Lauric acid-induced formation of a lyotropic nematic phase of disk-shaped micelles

Abstract

Addition of small amounts of lauric acid (LA) to a micellar solution of sodium dodecyl sulfate (SDS, 11.5 wt %) and cocamidopropyl betaine (CAPB, 3 wt %) has a dramatic effect on the rheological properties and phase behavior of the system. The viscosity increases by more than 1 order of magnitude up to a weight ratio LA/SDS = 0.17 and decreases for further LA loading. The decrease in viscosity is associated with the formation of a birefringent liquid crystalline phase. The evolution of the system from isotropic micelles in the absence of LA to lyotropic liquid crystals up to a weight ratio LA/SDS = 0.30 was probed by a combination of (23)Na NMR quadrupolar splitting, measurements of water and surfactant self-diffusion coefficients via (1)H-PGSE-NMR, and rheology. The evolution of the water self-diffusion coefficients indicates that LA induced a dramatic increase in the anisotropy of disk-shaped micelles. Birefringent samples always showed a well developed (23)Na quadrupolar splitting with a line shape typical of monodomain samples. This suggests that the whole sample is easily oriented within the spectrometer electromagnet, as usually observed for nematic liquid crystals. Sample spinning first destroys the alignment (only a single peak is discernible in the (23)Na NMR spectrum). Then, upon prolonged spinning, the alignment develops again. This indicates that the system is composed by disklike micelles aligning themselves with their normal perpendicular to the magnetic field. On the other hand, the linear viscoelastic response close to the nematic transition shows features usually observed in wormlike micellar systems (e.g., nearly Maxwellian behavior). To reconciliate the rheological data and the NMR evidence of disklike micelles, the formation of columnar stacks of disklike micelles is proposed. The rheology of the isotropic phase can therefore be interpreted in terms of entanglements of "living columnar stacks" of disklike micelles, and the nematic phase observed at high LA content could be attributed to a nematic columnar phase N(Col) formed by the alignment of such stacks.