Effect of tetrapentyl ammonium bromide adsorption on the electrostatic interactions in aqueous octaethyleneglycol decylether foam films

Foam films from aqueous solutions of octaethyleneglycol–decylether, C10(EO)8 are investigated in the presence of organic electrolyte (tetrapentyl-ammonium bromide (TPeABr) or inorganic electrolyte (KBr). The dependence of the diffuse electric layer potential on the surfactant and electrolyte bulk concentration is quantitafied from the data on the equilibrium films thickness obtained through interferometric measurements. The effect of the organic electrolyte on the film properties is compared to that of the inorganic. It is established that TPeABr affects the electrostatic interactions in the film in a complex manner: as an electrolyte as well as an ionic surfactant. Although being less surface active its adsorption on the film surfaces modifies their charge and adsorption layer composition. It is found that at higher TPeABr concentration (up to 1×10−3 mol l−1) the adsorption of the positively charged TPeA+ ions even results in recharging the surface and unstable (rupturing) films. The TPeABr effect is stronger at lower C10(EO)8 concentration (1×10−5 mol l−1), where the adsorption of the two components is compatible and leads to substantially lower values equilibrium film thickness (heq) and surface potential (ϕ0). The greater participation of TPeABr in the mixed adsorption layers at lower C10(EO)8 concentration is confirmed by data of their composition, which we have obtained by using the Molecular Interaction Parameter (βs=−3.24) between TPeABr and C10(EO)8 at the interface. The MIP can be derived from the surface tension isotherms of the separate components and their mixture.