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.
The world of surfactants from the perspective of titration
Jul 24, 2023
Article
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.
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.
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.