What are Surfactants.
The word Surfactant is an abbreviation, a shortened way to explain “surface-active agent.” Surfactants are typically products made up of molecules containing a hydrophilic, or “water-loving” end, plus a hydrophobic, or “water-fearing” end.
The text book definition of a surfactant is that it’s “a substance that tends to reduce surface tension of a liquid in which it is dissolved.” Surfactants are a chemical additive we use to reduce surface tension so that the particular product we ” formulate,” will perform a specific function- reducing or eliminating surface tension. Sometimes you will hear the phrase “it makes water wetter.”
Surfactants are typically going to be dissolved in to something to do a specific job, or play a certain “role” so that when the surface tension is reduced or broken, the function will be either completed, or be easier to complete. They are used in a plethora of applications and products. Surfactants are everywhere, and play such a large role in products we have around us daily. When we wake up in the morning they are in our soaps and facial cleansers, our shampoos, shaving creams, and even our toothpaste when we brush our teeth. Surfactants are in hard surface cleaners we use to wash our countertops, in the car wash soaps we use. They are in the insect repellants and sun screen sprays that protect us when we play outside. Surfactants are in all kinds of commercial and industrial products too. Surfactants are a main constituent in paints and coatings and adhesives. Just about anything you can think of that is sprayed on, applied, brushed on, wiped on, smeared on probably has some form of surfactant in the formulation.
SURFACE TENSION and SURFACTANTS
Surfactant(s) break down the surface tension of water or a liquid. So, why is that important? Surface tension is defined as “the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid.” The liquid “film” or tension must be reduced or broken, so that the water and the surfactant that are blended together, can do the work the product is intended for. When this “tension” is broken or greatly reduced the product can get in and get to work getting the cleaning job done. Next, but just as critical though – the cleansers and other ingredients will LIFT the soil particles off the surface (whether it is your skin or a counter top) and help the residue and dirt etc. so as to not re-deposit, and allow the particles to be rinsed away . Using the right surfactants, along with the right “additives ” like chelates, pH buffers, etc. will assist the surfactant(s) in getting the cleaning job done, and if blended correctly, helps the cleansing process get accomplished in an efficient AND economical way. This description of cleaning is just one of many ways that surfactants are used. There are many more – like emulsifying, and solubilizing where surfactants serve specific purposes in formulations.
Surfactants are often classified according to their commercial use. Classifying them this way is not very useful because many surfactants have several uses. An easier way to sort and classify them is based on their “type.” We can break them down by dividing them in to the common types of the surfactants and how they relate to water and how common they are based on the amount / tonnage of production annually which should indicate their level of popularity.
So, to get started let’s review how the surfactants are classed and categorized.
Surfactants can be broken down into three types:
Lipophilic: is Oil Loving
Hydrophilic: is water loving
Hydrophobic: is water hating. They repel water
Types/ Classifications of Surfactants:
Surfactants have what many term an “electrical charge” on the water-loving (hydrophillic ) end of the molecule, that distinguishes between the different types of surfactants. Based on this “charge” there are four different, further categories, these are: Anionic, Nonionic, Cationic and Amphoteric. Again, we are not only dividing the surfactants by the charge, but we are listing them in order of tonnage produced/ usage / popularity.
Anionic Surfactants are the most popular surfactants. Anionics are dissociated in water, so they have a negative/ water loving anion*, and a cation. A large part of this type of surfactant include the alkylbenzene sulfonates (detergents- not used in personal care usually), , lauryl sulfate (foaming agent), (fatty acid) soaps, di-alkyl sulfosuccinate (wetting agent), lignosulfonates (dispersants) etc…
The anionic surfactants possessing a negative charge on their hydrophilic end, charge helping the surfactant molecules interact with soil particles, lifting and suspending soils in “bubble-like” arrangements called micelles. Generally, they are great foamers, but they do not tend to not be as good at emulsifying oily soils as some other surfactant types.
In personal care the anioinics are really widely used, probably the most common. You can easily recognize them – sodium lauryl sulfate, ammonium sulfate etc… They are widely used in peronal care because they are good foamers, clean well, and are for the most part affordable. They make good shampoos, body washes, etc.. They also do well in formulations where there are other typcail soap makming chemicals like fattty acids, lye etc..
Some people report that anionics are irritating to theier skin. This is of course due somewhat to their efficient foaming and cleaning. The use or overuse of anionics like sodium lauryl sulfate in soaps that are used often will increase sensitivity so if you are using an anioinic surfactant minimize it in the formulation and add another effective bu less irritating surfactant as a co-surfactant to make the product have less potential to irritate the skin, maybe a suitable amphoteric like cocamidopryl betaine for exampe.
When reading a list of ingredients on your cleaning products, you can usually identify the anionic surfactants as those that have the following in their names:
- Sulfonate/ Sulfate
(For example, sodium lauryl sulfate (SLS) liquid and the SLS powder, Sodium lauryl sulfoacetate (SLSa) , ammonium sulfate, sulfosuccinates, sarcosinates, isethionates, and taurates, sodium gluconate are all anioinc surfactants)
Production wise Anionic surfactants account for about 50 % of the world production.
Nonionic Surfactants are next. They do not ionize (thus non ionic) in aqueous solution, because their hydrophilic group is of a nondissociable type, such as alcohol, phenol, ether, or amide. A large group of the nonionic surfactants are made hydrophilic (water loving) by the presence of a polyethylene glycol chain (PEG) , obtained by the addition of ethylene oxide (EO) . They are called polyethoxylated nonionics. Popular types in this group are nonyl phenol ethoxylates, alcohol ethoxylates.
Ethoxylated non ionics are very useful in many poplar applications like washes and cleansers. Since they are ethoxylated, and by the ethoxylation, like the PEGS, they are made more water loving or hydrophilic. So with the hydrophillic end this makes many of the the nonionics excellent solubilizers to get many different types of products to blend together – like fragrance oils that are not so water friendly to blend in to water based formulations. They can also help blend dissimilar products in to formulations that are not water based, but oil based.
As far as the lipophilic group is concerned, it is often of the alkyl or alkylbenzene type, the former coming from fatty acids of a natural origin. The additon of propylene oxide produces a polyether which is slightly hydrophobic. These are called polyEO polyPO block copolymers, which are most often included in a different class, e.g. polymeric surfactants. Cleaning and usage wise – these are typically good at cleaning particles grease and dirt easily and are usually not as expensive as some other types/ classes of surfactants.
In the past 20 years or so glucoside (sugar based) groups, have been introduced in the market, and have become very popular because of their low toxicity, and for the fact that they are seen as more environmentally friendly being from renewable resources like corn sugars. These glucosides are sometiems called alkyl polyglucosides, or just polyglucosides. Some very popular ones are Decyl glucoside, coco glucoside, lauryl glucoside. There are also some popular blends of the glucosides, or that use a glucoside as a main or co-surfactant. One very popular blend featuring decyl glucoside ( a good foamer) as a co – surfactant blended in with another nonionic the sodium lauroyl lactylate which offers unsurpassed skin moisturizing, and a smooth feel when applied- the blend is often called polyglucose / lactylate blend, with the INCI: Decyl Glucoside (and) Sodium Lauroyl Lactylate. Try it for a new cleanser. It is a great performer.
Some nonionics are high foamers (most are not) , like the anionics, while others amy not generate much foam. Because of their lower foam profile and strong emulsifying potential, these surfactants are the preferred choice when formulating many types of cleaners – from degreasers, to spot cleaners. And many are used in personal care (Not the nonyl phenol type though)
Unlike anionic surfactants, nonionics are typically, undiluted, thick liquids
Some particular types of Nonionic surfactants include:
* Ethoxylates (nonyl phenol and others)
* Alkoxylates (alcohol ethoxylates)
* Glucosides /Alkyl PolyGlucosides
Non ionic surfactants are probably the most useful surfactants for formulators of products if you consider ease of use, cleaning ability and effectiveness. However remember most nonionics do not foam as well and may not have good foam stability. Which is why they are usually blended with surfactants from other families “ or types, like blending with an amphoteric like cocamdiopropyl betaine for example.
Since non-ionics lack a “charge” we discussed, they can easily work with every other category of surfactants, which means they are very versatile and useful in many applications.
Some common nonionic surfactants used in personal care and bath and body recipes are the polysorbates like polysorbate 20, Polysorbate 80 , Glyceryl Stearate, Emulsifying Wax NF, E-wax, Glyceryl Oleate, ingredients with the prefix PEG, Ceteareths, Oleths, Sorbitans, and the Coco Glucoside, and Lauryl Glucoside mentioned above.
Another very useful nonionic that’s often overlooked is the PEG 40 Hydrogenated Castor oil. As previously mentioned since it is ethoxylated with 40 mols of ethylene oxide this product has many great features since it can easily solubilize so many products like fragrances, essential oils, etc and while also creating foam. It has many applications and is a very useful “tool” in the formulators arsenal of surfactants – check it out soon.
You may have noticed a few of the products mentioned as nonionics are not just liquids. Many products other than just liquids might carry the term “non ionic,’ as well as fit in to these other categories too. Cationics for example – there are many that are not liquids like Varisoft EQ 65 and BTM 25 and BTMS 50.
Lastly, about non ionics – there is a complete classification system built around a particular aspect of the Non Ionics called the HLB System, and that is a topic for another time and another blog entry. It is a very useful system used daily by formulators to solve problems about how to more easily blend products together for stability, and effectiveness.
About 45% of the overall industrial production of all Surfactants are Non ionic.
Cationic Surfactants are dissociated in water into an amphiphilic cation and an anion, most often of the halogen type. A large portion of this class corresponds to nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or several long chain of the alkyl type, often coming from natural fatty acids. These surfactants are in general more * Anion: negatively (-) charged ion which moves toward anode during electrolysis. * Cation: positively (+) charged. These surfactants – tend to be more expensive than anionics, because of a the expensive ways they are made – synthesis. They are often used mainly in two areas – 1) as bactericides (and a sub category – the antistatic agents,) 2) as positively charged substance like in corrosion inhibition. They are less common in cleaners
They are also found in many disinfecting hard surface cleaners since they do have antimicrobial characteristics.
Cationics are not found as often as other surfactants in peronal care, but they have their place. They are often well suited in formulations where you need a surfactant but foaming is not the reason it is needed. Sometimes cationics have other purposes besides foaming. Cationics are often used for softening and for their ability to work with other ingredients. A good example of where you may see a cationic surfactant is in a hair conditioner. The cationics are compatible with with amphoteric and nonionic surfactants will be compatible with cationic surfactants. However, because of the opposing charge cationic (positive) and anionic (negative) surfactants do not work together and will bceome a “goo” if combined.”
A few common cationic surfactants used in bath and body recipes are your chlorides, the benzalkonium, centrimonium, stearalkonium chlorides. Many common cationic surfactants are irritating to the skin so limit the use to just what is ncessary.
Cleansers containing cationic surfactants cannot be mixed with those containing oppositely charged anionic surfactants. The molecules would interact with each other, producing a gooey mess that drops out of solution.
When reading ingredients lists, look for the words “chloride” or “bromide” (as in alkylbenzene ammonium chloride, and the chlorides methioned above) to identify cationics.
When a single surfactant molecule exhibit both anionic and cationic dissociations it is called amphoteric (or zwitterionic). Amphoteric surfactants, our final type of surfactant, may be the least used overall (they are used a lot in personal care products though) and least talked about surfactants but they are very useful in certain products and applications.
Amphoteric surfactants have little use on their own, but work extremely well in enhancing the cleaning and foaming effect of both anionic and nonionic surfactants. They can serve as “coupling agents,” which hold the formulation’s surfactants, solvents and the inorganic salt components together.
Amphoterics are usually named in a way that easily indicates they are amphoterics, as in the trade name “Amphoterge.”
Other examples of amphoterics are betaines (cocamidopropyl betaine ) and amine oxides.
Most amphoteric surfactants are sensitive to pH, whereas they are cationic at low pH and anionic at high pH, with an amphoteric behavior at their intermediate pH.
In personal care products, amphoterics like cocamidopropyl betaine are commonly used and often blended in with nonionics and anionics. If you see a product like a sodium lauryl sulfate or sodium lauryl ether sulfate, you will often also see the co-surfactant as cocamidopropyl betaine or lauramidopryl betaine (or both) . This type of blend works well because you get the foaming, the cleansing, but the overall harshness of the anionics have been “balanced” with the amphoteric so the product has been made to be an efficient cleanser and not as harsh as the anioncs used alone.
Some amphoteric surfactants are more expensive versus the non ionics and anionics, and consequently, their use may be limited to special applications such as cosmetics and high quality cleaners where their high biological compatibility and low toxicity is of primary importance.