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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.

Basic structure of different surfactants.
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.