Phase behavior and molecular motion of aerosol OT in liquid-crystalline phases with water

Abstract

The phase diagram and molecular motion of sodium di-2-ethylhexylsulfosuccinate, or Aerosol OT, or AOT, with water mainly at 37 and 25°C, are explored. Results are reported on visual observations, polarizing microscopy, and 1H and 13C NMR. At 25°C the solubility limit of AOT in water is 1.4 ± 0.2 wt%, in agreement with the recent literature. Above the solubility, uniaxial negatively birefringent lamellar liquid crystals form. The biphasic region extends up to 20 ± 4 wt%, as determined from visual observations, microscopy, and mainly from 1H NMR linewidths. From 20 to about 75 wt% there is a lamellar liquid crystal whose birefringence changes to positive around 40 wt%. The phase behavior results are qualitatively consistent with those of Rogers and Winsor (1969) and Fontell (1973), although there are some quantitative differences. The surfactant molecular motion is quite fast below the solubility, becomes quite restricted in the lamellar liquid-crystalline phase, becomes fast again in the viscous isotropic phase (∼75 to ∼80 wt%), and becomes much more restricted above 80 wt%, where the phase is hexagonal inverse middle liquid crystalline. 13C T 1 values for the lamellar liquid crystal show a motional gradient common in isotropic lamellar liquid crystals. The water molecular motion becomes increasingly restricted in the lamellar liquid crystal. The water resonance broadens substantially in the viscous isotropic and is not seen in the hexagonal liquid crystal.