Lyotropic liquid crystalline phase behavior and structure of cesium n-tetradecanoate-water mixtures

Abstract

The compositional sequence and microstructure of liquid crystalline phases formed by binary mixtures of cesium n-tetradecanoate (CsTD) and water (deuterium oxide) were studied from 297 to 338 K and from 35 to 95 wt% surfactant. The experimental methods used were deuterium ( 2H) quadrupole nuclear magnetic resonance (NMR) spectroscopy and optical polarizing microscopy. The Krafft boundary in the CsTD-water system is about 2–5°C, more than 45° lower than that in the comparable sodium n-tetradecanoate-water system. The structured surfactant phases observed were micellar; normal hexagonal; ribbon, a viscous birefringent phase of biaxial symmetry; viscous isotropic; and lamellar. The ribbon phase structure was deduced from the biaxial deuterium NMR spectral lineshape and by the contiguity of hexagonal and ribbon phases. This phase has not been observed previously by quadrupole NMR spectroscopy in a simple binary surfactant-water system. The phase transition from hexagonal to ribbon (ca. 65 wt%) is apparently second order as indicated by the lack of an observable two-phase region. A higher order transition is consistent with the proposed continuous variation in microstructure from rodlike aggregates to those of ribbonlike symmetry. The viscous isotropic phase, which has been observed in mixtures of potassium dodecanoate or sodium octanoate with water, was stable at temperatures above 310 K, at a composition of about 70 wt% CsTD. The lamellar phase is stable to very high surfactant concentrations at modest temperatures, >95 wt% CsTD at 338 K. The change in counterion from sodium to cesium lowers the Krafft temperature, enabling the study of liquid crystalline phase behavior from 5 to 100°C.