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.
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