Impact of surfactant polydispersity on the phase and flow behavior in water: the case of Sodium Lauryl Ether Sulfate

This study delves into the impact of molecular polydispersity on the phase behavior of Sodium Lauryl Ether Sulfate (SLES) surfactant, aiming to deepen understanding of its implications for fundamental science and industrial applications. SLE3S is utilized as a model compound: a comprehensive characterization of molecular polydispersity is conducted using Gas Chromatography–Mass Spectrometry and Nuclear Magnetic Resonance spectroscopy, juxtaposing the findings with those for SLE1S. Our comprehensive investigative approach entails: (i) employing Time-Lapse dissolution experiments in microchannel geometries to observe the dissolution and phase transitions; (ii) utilizing polarized light microscopy, confocal microscopy, and Small Angle X-ray Scattering for microstructure identification assessments; (iii) conducting rheological evaluations at various concentrations and temperatures to determine their effects on the surfactant properties. The findings reveal that SLE3S, being more polydisperse, demonstrates complex phase behavior not observed in the less polydisperse SLE1S. Notably, SLE3S exhibits a unique concentration domain, corresponding to a concentration of about 60 %wt, where hexagonal (H), cubic, and lamellar (Lα) phases coexist, resulting in highly viscoelastic heterogeneous mixtures. This behavior is attributed to the local segregation of surfactant components with varying polarity, underscoring the crucial role of molecular polydispersity in the phase behavior of SLES surfactants.