To the Nature of Surfactant Adsorption onto Metallic Surfaces: Interaction with Metal or Hydrophobic Effect? Adsorption of Hexylamine on Platinum

Abstract

We investigated the adsorption of hexylamine (in its hydrated form, hexylammonium ion) on the solution-platinum interface in 1 M HClO4 in the presence of 0.1 M Fe2+ and 0.1 Fe3+ by potentiodynamic and impedance methods, and also at the solution-air interface by the maximum bubble pressure technique. We show that the physical adsorption of hexylamine at both interfaces is described by the Dhar-Flory-Huggins isotherm equation. The values of adsorption constant and free adsorption energy are close for both interfaces. For the solution-air interface, the hydrophobic effect is the main reason for the surfactant adsorption at this interface. Based on the closeness of the main adsorption characteristics for both interfaces, we suggest that the adsorption of hexylamine on platinum occurs mainly due to the hydrophobic interaction of hexylamine polar molecules with water polar molecules as well. It has been shown that for the solution–platinum interface, the share of the hydrophobic effect is about 60%, and that of the van der Waals interaction is about 40%.