| Summary: | 'Structured liquid' detergent products have received much attention as a means of providing liquid detergent compositions with special rheological and other properties. These include the ability to suspend particles and storage stability at ambient temperatures. The challenge is to prevent separation of the product into two or more layers (which requires a high viscosity) while controlling the rheology to allow a sufficient ease of pouring acceptable to the consumer. Liquid laundry detergent compositions are generally formulated with a variety of active ingredients, typically one or more anionic surfactants, often in combination with a nonionic surfactant and detergent builder materials such as electrolytes. The microstructure of these 'liquids' (determined by the product formulation, process conditions and procedures) strongly influences the macroscopic properties such as rheology. In this work, the microstructure, rheology and aging of a range of model structured liquid have been studied. The model systems are complex aqueous mixtures of sodium alkylbenzene sulphate (LAS), sodium alkyl ether sulphate (SLES) and primary alcohol ethoxylate (NEODOL 25-7). The dependence of the phase microstructure on sample composition was investigated by added different amount of electrolyte (tri-sodium citrate, TSC) in the model system. The physical appearance of these systems varied from transparent to milky depending on the concentration of the electrolyte. All were viscous 'gels'. Optical microscopy and SAXS have been used to elucidate the basic microstructure, its variation with electrolyte concentration and temperature on the angstrom to micrometer length scale. Deuterium nuclear magnetic resonance (2H NMR) spectroscopy on 2H2O-enriched samples has been used to provide information about the phase behaviour of the liquid crystalline systems. Changes in spectral line shape and water quadrupole 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 including Linkam optical shear cell, AR 2000 rheometer and 2H Rheo-NMR. A variety of rheological patterns including simple stress sweeps and oscillatory rheology have been investigated and provided information about the effect of shear, shear time and nature of deformation of the model structured liquids. In this context, the relaxation kinetics of the shear-induced structures has been investigated using 2H Rheo-NMR spectroscopy.
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