Phase behaviors and light scattering for dilute mixtures of C12E5 and lauric acid

Phase behaviors and light scattering patterns have been investigated for a water–C12E5–lauric acid system. Incorporation of small amount of lauric acid profoundly increases the existence region of the Lα phase. Light scattering pattern in the Lα phase shows diffraction peak much stronger than that for the sample without lauric acid. The spacing distance obtained from the peak position (∼250 nm) slightly decreases as the mole fraction of lauric acid in the total mixed solute (xa) increases from 0 to 0.175 where the total concentration of mixed solute (ctot) is kept constant (1.4 wt.%). The results are compared to the electrostatic theory of de Vries which takes into account the interplay between thermal undulations and electrostatic repulsion for salt-free lamellar phase. Addition of small amount of sodium chloride (0.1–1 mM) makes the diffraction peak broaden and increases the intensity of small angle scattering. Changes in phase behaviors and light scattering patterns along dilution line at a fixed mole fraction (xa=0.05) have been also investigated. In the concentration range 0.5<ctot<1.2 wt.%, the diffraction peak splits into two broad peaks, suggesting coexistence of two kinds of ordered phase.