Effects of Additives on the Flow Behavior of Lamellar-Structured Concentrated Surfactant Solutions

Consumer cleaning products including shampoos and detergents often contain a large amount of water, a critical component typically required at all stages of the product's lifecycle. The water we use in the shower is generally sufficient to quickly dilute personal care products and use them effectively. Changing their consistency from a slow-flowing liquid to a thick paste, effectively concentrating them, would have significant economic and environmental benefits.

Making concentrated formulations, however, is more complicated than simply removing as much water as desired. Conventional production methods cannot process concentrated surfactant-based materials without altering their microstructure and potentially compromising material performance. Unless multiscale material relationships for concentrated feedstocks are developed, industry will continue to waste valuable resources on inefficient and unsustainable manufacturing processes.

In this study, material relationships at low temperatures were determined for concentrated surfactant solutions using a combination of rheological experiments, cross-polarized microscopy, calorimetry, and SAXS. A lamellar structured 70 wt.% solution of sodium laureth sulfate in water was used as a model system. At cold temperatures, the formation of surfactant crystals resulted in extremely high viscosity. The bulk flow behaviour of multi-lamellar vesicles and focal conic defects in the lamellar phase was similar. Shear-induced crystallization at temperatures higher than the equilibrium crystallization temperature range resulted in an unusual complex viscosity peak. The effects of processing-relevant parameters including temperature, cooling time, and applied shear were investigated. Further, industrially relevant additives – sodium chloride, propylene glycol and cetostearyl alcohol – were separately added in varying concentrations (0.25-20 wt.%) and structure-property relationships were experimentally established. Sodium chloride and propylene glycol behaved as desiccants and plasticizers, respectively, while the fatty alcohols increased the surfactant bilayer thickness. Outcomes from this study help answer unexplained processing behavior for concentrated feedstocks and will have implications in improving the sustainability of consumer products.