Mixture of changing uniaxial micellar forms in lyotropic biaxial nematics

Abstract

Lyotropic nematics consisting of surfactant–cosurfactant water solutions may present a biaxial phase or direct U(+)↔U(−) transitions, in different regions of the temperature‐relative concentration phase diagram, for different systems and compositions. We propose that these may be related to changes of uniaxial micellar form, which may occur either smoothly or abruptly. Smooth change of cylinder‐like into disc‐like shapes requires a distribution of Maier–Saupe interaction constants and we consider two limiting cases for the distribution of forms: a single Gaussian and a double Gaussian. Alternatively, an abrupt change of form is described by a discontinuous distribution of interaction constants. Our results show that the dispersive distributions yield a biaxial phase, while an abrupt change of shape leads to coexistence of uniaxial phases. Fitting the theory to the experiment for the ternary system KL/decanol/D₂O leads to transition lines in very good agreement with experimental results. In order to rationalise the results of the comparison, we analyse temperature and concentration form dependence, which connects micellar and experimental macroscopic parameters. Physically consistent variations of micellar asymmetry, amphiphile partitioning and volume are obtained. To the best of the authors' knowledge, this is the first truly statistical microscopic approach that is able to model experimentally observed lyotropic biaxial nematic phases.

Keywords:

• biaxial nematics
• statistical model
• mixing and phase separation
• micelle form

Acknowledgements

The authors are grateful to the Brazilian agencies CNPQ, FAPESP, FAPERGS and CAPES for financial support. Monte Carlo simulations were performed at CESUP (Centro de Supercomputação da UFRGS). EFH thanks P. R. Krebs for helpful discussions and technical support concerning Monte Carlo simulations, Y. Levin for a brief discussion concerning micelle–micelle interaction mechanisms and P. S. Kuhn for reading the proofs.