Simple-Complex Fluid Transition in Microemulsions

Abstract

The aim of this work was to study the rheological behavior of water in-oil microemulsion formulated with AOT (sodium bis(2-ethylhexyl) sulfoccinate) in isooctane over a temperature range from 25°C to 55°C for the oil/AOT volume fraction φ _m =0.1 and various AOT and H₂O molar ratios, W₀, ranging from 0 to 45. The apparent viscosity was measured vs. shear rate by a rotational concentric cylinder viscometer. It was shown that above s⁻¹, the solutions utilized behaved as Newtonian liquids. The absolute viscosities were deduced from the shear stress- shear rate rheograms. For W₀ < 17, the solutions behaved as simple fluids, where the viscosities decrease as the temperature increases. However, an opposite behavior was observed for W₀ > 17; the viscosity remained constant and then increased above a critical temperature, T_c, depending on W₀. For the molar ratio W_0,c = 17; the studied viscosity remained constant, η(W_0,c) = (1.17 ± 0.04) cP, for the temperature range studied. Thus, a simple–complex fluid transition was evident in the studied system. For the simple fluids, W₀ < W_0,c, the total activation energy vs. W₀ showed three linear parts. The different slopes were attributed to the change of the microscopic structure of the formed micelles. For the critical molar ratio W_0,c = 17, the total activation energy vanished. So, according to the Eyring lattice model, the jump of the formed micelles from one site to another was not permitted and the entire molecular groups were in vibration and rotation motions. For W₀ > W_0,c, where the formed micelles were fully hydrated, the relative viscosity of the studied solutions vs. temperature was fitted according a polynomial law, where the exponent was found to be dependent on W₀.

Keywords:

• complex fluid
• reverse micelle
• rheology
• simple fluid
• transition

Acknowledgment

The authors gratefully acknowledge financial support from the Tunisian Ministry of Education, Research, and Technology.