Abstract
The correlation between molecular changes and microstructural evolution of rheological properties has been demonstrated for the first time in a mixed anionic/zwitterionic surfactant-based wormlike micellar system. Utilizing a novel combination of DLS-microrheology and Raman Spectroscopy, the effect of electrostatic screening on these properties of anionic (SLES) and zwitterionic (CapB) surfactant mixtures was studied by modulating the NaCl concentration. As Raman Spectroscopy delivers information about the molecular structure and DLS-microrheology characterizes viscoelastic properties, the combination of data delivered allows for a deeper understanding of the molecular changes underlying the viscoelastic ones. The high frequency viscoelastic response obtained through DLS-microrheology has shown the persistence of the Maxwell fluid response for low viscosity solutions at high NaCl concentrations. The intensity of the Raman band at 170 cm−1 exhibits very strong correlation with the viscosity variation. As this Raman band is assigned to hydrogen bonding, its variation with NaCl concentration additionally indicates differences in water structuring due to potential microstructural differences at low and high NaCl concentrations. The microstructural differences at low and high NaCl concentrations are further corroborated by persistence of a slow mode at the higher NaCl concentrations as seen through DLS measurements. The study illustrates the utility of the combined DLS, DLS-optical microrheology and Raman Spectroscopy in providing new molecular structural insights into the self-assembly process in complex fluids.
Highlights
Surfactant molecules can self-assemble to form vesicles or micelles, either spherical or cylindrical.The geometric constraints in surfactant packing and the bending elastic energy of their monolayers dictate the final morphology as hydrophobic and hydrophilic domains interact dynamically with each other and the surrounding medium to achieve the lowest energetic state [1]
This study provides the first detailed systematic characterization of the microstructure/rheology evolution in this specific wormlike system as a function of salt concentration and utilizes a combination of Dynamic light scattering (DLS), DLS-optical microrheology and Raman Spectroscopy to correlate microrheology, molecular structural changes and microstructure
This is indicative of a significant change in size of the micelles compared to just the sodium lauryl monoether sulfate (SLES) or cocamidopropyl betaine (CapB)
Summary
Surfactant molecules can self-assemble to form vesicles or micelles, either spherical or cylindrical. The ultimate goal of obtaining mechanistic knowledge is to rationally design and efficiently fabricate new classes of materials that have desired properties and functionality This insight can be generated through a combination of characterization techniques that allows for the correlation of rheological, microstructural and molecular level structural changes. Than conventional mechanical rheometry, which is fundamentally limited by inertia, and can access the very high frequencies required to measure the critical (short timescale) dynamics of such materials These advantages are especially relevant for wormlike micelles, which can exhibit relaxation mechanisms spanning a wide range of timescales. We combine DLS-optical microrheology with Raman Spectroscopy to investigate the impact of ionic strength on molecular structure and associated changes in the microstructure and rheology of a mixture of anionic (sodium lauryl ether sulfate (SLES)), and zwitterionic (cocoamidopropyl betaine (CapB)), surfactants in aqueous solution. This study provides the first detailed systematic characterization of the microstructure/rheology evolution in this specific wormlike system as a function of salt concentration and utilizes a combination of DLS, DLS-optical microrheology and Raman Spectroscopy to correlate microrheology, molecular structural changes and microstructure
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