Laboratory Rheo-SAXS Studies of Non-ionic Surfactants
Non-ionic surfactants are widely applied in the form of cleaning agents or detergents. Here we report on a combined rheological-scattering (Rheo-SAXS) study of an ethoxylate surfactant in water system. Measurements were performed using the SAXSpoint 5.0 laboratory SAXS/WAXS beamline with the unique and fully integrated Rheo-SAXS module. The obtained results prove a transition from lamellar to onion-like structures with increasing shear rate.
Non-ionic surfactants find wide applications in many fields: they are used as detergents, wetting agents, emulsifiers and solubilizers in cosmetics, personal and health care, textile and other industrial sectors. Non-ionic surfactants are a class of surface-active agents that do not disassociate into ions in aqueous solution. In comparison to ionic surfactants, some nonionic surfactants have several superior properties, e.g. superior cleaning performance, higher solubility, in particular in hard water and also a better chemical stability.
An important analytical method for analyzing the properties and behavior of surfactants is rheology. It helps to elucidate the flow and deformation of matter under shear as well as the viscosity. Thus, rheology allows relating a material’s internal structure to its mechanical properties on the macroscopic scale.
Aqueous non-ionic surfactant systems typically form molecular assemblies and structures at the nanometer level. Small-angle X-ray scattering (SAXS) is the perfect tool for analyzing these structures in solution. SAXS is a non-destructive method which provides representative and averaged results that are valid for a large sample volume. It is therefore perfectly suited for the structural analysis of materials like surfactants. Regular SAXS can characterize the nanostructure of a sample in its stational state. However, for materials like surfactants, the determination of their nanostructure under shear condition is more important.
Usually, such experimental set-ups combining rheology and in situ scattering – which are used to determine both, the flow behavior and the nanostructure of a material simultaneously – can be found only at large-scale facilities like neutron (Rheo-SANS)1 and synchrotron (Rheo-SAXS)2 sources. Dedicated rheo-scattering sample cells of benchtop rheometers have a long sample path which can only be penetrated by a strong neutron or X-ray beam, respectively.
A way to make this method work in practice on a laboratory SAXS system is to design a fully integrated Rheo-SAXS module comprising a special cell design with an optimized geometry, heating/cooling ability and a low-scattering cell.
The SAXSpoint 5.0 system from Anton Paar offers brilliant state-of-the-art X-ray sources and optics, scatterless slit collimation, a wide range of various sample environments and latest hybrid photon counting detector technology from Dectris. The unique Rheo-SAXS module comprises the DSR 502 rheometer head and an optimized Rheo-SAXS sample cell which can be temperature controlled in a wide range from -10 °C to +90 °C (optionally to up to +200 °C).
- M. Ito, Y. Kosaka, Y. Kawabata, T. Kato, Langmuir, 27, 7400–7409 (2011)
- L. Gentile, et al., J. Phys. Chem. B, 118, 3622-3629 (2014)
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