Characterization of wormlike micellar systems using DLS, rheometry and tensiometry

Alkyl polyethoxylates are surfactants widely used in vastly different fields, from oil exploitation to pharmaceutical applications. One of the most interesting characteristics of these surfactants is their ability to form micellar systems with specific geometry, the so-called wormlike micelle. In this study, dynamic light scattering (DLS), capillary viscometry, torque rheometry and surface tensiometry experiments were carried out in systems based on xylene, water, butanol (cosurfactant), and nona-ethyleneglycol-monododecyl ether (surfactant), with fixed surfactant:cosurfactant:oil composition (with and without oil phase) and varying overall concentration. The results showed that a transition from wormlike micelles to nanodrops was characterized by maximum relative viscosity (depending on how relative viscosity was defined), which was connected to maximum effective diameter, determined by DLS. Surface tension suggested that adsorption at the air–water interface had a Langmuir character and that the limiting value of the surfactant surface excess was independent of the presence of cosurfactant and xylene.

Scheme showing micelle formation as both oil and surfactant concentrations are raised: (a) at very dilute concentrations, surfactant is almost entirely in the bulk and oil at the surface; (b) hemimicelle formation; (c) system above CMC.Figure optionsDownload as PowerPoint slideResearch highlights▶ Wormlike micelle-nanodrop change was characterized by maximum effective diameter. ▶ This maximum coincided with the maximum relative viscosity obtained by viscometry. ▶ Turbulent behavior occurrence was used to rheologically characterize microemulsions. ▶ Adsorption isotherms suggested BuOH in nonionic systems behave as part of oil phase.