Study of the co-adsorption of hexane from the gas phase at the surface of aqueous C10EO8 drops

Alkanes, although not amphiphilic molecules, adsorb at the surface of water drops from the vapour phase. At the drop surface of the surfactant solution formed in hexane saturated air, a competitive adsorption of surfactant and hexane is observed. Experiments with drop profile analysis tensiometry are performed to quantify the adsorption of the non-ionic surfactant C10EO8 at the water/hexane vapour interface. The equilibrium surface tension isotherm shows a remarkable shift towards much lower tensions, even much below those values reached by micellar C10EO8 solutions. A thermodynamic model allows the description of the mixed hexane/C10EO8 adsorption layer at the water/vapour interface quite well. The incorporation of hexane molecules also significantly increases the dilation elasticity values, although the layers of hexane alone show only very low dilational elasticities and high viscosities. While a diffusional exchange of matter theory describes the visco-elasticity of the surfactant very well, it fails for the mixed adsorption layer.