Phase Behavior of Lyotropic Liquid Crystals in Linear Alkylbenzene Sulphonate (LAS) Systems in the Presence of Dilute and Concentrated Electrolyte

Abstract

A linear alkylbenzene sulphonate (LAS) system is studied in the presence of water and electrolyte (Na₂SO₄). The samples were studied, across a broad range of concentrations of surfactant and electrolyte, using optical microscopy, differential scanning calorimetry (DSC), and small angle x-ray scattering (SAXS). The results are contrasted with electrolyte free studies performed previously, and some startling differences are observed. An expected salting out phenomenon is observed as well as the presence of lateral phase separation. The latter phenomenon is remarkable, meaning that the lamellar phase consists of coexisting regions with different compositions rather than being homogeneous as previously thought. This behavior is dependent on the temperature and sodium sulphate concentration. This phase behavior has important implications for the formulation of detergents, and may be extended to help understanding of some biological phenomena. The phase diagrams are presented and the driving forces behind these phenomena are discussed, and a mechanism for lateral phase separation, as influenced by electrolyte, is proposed.

Keywords:

• Lamellar
• LAS
• lyotropic
• phase behavior

Acknowledgments

It is a pleasure to thank the EPSRC and Procter & Gamble for financial support. The authors are also grateful to the EPSRC Daresbury laboratory for access to synchrotron x-ray facilities.

Notes

From the Proceedings from Formula VI in Stockholm 2010.