Surfactant
Surfactants are surface-active substances, i.e., substances that reduce the surface tension of a liquid (usually water) and the interfacial tension with other liquids and solids. They therefore have a major influence on all types of interfacial contact and affect phenomena such as wetting, miscibility, and foaming. Surfactants occur naturally, in industrial products for virtually all areas of life, and in many industrial additives.
How are surfactants structured?
Surfactants are not a separate class of substances; they can have completely different chemical compositions. They are characterized by a two-part structure with a water-affine (hydrophilic) and fat/oil-affine (hydrophobic/lipophilic) area, i.e., surfactants are amphiphilic. In most cases, the hydrophilic part is small and short-chain, and the hydrophobic part is long-chain, which is why we refer to the "head" and "tail" of the molecule. In fact, this size ratio does not apply to all surfactants; the hydrophilic group can also be long-chain.
Why do surfactants reduce surface tension and interfacial tension?
Surface tension is caused by the attractive force (cohesion) between the molecules of a liquid. At the surface, this force does not act in all directions, which leads to tension along the surface. The situation is similar at oil-water interfaces because the attractive force between the phases (adhesion) is weak.
The chemically divided structure of surfactants causes them to accumulate at the interface, where they prefer to remain. The hydrophilic head faces the bulk phase of the water, while the hydrophobic tail faces outward or, in two-phase systems, toward the oil phase. The surfactant molecules displace the water molecules from the surface, weakening the cohesion there—and thus the surface tension.
How does surface tension depend on surfactant concentration?
As the surface becomes increasingly covered by surfactants, the surface tension initially decreases slowly and then more and more rapidly until it reaches saturation. This is because at the beginning, the distance between the adsorbed molecules is still large and therefore hardly any intermolecular interactions occur. The greatest drop in surface tension occurs in the region of almost complete saturation, and any further surfactant adsorption has a particularly strong effect on the surface tension. In this region, the dependence of the surface tension on the logarithm of the concentration is linear and has the greatest slope.
Above a certain concentration in the bulk phase, the surfactant molecules arrange themselves into agglomerates called micelles. Since the concentration of free surfactant molecules does not increase further, there is no further adsorption at the surface and the surface tension does not decrease further. The corresponding transition range is referred to as the critical micelle concentration (CMC), which is an important parameter for surfactants. Micelles are responsible for many of the useful properties of surfactants, e.g., their cleaning effect.
▶ Please also read the articles on the keywords micelle and critical micelle concentration (CMC).
What are surfactants used for?
Surfactants are used in countless processes and are a component of a wide range of industrial products. Depending on their intended use, surfactants are known by different names. These names are not a classification of the substances; one and the same surfactant can perform different functions.
Detergents (washing and cleaning agents)
The cleaning effect of surfactants is based on two complementary effects. On the one hand, the reduced surface tension ensures that water does not bead up but rather wets surfaces better and penetrates textiles more effectively. On the other hand, micelles bind fats and oils inside and mobilize them in the aqueous phase.
Emulsifiers
Emulsions are mixtures of oil and water that do not separate immediately, but only after some time and sometimes very slowly. When used as emulsifiers, surfactants reduce the interfacial tension and thus facilitate the division of a quantity of liquid into very fine droplets. Shelf life is also extended because the oil-water interface of the droplets is stabilized by surfactant molecules.
Wetting agents
Surfactants are used as wetting agents wherever aqueous solutions need to spread well on solid surfaces, e.g.
- In printing inks, paints, and inks, as well as coatings of all kinds
- In electroplating baths or etching baths and in electrolytic ore processing
- In pesticides
- In coolants/lubricants for mechanical manufacturing
Dispersing agents
Surfactants are also a component of many formulations in powder dispersions such as paints and varnishes. As dispersing agents, they improve powder wetting and prevent clumping.
Foaming agents
The formation of liquid foams is also an effect of the surface-active properties of surfactants. Whether as a bath additive, for firefighting, or for cleaning foams: depending on the desired product properties, suitable surfactants as foaming agents ensure fast or slow foam formation, coarse or fine-pored foam, and fast or slow decay.
How is the effect of surfactants measured?
The methods used to analyze the surface properties of surfactants and investigate their effectiveness are as diverse as the areas in which surfactants are used.
▶ Read more about this topic in our use case on surfactant characterization.
Surface tension
How efficiently and to what extent surfactants reduce surface tension is usually measured mechanically with force tensiometers, typically using the Du Noüy ring method or the Wilhelmy plate method. The related determination of the CMC (see above) is also carried out using these methods.
▶ In our video example , you can see how unwanted surfactant residues from cleaning processes can be detected by measuring the surface tension according to Wilhelmy.
Interfacial tension
The ring and plate methods are also used to investigate two-phase interfaces in order to evaluate the effectiveness of emulsifiers. Alternatively, measurements with a spinning drop tensiometer can be used, which is particularly suitable for measuring very small interfacial tensions.
Solid wetting
The effectiveness of a wetting agent can be determined by contact angle measurements. A drop is dispensed onto the substrate and its "roundness" is quantified.
Dispersibility
The Washburn sorption method reflects the contact angle and thus the wettability of a powder and is therefore used to formulate stable powder dispersions.
▶ Find out how powder wetting can be reliably determined using the Washburn Direct method in our application report AR293.
Foam formation
Time-dependent measurements of the amount of foam produced by bubbling or stirring, the decay dynamics, the bubble structure, and the liquid content provide a complete picture of the effectiveness of foaming agents and the properties of the foam formed.
▶ Read our application report AR279 on foam characterization of toothpaste formulations.
What types of surfactants are there and what are they used for?
Surfactants are categorized according to the nature of the hydrophilic group:
- Anionic surfactants: Compounds with an anionic group (e.g., carboxylates or sulfonates), often as alkali metal salts. Good foaming agents and cleaning agents, well suited for solvent-free degreasing; also frequently used as wetting agents to greatly reduce surface tension.
- Cationic surfactants: Compounds with a cationic group, e.g., quaternary amines. Good adsorption properties, e.g., on hair and textile fibers, which is why they are found in fabric softeners. Also antistatic and bactericidal.
- Nonionic surfactants: Compounds with nonionic, polar groups such as alcohol, ether, or ethoxylate. For low-foam applications and emulsion formation; unlike anionic surfactants, they are insensitive to water hardness.
- Amphoteric surfactants (also known as zwitterionic surfactants): Compounds with an anionic and a cationic group; often carboxylate and quaternary amine groups. Particularly skin-friendly and can be combined well with other surfactants; also effective as wetting agents and emulsifiers.
How quickly do surfactants work and what does this question mean for dynamic processes?
Surfactant molecules do not reduce surface tension abruptly. They need time to move to the surface (diffusion) and to adsorb there; the respective speed is described by the diffusion coefficient and the adsorption coefficient. In addition to external conditions such as temperature or concentration, these speeds depend on the size and spatial structure of the surfactant molecules.
▶ For the scientific background, please refer to our glossary articles on diffusion coefficient and adsorption coefficient.
The fact that the change in surface tension is time-dependent has a major impact on dynamic processes in which new surfaces are constantly being created. For example, when spraying paints or pesticides, only milliseconds pass between the formation of new droplet surfaces and contact with the material. If the surfactant is too slow, the surface tension may still be too high at this point and wetting may be poor.
To quantify the influence of the time factor, measurements of the dynamic surface tension are carried out using a bubble pressure tensiometer. The measurement records the course of the surface tension as a function of surface age and thus characterizes the dynamic behavior of a surfactant in solution.
Is the effect of surfactants limited to aqueous solutions?
Most surfactant applications are aimed at aqueous systems or water-oil phase boundaries. However, as amphiphilic molecules, many surfactants also dissolve in nonpolar organic liquids. Here, inverse micelles can form, in which the hydrophilic part is directed toward the interior of the agglomerate and can bind polar substances. One example of this is supercritical carbon dioxide for extraction processes, which can also dissolve polar substances with the help of surfactants.
