Reverse vesicles formed by polypentadeca- and polyeicosa-oxyethylene mono n-hexadecylethers in cyclohexane

Abstract

The self-assembly of polypentadeca- and polyeicosa-oxyethylene mono n-hexadecylethers (C16E15 and C16E20) in cyclohexane has been studied using small-angle X-ray scattering (SAXS), rheology measurements and polarised-microscopy (POM) observations. Both surfactants can dissolve in cyclohexane, which benefited from their low cohesive energy. A trace amount of water was used to help the formation of reverse aggregates. The SAXS measurements and POM observations all evinced the formation of reverse vesicles in the solutions. The solution viscosity showed the dependence on both water content, represented as the mole ratio of water to surfactant W0, and surfactant concentration Cs. The samples at Cs = 200 mmol L−1 and W0 = 15–20 had a visco-elastic response with elastic behaviour at high frequencies (G′ dominating over G″) and slightly viscous behaviour at low frequencies (G″ exceeding G′) which resulted in a crossover of G′ and G″ at below 0.1 rad s−1. The elastic plateau modulus G′P increased with increasing W0 or Cs. The effect of temperature on the rheological properties was examined. The value of flow activation energy, Ea, of C16E20 was lower than that of C16E15, which suggested that the former may be more likely to form reverse vesicles than the latter. The elastic plateau modulus G′P showed temperature dependence, which was explained in terms of the hydrogen bonding interactions between the water and the POE.