Surfactants: Cleansers and Emulsifiers

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For chemists, cleansers and emulsifiers belong to the same big family: the surfactants. Indeed, from the chemical point of view they are molecules that share similar structural characteristics and mechanism of action in solution. All surfactants are amphiphilic molecules, meaning that they get along with both water and oil. This is the key feature that allow them to act as cleansers and as emulsifiers: they are able to interact with both water and oil, therefore they can help washing away greasy dirt when used as cleansers, and stabilise oil phase and water phase when used as emulsifiers. 

In this article, I will call “cleansers” the surfactants that we typically use in cleanser products (shampoos and body cleansers), and “emulsifiers” the surfactants that we typically use in lotions and creams. 

The mess with commercial names

Before we speak about the types of surfactants, I must mention that in most cases you will not find them in supplier shops and formulas with their chemical name, but with a commercial name (examples: Emulsan, Plantapon, Tego Care, etc.). I don’t like to use the commercial name, because it differs from one supplier to another and in particular among different countries. In addition, as a chemist, the chemical name of the ingredient tells me a lot more than its commercial name. 

I will mention surfactants only with their chemical name, because I don’t always know the commercial name and because then you can understand which surfactant I’m talking about even if you’re in a different country and you buy from different suppliers. 

When you look for a surfactant in the supplier shop, just look for the INCI name of the surfactant and you will get the chemical name I use here. 

Chemical structure and classification

Surfactants have both a hydrophilic part and a hydrophobic part. The hydrophobic part is typically a hydrocarbon chain, similar to the one of fatty acids. The hydrophilic part can be a charged group (anionic, cationic, or zwitterionic groups, that is, a negatively charged one and a positively charged one), or a very polar and water-friendly group that does not bear a charge in aqueous solutions (1). 

The hydrophobic part of surfactants is typically a hydrocarbon chain, that is, a long carbon and hydrogen chain similar to the one we have seen in fatty acids. Indeed, many surfactants are derived from fats and triglycerides. 

The hydrophilic part of surfactants can be a charged group (negatively or positively) or a non ionic group. We typically speak of anionic, cationic, amphoteric and non ionic surfactants based on the hydrophilic head. Anionic, cationic, amphoteric and non ionic surfactants can be used as cleansers in hair shampoos, hair conditioners and cleansers for face and body. Some non ionic surfactants are employed as emulsifiers in lotions and creams. 

Anionic surfactants

Anionic surfactants are frequently used as cleansers in shampoos and body cleansers. Their hydrophilic head bears a negative charge in aqueous solutions. 

The most common groups of anionic surfactants are:

  • Carboxylates: -COO group. Examples: soaps, sodium lauryl glucose carboxylate.
  • Sulfates: -OSO3 group. Examples: sodium lauryl sulfate, sodium laureth sulfate, sodium coco sulfate, ammonium lauryl sulfate.
  • Sulfonates: -SO3 group. Examples: disodium laureth sulfosuccinate, sodium cocoyl isethionate, sodium lauryl methyl isethionate, sodium lauryl sulfoacetate.

Sulfates are traditionally the most commonly used anionic surfactants in cleansers. They have high foaming capacity and they are good cleansers. They are typically associated with amphoteric or non ionic surfactants to achieve milder formulations. 

The viscosity of sulfates is sensitive to electrolytes, which is why sulfate-based formulations are made more viscous by addition of salt (sodium chloride). 

Cationic surfactants

Cationic surfactants carry a positive charge in aqueous solutions and are mainly used as conditioning agents in conditioners and in 2-in-1 shampoos. They are used as conditioners because they can lower the surface friction between hair fibres and convey a hydrophobic protective surface to the hair. 

The cationic head is typically given by a nitrogen atom linked to four groups, that is, a quaternary nitrogen atom. Common cationic surfactants are:

  • Tetra alkyl or aryl ammonium salts: (R4N+)X. Examples: behentrimonium chloride, polyquaternium 10, cetrimonium chloride.
  • Alkylamines: for example stearamidopropyl dimethylamine.
  • Esterified quaternaries: called esterquats, they are esters with a positively charged nitrogen atoms and they are becoming more popular in recent years because of their better environmental profile. 

Amphoteric surfactants

Amphoteric species are molecules that bear functional groups that can be charged negatively and groups that can be charged positively. This is typically pH-dependent: within a certain pH range (typically very low), these molecule will be prevalently positively charged, whereas at high pH they will be prevalently negatively charged. In the middle, they will be both positively and negatively charged (they will be in their so-called “zwitterionic” form).

Typically, betaines are classified as amphoteric, although they are typically positively charged in the pH range of cosmetics. 

Other amphoteric surfactants are acyl ethylene diamines and their derivatives (acyl amphoacetates). 

Non ionic surfactants

Non ionic surfactants are never dissociated and do not show negative or positive charge in aqueous solution. Their hydrophilic moiety can be for example an ethoxylated group or a sugar. 

  • Ethoxylated alcohols: typically a fatty alcohol + a ethoxy group (PEGylated fatty alcohols). Examples are laureth, ceteareth, steareth surfactants (“eth” means ether, because they are actually ethers). 
  • Alkyl polyglucosides: sugar moieties linked to a long fatty chain. Examples are decyl glucoside, lauryl glucoside, and coco glucoside. 
  • Glycol and glycerol esters: examples are glyceryl oleate, glyceryl caprylate/caprate, and PEGylated versions. 
  • Sorbitan derivatives: for example polysorbates. 
  • Sugar and sucrose esters: for example sucrose cocoate and sucrose laurate. 

Ingredients typically used as emulsifiers belong to this surfactant category, too. Some of them are more hydrophobic and they are not soluble in water, because their lipophilic portion is prevalent in the molecule, whereas others are water soluble.

Cleansers

Mechanism of action

Hydrophobicity and hydrophilicity

Before we discuss how cleansers work, we should say a few words about hydrophilic and hydrophobic compounds. We often take these two concept for granted, but the reason why some chemical structures are water-friendly and others are not might be not so obvious for everyone. 

Why are some substances insoluble in water? There can be several reasons.

  • Some solid materials, like clay or glass, are insoluble in water because the molecular interactions between the molecules that build them are too strong and much stronger than the interactions with water. Therefore they cannot be “interrupted” and the molecules of these substances cannot move freely in water. These substances can be dispersed rather than dissolved in water, thanks to surfactants that help allowing it. 
  • Oils and waxes are not soluble in water because their molecules cannot interact much with each other, whereas water molecules interact very well with each other thanks to their polarity and ability to form hydrogen bonds. The oils are forced to separate from the water phase in order to minimise the contact area between the two species as much as possible. At the interface, the water molecules are attracted to the other water molecules that are below them in the bulk water phase, but they cannot interact with the oil molecules of the neighbouring oil phase. Thus, they do their best to stay together and this creates what is known as surface tension (which exists not only at the water-oil interface, but also at the water-air interface). 

Surfactants are able to lower this surface tension. They do it both at the water-oil interface and at the water-air interface, which is the reason why surfactant aqueous solutions are able to stabilise air bubbles and create foam (2).

Micelles

At very low concentration in water, surfactants tend to absorb to the surface of the liquid. As the surfactant concentration increases and reaches a critical concentration (CMC), they start to form aggregates called micelles (2, 3). When forming micelles, the surfactant molecules are organised such that the hydrophobic tails of the molecules are packed inside of the micelle and the hydrophilic heads are exposed towards the water medium. They have the ability to solubilise oils within their hydrophobic core, which is one of the possible detergency mechanisms (but not the only one). 

We are used to think about micelles as spherical structures, but they can actually assume different shapes according to the surfactant concentration, pH of the medium and ionic strength (presence of electrolytes). 

By increasing the surfactant concentration, micelles can reorganise and switch from spherical micelles to rods, followed by worms, packed rods and finally to a lamellar phase which is typical of liposomal vesicles. 

The viscosity of shampoos and cleansers is often given by this ability of micelles to pack into worms and entangle in each other (often triggered by the presence of salts – that’s why sulfates solutions become more viscous by addition of sodium chloride). Another trick to make sulfates solutions more viscous is the additioin of co-surfactants like betaines, that have a positive charge at the typical cleanser pH and can interact with sulfates and change the micelle packing into the worm-like structure. 

Cleansing mechanisms

Surfactants can remove oils from surfaces by different mechanisms. 

  • Rollup of the oil droplets: when oils are spread on the surfaces, the surfactant adsorbs on the surface and on the oil surface. The oil then rolls up into a droplet that is removed from the surface under agitation. 
  • Emulsification: the surfactant absorbs onto the oil-substrate surface and lowers the surface tension between oil and water, allowing its emulsification in water. 
  • Penetration: the surfactant diffuses into the oil, which then becomes part of the surfactant self-assembled system.
  • Solubilisation: the oil is incorporated into the inner hydrophobic core of the micelles (micelles are able to assemble and disassemble, and in this process they can incorporate oils in their structure). 

Foaming

Although the cleansing mechanisms of surfactants can occur also independently from foaming, the ability of a cleanser to make foam is the first thing that you evaluate when trying out a new shampoo. 

Surfactants help generating foam because they concentrate at the air-water interface and reduce the surface tension (4). 

When you have wet hands and rub them one onto the other one, little air bubbles are formed in the water and they are surrounded by the liquid. If there is no soap, the water surface tension closes the bubbles and you don’t even see them. If there is a soap in there, the bubbles are stabilised and can survive longer and become bigger, and you see the foam. 

Cleanser formulation

Amount of surfactant: the total active matter

Similarly to our discussion about the fat phase of creams, the first thing we should decide when formulating a cleanser product is how much surfactant we want to use. The amount of surfactant that is present in the cleanser is typically referred to as “active surfactant matter” or ASM. The calculation of how much surfactant is actually in the product is not as direct as with emollients, though: surfactants are not provided as 100% concentrated (=100% active). They are provided with a certain ASM, that is declared in the data sheet. 

For example, sodium laureth sulfate (SLES) is typically supplied as 27% active. If you formulate a detergent with only 10% SLES, the total active matter will be 2,7%. If you have more than one surfactant, you have to calculate the respective ASM and then make the sum. You can use calculators online like the one you can find at reference (5) or make your own Excel or Numbers spreadsheet like the one in the file attached (see at the end of the article)

Each type of cleanser product has an average ASM range. Here is what I found on the Making Skincare blog:

  • Face cleansers: 8-10%
  • Shampoos: 10-15%
  • Body washes: 15-20%

Keep in mind that this is valid for liquid cleansers. For solid cleansers (like solid shampoos), the ASM is much higher than this, because there is no water. Solid cleansers are basically super-concentrated versions of liquid cleansers, to which you will add water only while using them. Therefore they have ASM that can be like 10 times the one of the respective liquid product. 

Choice and combination of surfactants: primary and secondary surfactants

In a cleanser formulation, you will typically find primary surfactants (typically anionic) that do most of the hard cleansing work, and secondary surfactants (typically amphoteric or non ionic) that keep the detergent mild, help foaming and can have an influence on the rheology of the final product (6). 

Typical primary surfactants found in DIY formulas are:

  • Sulfates: sodium laureth sulfate (SLES), sodium lauryl sulfate (SLS), sodium coco sulfate (SCS, solid surfactant)
  • Sulfonates: sodium cocoyl isethionate (SCI, solid surfactant), sodium lauryl sulfoacetate (SLSA, solid surfactant)
  • Sarcosinates: sodium lauryl sarcosinate

Typical secondary surfactants in DIY formulas are:

  • Betaines: cocamidopropyl betaine
  • Acylamphoacetates: sodium and disodium cocoamphodiacetate
  • Glucosides: coco glucoside, lauryl glucoside, decyl glucoside
  • Sugar esters: sucrose cocoate, sucrose laurate

If you want to avoid sulfates (although I don’t see why you should) you can take a look also at reference (7), where you can find a nice informative table with some non-sulfate surfactants.

In the tables from the attached PDF file (at the end of the article) you can see a longer list of surfactants that I found in online suppliers shops from different countries (Glamour Cosmetics in Italy, Lotion Crafter in USA, Aliacura in Germany). As you can see, each country has its preferences in terms of surfactants: only few are sold by all three shops, whereas others are sold by only one of the three. 

Now, let’s assume we decided the ASM of our product and which primary and secondary surfactant we want to use. How do we balance them?

In manuals and papers, I found that typical shampoo formulation show a very high ratio between primary and secondary surfactant, like 10:1. You can also decide to lower this ratio and raise a bit the secondary surfactant quota, if you need to. For example, in solid shampoos I have seen that the ratio can be much lower, around 3:1 for example. In my solid shampoo formulas, the ratio between primary surfactants (SCI + SLSA) and secondary surfactants (betaine + glucoside) is typically 2.85:1. 

Emulsifiers

We already said a lot about emulsions and emulsifiers in the Making Creams series, therefore I will not say much here about what emulsifiers do. But now we kind of closed the circle and you know that those molecules who keep the cream stable are actually surfactants, just like the ones you have in the shampoo. However, they work much better as emulsifiers than cleansers. And they are non ionic. 

Let’s see briefly some of the emulsifiers you can use as DIY formulators. 

O/W Emulsifiers

The following emulsifiers are for oil-in-water emulsions and are typically used together with a fatty alcohol like cetyl alcohol or cetearyl alcohol to provide more body and stability to the emulsion. 

  • Methylglucose sesquistearate, methylglucose distearate: this is a waxy emulsifier that you can use in lotions and in creams. It is suggested to use it at 2-4% of the formula.
  • Cetearyl glucoside: hydrophilic emulsifier, can be used at 1-1,5% in emulsions.
  • Glyceryl stearate: use level 4-6%
  • Hydrogenated lecithin (Phospholipon 80): probably Germany’s favorite emulsifier, used at 1-3% as main emulsifier, 0.5-2% as co-emulsifier (for example coupled with sucrose stearate at 1%). The cool thing about lecithin is that it is very “skin-like” because it resembles the skin lipids, thus it can act not only as emulsifier but also as functional ingredient in moisturisers (8, 9). 
  • Sucrose stearate: hydrophilic emulsifier, use level 2-3%. Can be used for very light emulsions also alone, but must be coupled with a fatty alcohol (cetyl alcohol or cetearyl alcohol) to build body in creams (10). 

Suppliers also sell blends of surfactants that do not need other components to form stable emulsions:

  • Lamecreme: glyceryl stearate, glyceryl stearate citrate, 5-6%
  • Montanov 68: cetearyl glucoside + cetyl alcohol; 4-5% use level.
  • Olivem 1000: cetearyl olivate + sorbitan olivate; 2-8%
  • Xyliance: cetearyl wheat straw glycosides + cetearyl alcohol; 4-6%

This doesn’t mean that you cannot use co-factors like cetyl alcohol: at low use level, these emulsifiers will form thin and light emulsions. If you want thicker creams you should add the consistency factor anyway. 

W/O Emulsifiers

I never talk about them, but there are also some emulsifiers that are used in water-in-oil emulsions. These emulsifiers are more lipophilic than the ones listed above. Some examples are:

  • Lanolin: 2-15%, often coupled with lanolin alcohol (lanolin 2-4%, lanolin alcohol 3-4%)
  • Lanolin alcohol: 2-8%, often coupled with lanolin
  • Sorbitan olivate wax (Olivem 900), 7%

The stability of W/O emulsions is typically achieved by addition of waxes in the oil phase and by the combination of fats in order to achieve a broad melting point spectrum, such that the external oil phase provides a sort of network in which the water droplets are stabilised. Sometimes you can find W/O formulations in which the lipophilic emulsifier (those listed above) is coupled with a O/W emulsifier and with a salt like magnesium sulphate: this helps stabilising the emulsion and avoiding “sweating” of the final preparation (8). 

References

(1) Oldenhove de Guertechin (2017), Surfactants: Classification. In Handbook of cosmetic science and technology (eds. Barel, Payel, and Maibach)

(2) Lochhead (2017), Basic Physical Sciences for the Formulation of Cosmetic Products. In Cosmetic Science and Technology: Theoretical Principles and Applications (eds. Sakamoto, Lochhead, Maibach, and Yamashita)

(3) Nakama (2017), Surfactants. In Cosmetic Science and Technology: Theoretical Principles and Applications (eds. Sakamoto, Lochhead, Maibach, and Yamashita)

(4) Cornwell (2018), A review of shampoo surfactant technology: Consumer benefits, raw materials and recent developments, International Journal of Cosmetic Science, 40, 16-30

(5) Surfactant active matter calculator, Making Skincare https://makingskincare.com/surfactant-calculator/

(6) Yang (2017), Hair Care Cosmetics. In Cosmetic Science and Technology: Theoretical Principles and Applications (eds. Sakamoto, Lochhead, Maibach, and Yamashita)

(7) Sulfate-free surfactants, Making Skincare https://makingskincare.com/sulfate-free-surfactants/

(8) Käser (2019), Natrukosmetik selber machen – Das Handbuch (ed. Freya)

(9) Emulgatoren, Olionatura https://www.olionatura.de/kosmetikrohstoffe/emulgatoren

(10) Meet Sucrose Stearate, Skinchakra https://skinchakra.eu/blog/archives/370-Meet-Sucrose-Stearate.html

Attached files

I realised there was some weird problem with uploading the file directly via WordPress, so I decided to upload it on Gumroad. In the file bundle you can find the ASM calculator (for Excel and Numbers) and the list of surfactants from different suppliers. Gumroad is a selling platform but the files are downladable for free, you DON’T need to pay 🙂

Click here to redirect to Gumroad

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