Abstract
Here, we have investigated the synergistic growth of long wormlike micelles and their transformation into disklike micelles, which occurs in three-component solutions composed of sodium lauryl ether sulfate (SLES; anionic), cocamidopropyl betaine (CAPB; zwitterionic), and dodecanoic acid (HC12; nonionic). The solution rheology is characterized in terms of zero-shear viscosities and characteristic times for micellar breaking and reptation. Furthermore, the microstructure evolution, leading to the observed rheological behavior, is revealed by cryo-transmission electron microscopy (TEM) micrographs. In all cases, the CAPB-to-SLES ratio is fixed, whereas the fatty acid concentration is varied. At a certain HC12 concentration, the solution viscosity passes through a maximum. The cryo-TEM imaging indicates that wormlike micelles appear before the peak, grow further up to the peak, and finally transform into disklike aggregates (a very rare micellar structure) after the peak. The transformation of worms into disks leads to a drop in viscosity because the length-to-thickness aspect ratio of the disks is significantly lower than that of the worms. In this article, we elucidate the structure-rheology relations in micellar solutions that might be applied for the design of personal-care and household formulations.
Highlights
Amphiphilic molecules, when dissolved in water, self-assemble into different structures and mesophases.[1]
We will study the effect of a third component, namely, a medium-chain fatty acid (HC12), which in its nonionic form can reduce the intramicellar electrostatic repulsion and promote further micellar growth.[23−26,34] In Figure 2, we examine the rheological properties of our ternary solutions in the steady shear regime
We investigated the effect of fatty acid (HC12) on the solution rheology and microstructure at a fixed 4:1 cocamidopropyl betaine (CAPB)/sodium lauryl ether sulfate (SLES) molar ratio
Summary
Amphiphilic molecules (surfactants), when dissolved in water, self-assemble into different structures and mesophases.[1] Just above the critical micelle concentration (CMC), spherical micelles represent the dominant form of self-assembly.[1] At higher surfactant concentrations, above the so-called second CMC,[2] cylindrical, wormlike or disklike micelles appear in the solution. By increasing the surfactant concentration further, the amphiphilic molecules form liquid-crystalline mesophases, which typically appear in the following order: hexagonal, cubic, lamellar, inverted cubic, and inverted hexagonal.[1,3]. Rehage and Hoffmann,[7,8] followed by many others,[9−34] have shown that the zero-shear viscosity of such worm-forming solutions can pass through one or even two peaks as a function of the concentration of additive
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