Date Published: October 8, 2014
Publisher: Public Library of Science
Author(s): Zhihu Yan, Caili Dai, Haishun Feng, Yifei Liu, Shilu Wang, Victor M. Ugaz.
The viscoelastic properties of worm-like micelles formed by mixing the cationic surfactant N-hexadecyl-N-methylpiperidinium bromide (C16MDB) with the anionic surfactant sodium laurate (SL) in aqueous solutions were investigated using rheological measurements. The effects of sodium laurate and temperature on the worm-like micelles and the mechanism of the observed shear thinning phenomenon and pseudoplastic behavior were systematically investigated. Additionally, cryogenic transmission electron microscopy images further ascertained existence of entangled worm-like micelles.
It is well known that amphiphilic surfactants exhibit an impressive range of polymorphic self-aggregates in solution, such as micelles, micro emulsions, liquid crystals and vesicles –. Among these organized aggregate structures, worm-like micelles are spontaneously formed entangled networks that impart excellent viscoelastic properties to the surfactant solutions similar to those of flexible polymer solutions. However, unlike flexible polymers that are connected by strongly covalent bonds, worm-like micelles are held together by weaker intermolecular forces that can reversibly break and recombine easily. Owing to this unique characteristic, over the past two decades, worm-like micelles have become popular in many applications, such as fracturing fluids, heat-conducting fluids, and personal care products –.
In summary, with the aid of sodium laurate, the aggregation behavior of worm-like micelles in aqueous solution was observed using N-hexadecyl-N-methylpiperidinium bromide as the cationic surfactant. Rheological measurements revealed that the main factors affecting the viscoelastic properties of worm-like micelles are temperature and the concentrations of sodium laurate. The scission recombination mechanism combined with the higher diffusion rate of the cylindrical part of the micelles leads to pseudoplastic behavior and shear thinning in the steady shear rheology experiments. For the C16MDB/SL mixed solutions, the estimated values of several important rheological parameters as functions of the solution composition and temperature are calculated. At the same time, the flow activation energy and the scission energy of worm-like micelles formed in 70 mM C16MDB/40 mM SL solution were compared with other published results. Cryo-TEM imaging visually observed the formation of entangled micelles, which supports the rheological experimental results. We believe that this work has further explained the mechanism of formation of worm-like micelles with high viscoelasticity and serves as a possible application of ILs in colloidal systems.