Cesium dodecyl sulphate phase behavior in aqueous solutions and comparison with the sodium dodecyl sulphate/water phase diagram

Abstract

The goal of this Master’s Thesis has been to interpret the phase behavior of the cesium dodecyl sulphate/water system and compare the findings with the analogous findings of the SDS/water system. The synthesis of CsDS has been examined through 2 different methods: recrystallization and ion exchange. The phase behavior of the CsDS/water system has been examined, using several characterization methods. Macroscopic observation with cross-polarized light was used to interpret the solubility curve in the concentration region 1 wt % - 30 wt % CsDS. X-ray Diffraction was used in order to identify the unit cell crystal structure of CsDS and deduce conclusions about similarities and differences among different CsDS samples, unheated and heated. Cryo-TEM was used in order to observe the CsDS micellar nanostructures in an aqueous CsDS solution at a concentration higher than the CMC of CsDS for the given temperature. Optical microscopy with cross-polarized light was used in order to check for presence of liquid crystals, as well as to verify the solubility curve observed by the macroscopic observation. Small Angle X-ray Scattering was used in order to identify the phase of the liquid crystal structure (hexagonal or lamellar) in the intermediate concentration region (30-50 wt %).This study also includes the examination of possible organic and/or inorganic impunities in the CsDS/water phase behavior in the low concentration region (5-40 wt%) using macroscopic observation with cross-polarized light. The studies have indicated certain differences between the Cs+ and the Na+ system. The solubility curve of the CsDS/water system in the concentration region 1 wt % - 40 wt % CsDS is relatively flat, characterized by higher Krafft temperatures than the SDS/water system, for the same concentration region. This can be explained by the differences in the hydrated radii between the alkali metals, Cs+ and Na+ .The larger ionic radius of Cs+ attracts weaker the electrons of the water molecules. This corresponds to a smaller hydrated radius for Cs+ compared to Na+ and therefore to less hydrophilicity and lower solubility (higher Krafft temperature) for a given concentration, for the CsDS/water system compared to the SDS/water system. Another difference is the width in terms of concentration of the two-phase region of liquid crystals and micelles. This is larger for the Cs+ system and it indicates stronger van der Waals forces between the less ordered micellar phase and the more ordered liquid crystal phase. Both ellipsoidal micelles and threadlike structures were observed on the same grid in a CsDS/water solution, using Cryo-TEM. The ellipsoidal micelles that were observed for the CsDS/water system are twice as large as the ones observed for the SDS/water system, while it is very likely that the threadlike structures are related to some birefringent, intermediate liquid crystals that were observed by the macroscopic observation with cross–polarized light, when the heating rate was larger than 0.2[°C/min]. Therefore, it is concluded that heating rates larger than that value can affect the structures observed macroscopically, as well as microscopically. The X–ray Diffractograms of all CsDS samples had similar 2·θ diffraction angles which may indicate similar unit cell crystal structures. The diffractograms of the unheated CsDS and SDS crystals also exhibit similarities in their diffraction angles in the diffraction angle examined.