High-Pressure SANS and NSE Experiments in Microemulsion Systems
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Abstract:
Small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy have been used to elucidate static and dynamic structures of a microemulsion system composed of a nonionic surfactant, water, and oil. Using the contrast variation neutron scattering technique, static structure parameters are evaluated, and the molecular volume change with pressure is calculated. The bending elastic modulus increases with increasing pressure. This tendency is similar to the microemulsion system composed of anionic surfactant, water, and oil. Therefore, a universal feature of the surfactant membrane as a response to pressure is clarified, that is, the surfactant membrane becomes rigid at high pressure due to the increase of density of the hydrophobic tail of surfactant molecules and their aggregates.Keywords:
Microemulsion
Neutron spin echo
Microemulsion
Radius of gyration
Hydrodynamic radius
Small-Angle Scattering
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The properties of bicontinuous microemulsions, consisting of water, oil and a surfactant, depend to a large extent on the bending moduli of the surfactant containing oil–water interface. In systems with CiEj as surfactant these moduli can be modified by the addition of diblock copolymers (boosting effect) and homopolymers (inverse boosting effect) or a combination of both. The influence of the addition of homopolymers (PEPX and PEOX, X = 5 or 10 kg/mol molecular weight) on the structure, bending modulus and dynamics of the surfactant layer is studied with small angle neutron scattering (SANS) and neutron spin-echo spectroscopy (NSE). Besides providing information on the microemulsion structure, neutron scattering is a microscopic probe that can be used to measure the local bending modulus κ. The polymer addition gives access to a homologous series of microemulsions with changing κ values. We relate the results obtained by analysis of SANS to those from NSE experiments. Comparison of the bending moduli obtained sheds light on the different renormalization length scales for NSE and SANS. Comparison of SANS and NSE derived κ values yields a consistent picture if renormalization properties are observed. Finally a ready to use method for converting NSE data into reliable values for κ is presented.
Neutron spin echo
Microemulsion
Quasielastic neutron scattering
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Neutron spin echo
Raft
Quasielastic neutron scattering
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Microemulsion
Neutron spin echo
Dynamics
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Time-resolved small-angle neutron scattering (SANS) was used to follow the formation of a microemulsion after rapid stopped-flow mixing of a lamellar phase (Lα), comprising the non-ionic surfactant TX-100 and water, with toluene. Initially after mixing, an oil-swollen Lα phase was formed. Between 4 and 160 s the Lα system evolved into a dense water-in-oil (w/o) microemulsion, during which time the two phases coexisted. After 160 s, the system was a pure single-phase microemulsion. SANS spectra were fitted using a linear combination of the phenomenological Teubner–Stey microemulsion and the Nallet lamellar models. The characteristic repeat distance of water domains in the microemulsion was seen to decrease over the first 70 s of the transformation as more of the microemulsion formed; conversely the domain correlation length appeared to increase.
Microemulsion
Lamellar phase
Small-angle X-ray scattering
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Microemulsion
Gelatin
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Neutron spin echo
Small-Angle Scattering
Biological small-angle scattering
SIGNAL (programming language)
Echo (communications protocol)
Quasielastic neutron scattering
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The ability to modulate the size, the nanostructure, and the macroscopic properties of water-in-oil microemulsions is useful for a variety of technological scenarios. To date, diverse structures of water-in-alkane microemulsions stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been extensively studied. Even though the decisive parameter which dictates the phase behavior of micremulsions is the nature of the continuous phase, relatively very few reports are available on the structure and interactions in the microemulsions of aromatic oil. Here, we present a fundamental investigation on water-in-xylene microemulsions using small-angle neutron scattering (SANS) at a fixed molar ratio (ω) of water to AOT. We elucidate the microstructural changes in the water-AOT-xylene ternary system at dilute volume fractions (Φ = 0.005, 0.01, 0.03), where the droplet–droplet interactions are absent, to moderately concentrated systems (Φ = 0.05, 0.10, 0.15, and 0.20), where colloidal interactions become important. We also characterize the reverse microemulsions (RMs) for thermally induced microstructural changes at six different temperatures from 20 to 50 °C. Depending on the magnitude of Φ, the scattering data is found to be well described by considering the RMs as a dispersion of droplets (with a Schulz polydispersity) which interact as sticky hard spheres. We show that while the droplet diameter remains almost constant with increase in the volume fraction, the attractive interactions become prominent, much like the trends observed for water-in-alkane microemulsions. With increase in temperature, the RMs showed a marginal decrease in the droplet size but no pronounced dependence on the interactions was observed with the overall structure remaining intact. The fundamental study on a model system presented in this work is key to understanding the phase behavior of multiple component microemulsions as well as their design for applications at higher temperatures, where the structure of most RMs breaks down.
Microemulsion
Small-angle X-ray scattering
Small-Angle Scattering
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Microemulsion
Neutron spin echo
Decane
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Neutron scattering is a powerful method to investigate hierarchical structure and dynamics of soft matter. In this article, several experimental results of surfactant systems, binary liquid mixtures with antagonistic salt, lipid membrane and hydration water by means of Small-Angle Neutron Scattering, Neutron Spin Echo and Quasi-Elastic Neutron Scattering were introduced.
Neutron spin echo
Neutron reflectometry
Soft matter
Biological small-angle scattering
Quasielastic neutron scattering
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