Factors Influencing the Crystallinity and Structure of Sodium Lauroyl Isethionate

Sodium cocoyl isethionate (SCI) is an anionic surfactant widely employed as a foaming and cleansing agent in cosmetic formulations. It comprises a polar sulphonate headgroup connected through an ethyl spacer and ester linkage to an alkyl chain containing 8–18 carbon atoms. Sodium lauroyl isethionate (SLI) was selected as a model compound because its C12 chain is the predominant chain length in commercially manufactured SCI. This research investigated the physicochemical and structural changes occurring in SLI during high-temperature processing and subsequent storage. Static and dynamic laboratory-scale processing conditions were combined with thermal, spectroscopic, and crystallographic analyses to improve understanding of the behaviour of synthetic detergents during manufacture.

SLI was synthesised using excess lauric acid (LA) and recrystallised from methanol to an average purity of 98 wt.%. To simulate manufacturing conditions, SLI was isothermally aged under relevent conditions to assess for potental materials changes using thermal and chemical methods. Structural analaysis was also conducted via pXRD and temperature resolved SAXS to examine changes to crystallinity.

The influence of process relevent ageing on hydration was evaluated using dynamic vapour sorption and thermal analysis. Aged material was prepared and compared with unaged material to determine how structural changes may influence moisture sorption.

Finally, binary SLI–LA mixtures were prepared to represent commercial SCI, and a phase diagram was constructed to detail the relationship between blend composition and the thermal and structural properties.

Collectively, these results establish relationships between thermal history, lamellar organisation, hydration, and fatty-acid content in SLI, providing a structural basis for understanding SCI behaviour during manufacture and storage.