Real-time monitoring of oil-induced micellar transitions in viscoelastic surfactants by small-angle X-ray scattering

Hypothesis: Viscoelastic surfactant solutions with entangled wormlike micelles (WLMs) display dramatic changes in rheological properties when exposed to hydrophobic substances. This change is key in the oil and gas industry and for drug delivery. The changes in viscoelastic properties are believed to be a result of changes in micellar shape due to the oil solubilization. The time dependence of the process has practical importance, yet its mechanism is unknown. We set out to map the structural changes with time using small-angle x-ray scattering (SAXS). Experiment: A surfactant system with erucamidopropyl hydroxypropyl sulfobetaine as the active ingredient was homogenized with three concentrations of n-decane (70 mM, 140 mM, 280 mM) in 600 mM CaCl₂. The samples were monitored with time using SAXS. Model fits were used to determine the structures present in the sample at each time interval. Findings: The entangled WLMs disappeared while spherical decane-swollen micelles formed with time, explaining the sharp decrease in viscosity. It was shown that the time at which the spherical swollen micelles appeared depends on the concentration of n-decane. This insight allows control of the spherical micelle appearance time, which is important for the successful application of WLMs in sectors that require WLMs to persist for a set time before they transform.