Surfactant (I): What is surfactant?
1. Introduction
In this series of articles on surfactants, we will learn the basics behind one of the common ingredients used in the personal care and cleaning industry. In Part 1, we will first look at what surfactants are and then how they work. In the next part of this post, we’ll cover the four surfactant groups (anionic, cationic, nonionic, and amphoteric) and some common surfactant subgroups. We will also delve into natural surfactants and environmentally friendly alternatives. +
2. What are surfactants?
Surfactants are one of the most commonly used ingredients in the personal care industry. From make-up preparations to shower gels or shaving foams, from wipes to conditioners, almost all cosmetic preparations and products contain one or more surfactants. The cleaning industry is also known to use surfactants extensively, but the use of surfactants is not limited to the cleaning and personal care industries. Surfactants are widely used in industries such as paints and coatings, pharmaceuticals, agrochemicals, industrial chemicals and petroleum extraction. While there are many different surfactants, there are some that almost everyone will recognize (e.g., sodium lauryl ether sulfate (SLES) and sodium lauryl sulfate (SLS)), and some surfactants have exotic names, such as sarcosine salts or amine oxide.
Surfactants _ _ _ _ are added to formulations for a variety of reasons. Anionic surfactants (negatively charged) are often added because they have effective cleansing or foaming properties, while cationic surfactants (positively charged) are added because they have conditioning properties. Then there are nonionic surfactants (uncharged) and amphoteric surfactants (amphiphilic molecules), which are often used in combination with other surfactants to enhance their performance. Some are used only to improve cleaning, others to emulsify the active agent, and still others are used only to stabilize formulations or improve viscosity. These functions may seem very different, but they all rely on a simple mechanism;
Surfactants all work by reducing the surface tension of a liquid. While there are many different types of surfactants, they all have the same basic chemical structure that allows them to do this. Surfactants are generally made up of two parts:
Hydrophilic head: this small part of the molecule usually prefers water.
Hydrophobic tail: this part usually has a long tail and hates water.
3. How surfactants work
Now that we know what surfactants look like, let’s talk about how they work. When reading about surfactants, you will often read things like surfactants reduce surface tension or interfacial tension. Although these terms are sometimes used interchangeably, it is important to distinguish them correctly to prevent confusion.
3.1. Surface Tension and Interfacial Tension
The difference between these two forms of tension is the nature of the tension. Imagine a beaker containing a liquid. The top layer of the liquid comes into contact with the air, causing the water to develop tension, which is called surface tension. In other words, surface tension applies to both liquids and gases. Now imagine that you add another liquid or solid to the beaker. These new substances also create an attraction to the liquid, and this tension is called interfacial tension. This interfacial tension can exist between two liquids, between a liquid and a solid, or even between a solid and a gas.
Some surfactants are good oil/water surfactants, while others are good water/oil surfactants. Also, some work better at the liquid/liquid interface while others work better at the liquid/solid interface. It is important to know that there is a difference between the two and, in a broad sense, what that difference is. However, this is also not the focus of this paper; therefore, from now on, we will refer to surface tension and interfacial tension collectively.
3.2. The role of the surfactant molecule
Surfactant molecules have hydrophilic heads and hydrophobic tails. This structure makes them behave in a certain way, which is why they are so useful in such a wide range of formulations.
Surfactant molecules have two parts, and they want to orient themselves in a way that best suits both parts of the molecule. This means that, ideally, the hydrophilic part is in the water and the hydrophobic part is outside the water. When a surfactant is added to water, the surfactant first collects on the surface with its tail sticking out of the water. When a certain concentration of surfactant is reached, their tails will no longer be able to stick out of the water, and some of them will be forced to sink underwater and into solution. If more surfactant is added, a critical micelle concentration (CMC) will be reached. Above this surfactant concentration, the solution
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