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Rheology, Cryo-TEM and Small-Angle Neutron Scattering of Highly Viscoelastic Wormlike Micellar Solutions

Vania Croce

Vania Croce (previous student, left the group in 2004)

In this project the structure and rheology of wormlike micelles formed by aqueous solutions of erucyl bis(hydroxyethyl) methyl ammonium chloride (EHAC) with potassium chloride (KCl) has been studied. In order to understand the strong viscoelastic response of these systems, steady state rheology, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (Cryo-TEM) experiments were performed. The effect of surfactant concentration, added salt and temperature were investigated.

The interest in these systems is the marked increase in viscosity that appears on adding salt, which above a higher salt concentration (>6wt%) then reduces again. The effects are also strongly temperature dependent. The surfactant solutions have gel-like behaviour at room temperature and become Maxwellian, as the temperature is increased. Small-angle neutron scattering and Cryo-TEM begin to correlate these bulk rheology results with the nanoscale structures formed in solution. Both techniques indicated the formation of wormlike micelles. Salt-free solutions of surfactant form spherical micelles with a radius of 35 Å. Analysis of the scattering intensity curves in the high Q range in solutions containing salt, using the Kratky-Porod wormlike chain model, revealed a cross-sectional radius of gyration (Rx,s) of 21 Å. Additionally, Cryo-TEM images demonstrated the changes in the structure of the entangled network upon the addition of salt. At high salt concentrations (>6wt%) branched wormlike micelles are seen in the micrographs.

In conclusion, it can be said that the increase in viscosity is due to the formation of wormlike micelles and the decrease due to changes in the properties of the branching network.