Surfactants part 3 - Surfactant subtypes

In Surfactants part 1 - What is a surfactant of this series of blog posts we discussed what surfactants are, how they work, and what their different functions are. In Surfactants part 2 - Types of surfactants we looked at the different types of surfactants. In this third blog post, we will go further in depth in the chemical nature of some of the more common surfactant subgroups like, amine-oxides, alkyl poly glucosides (APGs), docusates, betaines, and amino acid surfactants.

In parts one and two of this series of blog posts, we looked at the general structure of surfactants, how they are made up of a hydrophobic tail and a hydrophilic head, and how based on the charge on the head four groups can be created, anionic (negative charge), cationic (positive charged), nonionic (not charged) and amphoteric (positive and negative charge).

While it is of great importance to know whether a surfactant is carrying a certain charge, that is only half the story. It is of equal importance to know the actual structure of the molecule; not all surfactants with a specific charge perform the same and changes in a formulation affect surfactants differently, even if they are charged the same.

For anionic surfactants, an easy way to differentiate between the subgroups is to look at the hydrophilic head and examine which atom allows it to be negatively charged. There are 3 commonly used possibilities, Sulfur (S), Phosphorus (P), and Carbon (C). These atoms on their own would all be positively charged, which is why they attract negatively charged Oxygen (O). Simply put, however, these three molecules don’t have enough space around them to satisfy the needs of all oxygen atoms, which is why ultimately all of these will have one negatively charged oxygen attached to them, giving the molecule a negative, anionic charge.

The sulfate surfactants are very commonly used. Ingredients like Sodium Lauryl Sulfate (SLS), Ammonium Lauryl Sulfate (ALS), or Dioctyl Sodium Sulfosuccinate (Docusate) can be found in almost every cleaning agent, shampoo, toothpaste, soap, or anything in between. They are also quite commonly used as a surfactant in some laboratory and pharmaceutical applications, as well as in the food industry as an additive.

Sulfate surfactants are high to very high foaming, with Lauryl Sulfates being exceptionally foamy. Like other anionic surfactants, sulfates can be a bit harsh, with some having the potential to be slightly irritating if used regularly.

Some relatively mild, yet still very well foaming, sulfate surfactants are the isethionates, such as Sodium Cocoyl Isethionate pictured above, and the very similar taurates, like Sodium Methyl Cocoyl Taurate. Both of these subgroups will work in hard water unlike a lot of other surfactants. They are often used in combination with amphoteric surfactants to improve their relatively low solubility.

Phosphate surfactants are often found in the form of phosphate esters. These surfactants are often not suitable for use in cosmetics but are perfect for a wide range of cleaning applications. They are generally made from (often natural) fatty alcohols which are phosphorylated to replace the alcohol with the phosphorus, giving the molecule a negative, anionic charge.

The advantages of phosphates lie mostly in their stability and solubility. These molecules are stable over an often very wide pH range while simultaneously being very soluble. Phosphate esters are mostly medium foaming, but they can range from low to high. They are often the surfactant of choice for wetting and emulsifying agents in non-cosmetics applications. Another advantage can be that phosphate esters provide great corrosion inhibition.

Carboxylates are traditionally amongst the most widely used surfactants. Many soaps, shampoos, deodorants, and even pharmaceuticals and food additives make use of these surfactants. Very common examples include Sodium Stearate and Sodium Lauroyl Sarcosinate (Sarcosyl). These surfactants are made through saponification of fats and oils. This means adding a base to the fats and oils, which then gives a carboxylate.

Carboxylates are often less harsh and lower foaming than sulfates. Their milder characteristics make them ideal for use in a wide variety of personal care and cosmetics products.

Amino acid surfactants (AASs) are a special case in the anionic surfactants. As the name suggests, AASs are based on amino acids. These organic molecules form the building blocks for all living things. Amino acids also have a very self-explanatory name: these molecules have an amine group on one side of the molecule and a carboxylic acid group on the other side. In living systems, this makes them very versatile building blocks, as other molecules can be attached very specifically through different processes on either side of the molecule. The same is true when they are used as surfactants. There are many, many amino acids but only a handful are really used to make surfactants. Amino acids all share the same backbone (amine-carbons-acid), but they differentiate by what is attached to the carbon atoms in the backbone. This makes the group of AASs a very specific group of surfactants while also being very diverse and broad. They all share similar properties like being very mild and very biodegradable. Most AASs have a hydrophobic tail attached to the amine side of the molecule while leaving the acid side free. This gives the molecules a look similar to that of a traditional surfactant with a long hydrophobic tail on one side and a hydrophilic head on the other. Others, however, don’t use the traditional long fatty tails, these molecules will therefore rely more on the natural polarity that exists in amino acids.

Frequently used amino acids are glutamic acid, glycine (sarcosine is a glycine derivative), arginine, aspartic acid, and alanine. While these are very different amino acids, the surfactants they make are all quite similar. They offer very mild cleansing properties, allowing them to be used in products even for sensitive skin. As they are made from organic building blocks, they also boast very low toxicity while maintaining high efficacy. To add to this, they can be very moisturizing. Furthermore, AASs with two acid groups, like those based on the amino acid glutamic acid (Disodium Cocoyl Glutamate, Sodium Lauroyl Glutamate, or Tetrasodium Glutamate Diacetate), can also act as excellent ecofriendly alternatives to traditional chelating agents and water softeners.

Cationic surfactants are less widely used than their anionic counterparts. There are also fewer options to choose from. Almost all cationic surfactants are positively charged because they contain an amine function in their structure, this is a basic nitrogen atom. We can differentiate these into two subgroups: permanently charged and pH dependent surfactants.

These are the vast majority of cationic surfactants used in the personal care, cosmetics, and cleaning industries. As the name suggests, these molecules contain an amine (nitrogen atom) that has a permanent positive charge. Most commonly, these are quaternary ammonium compounds. This means that in the hydrophilic head of the surfactant molecule a nitrogen atom is present that has 4 connections to other atoms, which is one more than it would like and therefore it is positively charged. Some commonly used permanently charged cationic surfactants include Cetrimonium Chloride (CTAC), Cetrimonium Bromide (CTAB), and Behentrimonium Chloride (BTAC).

These cationic surfactants are, as the name suggests, only cationic under certain pH conditions. Amines that are not normally charged gain a positive charge if exposed to a high pH (often over 10). While these can be extremely useful in certain niche conditions, the high pH necessary for these surfactants to work makes them unsuitable for personal care and most cleaning products.

Nonionic surfactants are a very versatile group of surfactants. They gain their surfactant workings through their ability to form water bridges between water molecules and their hydrophilic heads. Because they have no charges, they can be mixed with all other kinds of surfactants. Nonionic surfactants are less easily differentiated based on structure alone as they all look very similar, which is why, for these surfactants, we will make subgroups based on the base materials used to make the surfactants. The subgroups we will discuss are the Alkyl Poly Glucosides (APGs) and the Ethoxylates.

The subgroup of Alkyl Poly Glucosides (APGs) are plant based surfactants and a naturally derived group of surfactants. They consist of multiple (poly) sugar (glucose) molecules which make up the hydrophilic head, with a hydrophobic alkyl tail. The tails are often made from fatty alcohols. These, as well as the glucoside that makes up the head, are almost always plant based and therefore highly biodegradable. For this reason, APGs are often the surfactant of choice for green, ecofriendly products. Their nonionic nature makes them mild and safe even for sensitive skin. They can also very easily be combined with a small amount of co-surfactant like an anionic (more cleansing) or a cationic (more conditioning) surfactant. Commonly used APGs are Caprylyl/Capryl Glucoside, Caprylyl/Decyl Glucoside, Lauryl Glucoside, Decyl Glucoside and Coco Glucoside.

Ethoxylates are a very wide range of surfactants that have one thing in common, they’re ethoxylated. Ethoxylation happens when ethylene oxide is added to another compound. This compound could be a fatty acid, a fatty alcohol, an alkylphenol, amines, or even amides. They all behave in similar manners with only very minor differences. Oftentimes, when one is debating on using an ethoxylate surfactant, the choice should be made more on characteristics like biodegradability, toxicity, and the other chemicals in the formulation. The differences in the surfactants stem from the different compound that is reacted with the ethylene oxide. These different compounds may affect viscosity, solubility, and biodegradability, to name a few.

Amphoteric surfactants, or zwitterionic surfactants, are the last of the groups to discuss. As described in the last blog, these surfactants have both anionic and cationic centers, making them equally as versatile as nonionic surfactants. The cationic centers are almost always based on amines, while the anionic centers have about as many possibilities as regular anionic surfactants. Amphoteric surfactants are often natural and organic compounds. These molecules can be commonly found in many vital biological processes. An example of these molecules in the human body are the phospholipids, which make up our cell membranes.

Amine oxides are the first subgroup of the amphoteric surfactants we will discuss. As the name suggests, the cationic center is an amine, and the anionic center is an oxide. They can be used for different applications, from cleaning and detergent uses to conditioning. This makes them very viable in a wide range of products in both personal care and cleaning industries. Another very interesting aspect about this group of compounds is that they boast antimicrobial properties. Common amine oxides are Lauramine oxide and Myristamine oxide.

The last subgroup we will discuss is the betaines. The name betaine used to refer to a specific molecule (trimethylglycine), but these days betaines are technically any molecule that has a negative center and a positive center which are not adjacent. The betaines used in the personal care and cleaning industry also fit this description but are rarely ever derived from trimethylglycine. A good example of a betaine is the Cocamidopropyl Betaine (CAPB). This ingredient is often used in soaps, shower gels, shampoos, and many other products. Its cationic center is a quaternary ammonium, and its anionic center is a carboxylate. These are common for betaines. The name ‘cocamidopropyl’ can be split into two parts: ‘-amidopropyl’ refers to the unit linking the hydrophilic betaine to the hydrophobic tail, and ‘coc-’ comes from coco, or coconut, which refers to the origin of the hydrophobic tails. Since coconut oil is a natural product, it’s quite time consuming and expensive to extract a specific betaine from the mixture. As such, a lot of products are simply sold as ‘coco’, even though they are made up of a mixture containing Lauramidopropyl Betaine and many other parts. Like all amphoteric surfactants, betaines are quite mild when used as a detergent and they can also be used as a conditioning agent. Betaines also make for excellent co-surfactants.

While the subtypes mentioned in this blog post is by no means meant to be an exhaustive list, it does give a good overview of how different similar molecules can act. It is therefore always advisable to do extensive research into a specific product, or at least into the specific subtype of surfactant.

In the next and last blog post in this series, Surfactants part 4 - Natural surfactants we will discuss the upcoming world of natural, green, and ecofriendly surfactants.