Viscoelastic Properties of Micellar Lyotropic Nematic Liquid Crystals: Exploring the Impact of Temperature and Surfactant Concentration

The bulk elastic behavior of a nematic liquid crystal (LC) is commonly described by three elastic constants, involving splay (K11), twist (K22), and bend (K33) director deformations. While the elastic properties of thermotropic nematic LCs are well-understood, knowledge of the elasticity of lyotropic liquid crystals (LLCs) is still quite limited. In particular, for micellar systems, which represent the largest and most ubiquitous class of LLCs, no systematic measurements of all three elastic constants have been reported so far. By means of light scattering, this study presents the concentration and temperature dependence of the three elastic moduli and their corresponding viscosities (ηsplay, ηtwist, and ηbend) for a lyotropic nematic LC, formed by the surfactant N,N-dimethyl-N-ethylhexadecyl-ammonium bromide (CDEAB) and the cosurfactant 1-decanol (DOH), assembling in water into disk-shaped micelles. At increasing surfactant concentration, a pretransitional increase in the twist and bend viscoelastic parameters is found, indicating a strong divergence near the transition to the higher ordered lamellar phase. In contrast, the splay viscoelastic coefficients show an overall increasing, yet noncritical, behavior. Furthermore, all three elastic constants and viscosities decrease linearly with an increasing temperature. These findings, which add new insights into the viscoelasticity of micellar LLCs, are compared with experimental results on both thermotropic and other classes of lyotropic nematic phases and discussed in light of existing theoretical concepts.