and dispersed well enough these products can be made stable through stearic hindrance – the oil droplets are caught up, as if in a fishing net! Sometimes bentonite clays can also be used in this way to bring opacity to the formula and to provide an electrical repulsion layer into the structure to help repel agglomeration of the dispersed phase.
Another option in this space is a modified acrylate copolymers such as Acrylates/Beheneth-25 Methacrylate Copolymer. Although this particular polymer is best suited to surfactant formulations it does have a role to play in leave-on skincare as it has the benefit of being tolerant to a relatively high level of salt and other water-phase destabilisers making it a versatile choice for the ‘emulsifier-free’ concept developer. The fact that the polymer can stabilize a reasonably sized oil phase while remaining ‘emulsifier free’ paired with its low use levels add to the cost efficiency of this solution. Another interesting feature of these polymeric ‘emulsifier-free’ ingredients is their ability to be sprayed which opens up new doors for the formulator and marketing department.
But what if you are looking to create something a bit richer and with an oil phase more typical of a traditional moisturizer?
Modified lecithin chemistry has become accepted technology in the ‘no emulsifier’ space, especially because these lecithin fractions often have lipid enhancing properties and can be sold on their ‘skin compatibility’ and moisture binding powers. In addition, the phospholipid structure also lends its self to active delivery given how similar in structure they are to human cell membranes (well at least in terms of their chemical constituents). Lecithin chemistry tends towards forming a liquid crystal network in the continuous phase, which both traps and interacts with the dispersed phase in an ultra-stable and skin-compatible three-dimensional structure. A number of companies are now offering a range of modified lecithins suitable for everything from light hypoallergenic milk formulations through to super-rich balms which should please marketing departments the world over but from a chemists perspective it is hard to see how these ingredients have managed to escape the ‘emulsifier’ tag.
Ingredient Origin and Ethics.
Another important consideration for the general public and formulators alike is ingredient sustainability and/or ethics. There is a steady yet growing interest in ‘palm oil free or sustainable palm’ concepts the achieving of which is surprisingly difficult. That said, making an emulsion without adding any palm derived ingredients used to be easy – just use petroleum derived chemicals – but these days that is just as unacceptable for a growing number of brands. The reality is, our enthusiasm for embracing the natural revolution has increased demand for vegetable based feedstock and those of us who have been in the industry for a while know that means either palm (orang-u-tan habitat) or Rapeseed/ canola (pesticides and bees).
So today while it is still not that easy to create a palm free emulsion it is not impossible especially given that our previous ‘emulsifier free’ examples are predominantly palm free (lecithin phospholipids are frequently made from Rapeseed, Egg, Soybean and/or Sunflower). In fact, it would be fair to say that the hardest thing about making a palm free emulsion today is not over which emulsifier to choose but in which supporting ingredients can be used to increase stability, viscosity (without gumminess) and overall skin feel.
INCI Name – It Has To Look Good On The Label!
Still under the guise of marketing issues, the issue of INCI names also comes up here, while ingredient maufacturers do have to abide by a ‘truth in marketing’ legislation when they apply for their INCI names (they can’t just make them up) the reality that a natural sounding INCI will sell more and have better shelf appeal than a more chemical sounding ingredient. This reality has really hit home and that can be both good and bad. In some cases we have very average ingredients (in terms of performance) becoming popular because they have a nice name while outstanding ingredients (that are still natural in many cases) are overlooked.
Show Me The Money!
Another important factor in the marketing basket is price. Emulsifiers can make quite a dent in the formula budget and when comparing something natural to a stock-standard petroleum based emulsifier you can be looking at anything from three to five times the price which of course has to be justified. In many ways this is the acid test – will customers put their money where their ethical mouths are or do we have to sell them another benefit? Blending different technologies together can be a good way to increase performance while managing price and this philosophy has kept many of the older emulsifiers options such as glyceryl stearate SE alive and selling well.
Performance benefits of different technology will be looked at more in the chemistry section but as we are starting to see from some of the claims relating to emulsifier free formulations, the ingredient that holds the oil and water together is, in many cases expected to do so much more besides. When the Olive derived emulsifier Olivem 1000 first came onto the market there was a great deal of interest in its ability to act as an active delivery system as well as the ingredient that just happened to make an emulsion possible. That benefit contributed to the ‘emulsifier free’ marketing tagline (it isn’t an emulsifier, it is an active delivery system) while also helping the cosmetic chemist deliver oil-soluble actives deep into the skin (theoretically). This important dual functionality contributed to the immediate success of this technology, success that continues today in spite of the ingredients relatively high price point compared with older technology or ingredients like cetearyl alcohol which also form liquid crystal structures for a fraction of the price! In terms of intergrating new emulsifier technology into the laboratory these days the real question isn’t the price but how many benefits one gets for that price? Water resistance, barrier protection, long-wear characteristics, increased dispersion of actives, viscosity boosting and rheological benefits are all possible thanks to a combination of science and nature!
3) Physical Character.
Something that we may overlook when considering price is the emulsifier’s physical form and this is because we often just think of emulsions as hot process items and so the form of the emulsifier is not really a talking point. But it doesn’t have to be that way. Electricity prices are rising in many countries and even those with cheap power aren’t in the habit of wasting it so liquid emulsifiers that can be used in cold-process applications can help the formulator to tick a few boxes from sustainability through to the economic benefits of saving time and money. The range of liquid emulsifiers is steadily growing and is worth a look, especially for markets that demand an ultra-light touch finished product with little to no wax or butter content.
Of course we couldn’t talk about emulsifier selection without talking chemistry. The emulsifier is the heart of the formula and while it may often seem like, today the chemistry has all been done for you, as I mentioned in the beginning, there are some very challenging problems that await the professional cosmetic chemist that only an appreciation of the underlying chemistry will help solve!
Anionic emulsifiers are, in some ways the old-fashioned cousins when it comes to emulsifier technology given that soap and borax type emulsions were where the industry started but that doesn’t mean they don’t deserve a second look.
Soap-based emulsifiers can be extremely useful in cleansing formulations but can also form part of a very elegant high-end anti-ageing formulation just as long as your selection of actives is chosen carefully.
Anionic emulsifiers carry a net negative charge in solution and because of that they, like the cationics are sensitive to electrolytes. Anionics benefit from the presence of a little monovalent salt or acid up to a point (as the increased saltiness/ acidity increases the critical micelle concentration and activity of the water phase) but above that the formula can critically fail in a similar way to that which we see occurring in over-salted surfactant blends. Salt content can creep up on you in an active formula as ingredients such as Aloe, Sodium PCA, Seaweed Extracts, Sodium Hyaluronate and even some herbal actives can push the limits and as such, freeze/ thaw stability is an essential part of early anionic emulsion stability testing.
In terms of skin irritation potential, the old pharmacopeia driven Sodium Lauryl Sulfate and its ‘harsh on skin’ reputation has largely been replaced and when we talk about anionic emulsifiers we are now more often talking of the elegant phosphate esters renown for their skin compatibility or gentler surfactants such as the lactylates or glutamates making it entirely possible to make an anionic emulsion that is skin-kind and gentle.
Cationic technology for skincare arose from the wool industry and then transferred into hair care. As hair and skin are both keratin derived it didn’t take long for the benefits of cationics to be harnessed in moisturiser technology. Cationics function very well in formulations that are desired to stay on the skin for a long time such as sunscreens, long-wear make-up and barrier creams. This is because their positive, cationic charge adheres them strongly to the surface of the skin, resisting wash off and wear. A practical example of where Cationic emulsifiers have proved very helpful is in preventing sand from sticking to a freshly-sun-screened body thanks also to its anti-static capacity. Like anionics, cationics are also sensitive to what is going on in the water phase and cope best with a relatively quiet external phase rather than one loaded with additives.
Due to their natural capacity for skin adhesion cationic emulsifiers can be more likely to irritate than other chemical families but that said, in many cases the formulator can work around this – formulating to an acidic pH is advisable. New generation cationic emulsifiers tend to favour long hydrophobic tail (s) as these have the effect of reducing the charge density of the headgroup and thus minimizing irritation potential. This, of course, has to be balanced by the ingredients capacity to form and hold an emulsion. As with anionic emulsifiers, cationics do benefit from a little monovalent salt as this can boost the CMC and as a consequence of that, the viscosity, but beyond a certain point the salt becomes detrimental to stability.
It is important to note that in using a cationic emulsifier system the formulator does rule out the use of pretty much all grades of carbomer and even anionic thickening agents such as xanthan, unmodified guar and tragacanth.
Non-ionics remain the first choice go-to emulsifiers for the majority of applications due to their flexibility and low potential for chemical interaction. It is often a non-ionic emulsifier blend that is chosen first when creating creams with high activity levels or hard-to-stabilise ingredients such as salicylic acid, AHA’s, Zinc Oxide or high strength vitamin C. It was ingredients such as Seppic’s Montanov 68 that first got us hooked on the self-emulsifying blend of non-ionic and freed us from the trials of calculating HLB in a complex oil-phase world. The key benefit of a non-ionic emulsifier is its robust salt tolerance. In fact, the addition of a little non-ionic is recommended in ionic emulsions as the mixed micelles that will form tend to display a dramatically enhanced salt tolerance over the ionic alone. While the presence of self-emulsifying blends has made things easier it doesn’t for a moment mean we shouldn’t consider what is going on inside of our product.
The HLB system is alive and well in the non-ionic world and gives us a great insight into where the emulsifier will orient its self and whether it is able to bring any other features to the product. That said, it is also important to mention the existence of more than one HLB system so that one can compare like with like. Between 1949-1954Griffin developed a pretty robust yet simple HLB system and that is the standard upon which the Span and Tween pairings from ICI were arranged. The method produced a scale ranging from 0-20 indicating what percentage of the emulsifier was hydrophilic. The number given was the percentage hydrophillicity / 5 (so the maximum number 20 related to a molecule being 100% hydrophilic). This simple system was expanded upon by Davie’s in 1957 who, thought that some weight should be given to the functionality of the chemical groups on the molecule. This makes sense given the variety of structures available to give a hydrophilic character. This method is widely used today and is one of the reasons that ionic emulsifiers can be assigned a HLB value, it is also the reason that HLB numbers in their 30’s are found (the maximum HLB in this system is 40).
In both systems both the emulsifier and the oils to be emulsified have a HLB attached to them – the emulsifiers have a real HLB whereas the oils have a required HLB. It is widely accepted that the best emulsifier pairings are formed when a high HLB emulsifier is combined with an emulsifier with a low HLB rather than just selecting the emulsifier with the exact HLB you want to achieve. This combination effect serves to best fill the interface surrounding the continuous and dispersed phase, leaving less room for gaps and therefore increasing stability.
Liquid Crystal Emulsifiers
Liquid Crystal Emulsifiers work on the principal of forming a lamella network in the cream, which most closely mimics the skin barrier thus facilitating the effective delivery of actives. Lecithin naturally works this way as does Olivem 100 and various other combinations available to purchase today.
Because of their skin-like structure, liquid crystal emulsifiers are often desired because of their beautiful aesthetics. While it is possible to create a range of textures using almost any emulsifier, depending on what goes into the rest of the formula, it would be reasonable to say that the liquid crystal generating emulsifier is the most fool-proof way of creating a beautiful texture without too much additional work.
Polymeric emulsifiers are a really good option for those looking for elegant and quick cold-process solutions. Often sold as liquid polymer suspensions these emulsifiers can create anything from lightweight sprayable milks through to thicker, richer creams depending on what they are paired with. While not for the natural market these can be quite a sustainable option due to their low addition rate, the speed with which they can form an emulsion and their cold-processing capabilities. That said, polymeric emulsifiers won’t work in every situation. Often these emulsifiers are acrylic acid polymers that tend to form complexes with cationic species. In addition, the general salt tolerance of acrylates is low (reflecting the overall intolerance of ionic substances). But one large advantage over ionic and, to a certain point the non-ionic emulsifiers is their ability to form highly stable emulsions with a very low level of polymer with a non-polar or very slightly polar oil phase. This makes them the perfect emulsifier for a silicone-based emulsion, even those containing cyclomethicone.
4.a) The Chemistry of the Water Phase.
I have mentioned a few de-stabilising and stabilizing factors while talking about emulsifier chemistry and marketing considerations but now it is time to really focus on this. An emulsion is a combination of two immiscible phases held together by what to many looks and feels like magic! What is really going on is a physical rearrangement of components, all trying to get themselves into a position that means they are exerting the lowest possible amount of energy – those dispersed phase droplets are lazy! The dispersed and continuous phases are influenced by everything that comes into the formula and some things are more disruptive than others.
Lowering the Surface Tension.
The surface tension between oil and water is so high they don’t mix unless you add a surfactant (emulsifier in this situation). We understand that and have discussed the different types of emulsifier that we could add but do we understand what other ingredients do to the products surface tension?
Preservatives and solvents can dramatically alter the surface tension between the oil and water droplets. As we see with our emulsifier, some reduction in surface tension is required to facilitate the development of an emulsion but in other cases the changes are catastrophic and can result in viscosity and emulsion collapse.
Glycerin, Propylene Glycol and Ethanol are common additives in a cosmetic product and can all impact on surface tension. All of these ingredients decrease the polarity of the water phase and the influence of that decrease changes depending on the quantity of additive present although the relationship between dose and effect is not strictly linear. Reducing water phase polarity loosens the grip the water has on the dispersed phase somewhat by toning down the intramolecular forces that make water behave as it does – Van Der Waals, Dipole-Dipole and hydrogen bonding. While all emulsifiers depend on these forces to some degree to stabilize the product, the Ionic emulsifiers depend on them more strongly and are therefore most likely to be influenced by their presence.
A Flood Of Ions
We often talk about salt and saltiness in formulating but what we are really talking about much of the time is the ionic strength of (often) our continuous phase. While we all accept that swapping Demineralised Water for sea water might cause us formulary issues, we are less likely to accept that the actives we carefully measure into our water phase are doing the same. Be they acids, bases or salts an ion rich water phase can cause havoc for the stability of a product.
Ionic charge in the water phase can help increase intramolecular bonding and can also help in the formation of an electronic double layer around the dispersed phase which increases stability but things can go too far, especially with divalent salts such as zinc and with strong acids and oxidizing agents such as glycolic and peroxide. In general the cosmetic chemist is looking to minimize chemical reactions in their formula and so any addition of ions should be thought of as fuelling the fire of chemical rebellion. Every emulsion has its limits plus adding too many ions into the continuous phase will give the product a sticky/ tacky/ salty feel when applied.
4B) Tricky Oil Phases.
When compared to vegetable oils, silicones demonstrate practically no polarity as well as a very different chemical structure – flexible chains vs bulky triglycerides. Because of these differences silicone fluids such as dimethicone and cyclomethicone tend to mix poorly or not at all with vegetable oils in the same formula. If the formulator wishes to create a silicone rich emulsion, the influence of this low polarity should be considered and steps to reduce the polarity of the continuous phase would increase stability and take some of the pressure off the emulsifier as the surface tension between the two phases would be lessened.
Silicone emulsifiers are available from the major silicone manufacturers should be the first port of call for all those looking to create a silicone-dominant emulsion, especially where the silicone phase will be large or even dominant. That said, with careful consideration it is also possible to create a silicone-rich emulsion with non-silicone emulsifiers if the chemistry of the whole product is considered and accounted for.
|Solvent||Polarity||Solubility Parameter (A difference of <2 indicates mutual structural solubility). Based on the theory of ‘like dissolving like’ Journal SCC 1985.
|Cetyl Alcohol / Stearyl Alcohol||8.94 – 8.90|
|White Mineral Oil||7.09|
Is your emulsion going to be salty, acidic, basic or contain a high proportion of solvents that are less polar than water or does it contain Hydrogen Peroxide? If so the best starting point is a non-ionic.
At this point I think it’s best if we head into the lab and do some experimenting.
Catch up with those experiments by clicking through here.
- The stability of emulsions in the presence of additives. GA[I]1#, B. JOVANOVI]2# and S. JOVANOVI]2# 1Institute for Plant Protection and Environment, Teodora Drajzera 9, YU-11000 Belgrade and 2Faculty of Technology and Metallurgy, Karnegijeva 4, YU-11000 Belgrade, Yugoslavia, 2001. http://www.doiserbia.nb.rs/img/doi/0352-5139/2002/0352-51390201031G.pdf
- Solute and Solvent Structure/ Polarity. The Pharmaceutics and Compounding Laboratory. UNC Eshelman School of Pharmacy: http://pharmlabs.unc.edu/labs/solubility/structure.htm
- Using Solubility Parameters in Cosmetics Formulation. C.D Vaughan, Cosmair Inc, 285 Terminal Ave, Clark, NJ, 07066. Jan 1985 http://journal.scconline.org/pdf/cc1985/cc036n05/p00319-p00333.pdf
- Introducing Pemulen Polymeric Emulsifiers. file:///Users/amandafoxon-hill/Downloads/TDS-114_Introducing_Pemulen_Polymeric_Emulsifiers%20(2).pdf
- Emulsion Formation, Stability and Rheology. Tharwat F Tadros. https://www.wiley-vch.de/books/sample/3527319913_c01.pdf
- Cationic emulsifiers: An emerging trend in skin care. Angela Paez and Anna Howe, Degussa Goldshmidt Chemical Company. http://personal-care.evonik.com/product/personal-care/Documents/cosmetic-toiletries-manufacture-worldwide-cationic-emulsifiers-for-skin-care-applications.pdf
- Macol CSA 20 Polyether. BASF data sheet. http://worldaccount.basf.com/wa/NAFTA~en_US/Catalog/ChemicalsNAFTA/doc4/BASF/PRD/30093528/.pdf?asset_type=pi/pdf&language=EN&urn=urn:documentum:eCommerce_sol_EU:09007bb28006d4c2.pdf
- Emulsions – Part 2. Klaus Tauer. MPI Colloids and Interfaces. Am Muhlenberg, D-14476 Golm, Germany. http://www.mpikg.mpg.de/886743/Emulsions_-2.pdf
- A Quantitative Kinetic Theory of Emulsion Type 1. Physical Chemistry of The Emulsifying Agent. J T Davies, University Lecturer in Chemical Engineering, Cambridge. http://www.firp.ula.ve/archivos/historicos/57_Chap_Davies.pdf
- Intermolecular Forces. Forces. http://chemed.chem.purdue.edu/genchem/topicreview/bp/intermol/intermol.html