Dilational Rheological Properties of Non-Ionic Surfactants at the Water-Decane Interface: Effect of Unsaturated Hydrophobic Group

The interfacial tensions and dilational properties of adsorbed films of two non-ionic surfactants with different hydrophobic groups, polyoxy-ethylene sorbitan stearate (Tween 60) and polyoxy-ethylene sorbitan monooleate (Tween 80), at the water-decane interface have been investigated by the drop-shape analysis method. The effects of dilational frequency and bulk concentration on the interfacial properties were expounded. The influence of low temperature on the interfacial tensions and dilational properties have also been researched. The experiment results show that the interfacial activity of Tween 80 is rather large compared with Tween 60. The minimum area per molecule at the water-decane interface (A(min)) value of Tween 80 is little large than that of Tween 60, which is due to the steric effect of unsaturated double bond in Tween 80 molecule. The dilational data show that the ethylene oxide groups of non-ionic surfactant form a stable sub-layer, which results in the increase of modulus and the decrease of phase angle for both Tween 60 and Tween 80 than those of common ionic surfactants. Moreover, the unsaturated hydrophobic group of Tween 80 is much flexible, which is easily crosslinked and entangled. Therefore, dilational modulus of Tween 80 is higher and phase angle is lower than that of Tween 60. Low temperature decreases the flexibility of unsaturated hydrophobic group and lessens the influence on the interaction of saturated hydrophobic group. Saturated surfactant molecules of Tween 60 almost lose temperature response.