Mixed Surfactant Lamellar phases. Studies under shear

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Abstract

'Structured liquid' detergent products have received much attention as a means of providingliquid detergent compositions with special rheological and other properties. These includethe ability to suspend particles and storage stability at ambient temperatures. The challengeis to prevent separation of the product into two or more layers (which requires a highviscosity) while controlling the rheology to allow a sufficient ease of pouring acceptable tothe consumer. Liquid laundry detergent compositions are generally formulated with avariety of active ingredients, typically one or more anionic surfactants, often incombination with a nonionic surfactant and detergent builder materials such as electrolytes.The microstructure of these “liquids” (determined by the product formulation, processconditions and procedures) strongly influences the macroscopic properties such asrheology.In this work, the microstructure, rheology and aging of a range of model structured liquidhave been studied. The model systems are complex aqueous mixtures of sodium alkylbenzene sulphate (LAS), sodium alkyl ether sulphate (SLES) and primary alcoholethoxylate (NEODOL 25-7). The dependence of the phase microstructure on samplecomposition was investigated by added different amount of electrolyte (tri-sodium citrate,TSC) in the model system. The physical appearance of these systems varied fromtransparent to milky depending on the concentration of the electrolyte. All were viscous“gels”. Optical microscopy and SAXS have been used to elucidate the basicmicrostructure, its variation with electrolyte concentration and temperature on theangstrom to micrometer length scale. Deuterium nuclear magnetic resonance (2H NMR)spectroscopy on 2H2O-enriched samples has been used to provide information about thephase behaviour of the liquid crystalline systems. Changes in spectral line shape and waterquadrupole splittings are presented and discussed as a function of sample composition,temperature and aging process.The shear-alignment process of the lamellar microstructure has been examined includingLinkam optical shear cell, AR 2000 rheometer and 2H Rheo-NMR. A variety ofrheological patterns including simple stress sweeps and oscillatory rheology have beeninvestigated and provided information about the effect of shear, shear time and nature ofdeformation of the model structured liquids. In this context, the relaxation kinetics of theshear-induced structures has been investigated using 2H Rheo-NMR spectroscopy.

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