Effect of Inorganic Salts on the Phase Behavior of an Aqueous Mixture of Heptaethylene Glycol Dodecyl Ether

Abstract

The effect of LiCl, NaCl, and CsCl on the phase behavior of an aqueous mixture of heptaethylene glycol dodecyl ether (C12E7) was investigated by means of differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and Fourier transform infrared (FT-IR) spectroscopy. Phase diagrams for the mixtures containing the inorganic salts with a concentration of 1.0 M were constructed on the basis of the DSC experiments. The addition of the salts induced an expansion of the lamellar (Lα) phase region toward higher temperatures and a shrinkage of the normal hexagonal (H1) and the bicontinuous cubic (V1) phase regions toward lower temperatures. The effectiveness of the salt species followed the sequence of CsCl < NaCl < LiCl for the expansion of the Lα phase region and of LiCl < CsCl ≤ NaCl for the shrinkage of the H1 and V1 phase regions. The influence of different salts on the phase behavior of the mixture is qualitatively interpreted in terms of the hydration of ions. The SAXS and FT-IR results revealed the salt effect on the molecular assemblies in the Lα phase and H1 phase and on the conformational changes of the surfactant molecules in the two mesophases, respectively.