The world of surfactants from the perspective of titration

What do personal care products like shampoo and cleaning products such as detergents have in common? They contain surfactants which facilitate the mixing of generally immiscible substances such as oil and water. The word «surfactant» itself is composed from the term «surface-active agent», and this describes the mode of action precisely: surfactants decrease surface tension and allow the formation of a dispersion between different substances. Surfactant determination is crucial for the quality control of everyday products (e.g., shampoo, detergents, toothpaste, disinfectants) as well as raw materials. When reliable determination of the surfactant content is required, potentiometric titration is the best choice. Titration of surfactants can be challenging since many factors (described in more detail in this article) have to be considered. Metrohm has ample experience in method development and offers various titrators and sensors well-suited for reliable surfactant content determination.

Figure 1. Basic structure of different surfactants.

Introduction to surfactants

All surfactants consist of a polar and apolar part, or more precisely a hydrophilic (water-attracting) and hydrophobic (water-repelling) part. The hydrophobic segment is very often a (long-chained) alkyl group, while the hydrophilic part varies according to the nature of the surfactant, either anionic, cationic, or non-ionic (NIO). Anionic surfactants have an anionic functional group such as a carboxylate or sulfate, cationic surfactants mainly have quaternary ammonium groups as the functional group, and NIO surfactants neither contain dissociable functional groups nor do they form charged molecules. The different structures of these surfactant types are depicted in Figure 1.

NIO surfactants based on POE adducts form a pseudo-cationic complex with barium chloride. This complex is then titrated with the anionic surfactant STPB.

Figure 2. NIO surfactants based on POE adducts form a pseudo-cationic complex with barium chloride. This complex is then titrated with the anionic surfactant STPB.

Measurement of surfactant content by titration

Each of these various surfactant types can be determined by potentiometric titration. The principle behind this is a precipitation titration, meaning that anionic surfactants are titrated with a cationic surfactant and vice versa.

But what does this mean for NIO surfactants which do not dissociate into charged functional groups?

NIO surfactants containing polyoxyethylene (POE) groups require an extra step. Upon addition of BaCl₂, they form a pseudo-cationic complex (Figure 2). This complex is then titrated with sodium tetraphenylborate (STPB).

NIO surfactants based on alkyl glycosides (AG) and alkyl maltosides (AM) need an additional sulfonation step before they can be titrated just as anionic surfactants in a two-phase titration.

Aqueous vs. two-phase titration for surfactant analysis

Depending on the sample, the surfactant content is either determined in aqueous media or in a mixture of water and organic solvents.

In contrast to anionic and cationic surfactants, NIO surfactants can only be determined in pure aqueous media. However, this is not the only special aspect. A calibration factor has to be determined first for NIO surfactants, since the previously mentioned formation of the pseudo-cationic complex with BaCl₂ is not strictly stoichiometric. As shown in Figure 2, the bivalent barium cations are surrounded by the POE chain in a helical configuration. This structure is very flexible, and the inclusion of the cation is not specific. The NIO content is then expressed as x% of the standard equivalents. A good standard to use is Tergitol 15-S-9. In the past, Triton X-100 was a typical standard, but this chemical was considered to be a substance of very high concern by REACH.

Aside from titration in aqueous media, anionic and cationic surfactants can also be determined via two-phase surfactant titration. Typically, chlorinated solvents are used as organic solvents, based on the originally developed Epton method. In addition to using chlorinated solvents, which are potentially harmful to the environment and human health, the Epton method is time-consuming and uses color indicators which are a likely error source when it comes to the reliability of the results. However, Metrohm has proven in many applications that it is possible to replace chlorinated solvents with methyl isobutyl ketone (MIBK):EtOH (1:1) along with the use of sensors for more reliable endpoint detection.

The principle of two-phase surfactant titration is illustrated in Figure 3. On the left, the situation in the beginning of the titration is shown before the equivalence point (EP) is reached. Anionic surfactants are present in the aqueous phase (in blue). Upon titration with a cationic surfactant, precipitate is formed and migrates into the organic phase (in grey). As soon as the EP is reached, no anionic surfactants are present in the aqueous phase anymore (Figure 3, center). At the end of the titration after the EP is reached, an excess of cationic surfactant is present in the aqueous phase (Figure 3, right). Vigorous stirring is essential to mix the two phases properly and to ensure the migration of the surfactants between the phases.

Figure 3. Left: Situation before the EP, anionic surfactants are present in the aqueous phase. Center: Situation at the EP, precipitate moves to the organic phase and no anionic surfactants are left in the aqueous phase. Right: Situation after the EP, excess of cationic surfactants are present in the aqueous phase.

Choosing the correct pH when titrating surfactants

In addition to the choice of the media, which is mainly determined by the sample itself, the correct pH value plays an important role for a successful surfactant titration. For NIO surfactants, it is generally not necessary to adjust the pH to a defined value. Nevertheless, it is recommended to perform the titration in a pH range between 3–9.

In contrast, the pH has to be adjusted for anionic and cationic surfactants. For many anionic surfactants, bearing e.g., a sulfate group, the surfactant titration should be performed at pH 2–4. However, anionic surfactants with a carboxylate group should be titrated at pH 10–13. Many cationic surfactants are titrated at pH 3–5.

The adjustment of the correct pH can easily be performed with a SET pH titration with either an acid or a base, prior to the actual surfactant titration.

How to choose the right electrode for surfactant titration

Beside the consideration of the media and the correct pH value for a successful titration, a proper electrode also has to be used. Metrohm offers several electrodes, with suitability depending on the sample media (Figure 4).

For aqueous surfactant titrations, the NIO surfactant electrode, the Ionic Surfactant Electrode, and the Cationic Surfactant Electrode are available. While the first one is exclusively for NIO surfactant titrations, the other two electrodes can be used for both anionic and cationic surfactant titration. If only cationic surfactants are being titrated, it is recommended to use the cationic surfactant electrode. All three electrodes are coated with a polymer membrane which gets destroyed upon contact with solvents such as toluene or chloroform.

For two-phase surfactant titrations, either the Surfactrode Resistant or the Surfactrode Refill can be used. The Surfactrode Resistant must be used when chlorinated solvents are present. The Surfactrode Refill does not tolerate chlorinated solvents, and the active material of this electrode can be refilled, which basically ensures an unlimited lifetime.

All electrodes mentioned in this section must be used in combination with a reference electrode, such as the LL ISE reference electrode (Figure 4).

Figure 4. Metrohm offers several electrodes for the determination of surfactants in various matrices. From left to right: NIO surfactant electrode, Ionic Surfactant Electrode, Cationic Surfactant Electrode, Surfactrode Resistant, Surfactrode Refill, and LL ISE reference electrode.

Figure 5. Titration curve of the determination of an anionic surfactant in toothpaste with the Surfactrode Refill.

How to perform a surfactant titration

After pH adjustment, the surfactant titration is performed. It is no different from any other (precipitation) titration.

An example of anionic surfactant determination in toothpaste with a two-phase surfactant titration is shown in Figure 5.

The toothpaste determination shown here is performed with an OMNIS Titrator which minimizes user contact with potentially harmful chemicals. The dosing of the titrant is performed with an OMNIS cylinder unit with a resolution of 100,000 steps, giving both reliable and repeatable results. The benefit of using the Surfactrode Refill for this analysis is that the active material can be refilled. This essentially results in a «new» electrode every time the active material is filled up.

Summary

We interact with surfactants every day, often without even noticing, but they certainly improve our daily lives. The determination of the correct surfactant content in samples via potentiometric titration can be challenging since many aspects have to be taken into account. But with the correct choice of the pH, a proper electrode, and the implementation of a few tips and tricks, the determination of surfactant content with titration is both reliable and accurate.