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%.

Investigation of Hexylamine Adsorption on Gold in Perchloric Acid

“…the obtained adsorption parameters were compared with the adsorption parameters of hexylamine on platinum [29] and the air-solution interface [29].…”

Study of Hexylamine Adsorption on Platinum by Chronoamperometric Method

“…45 Gibbs free adsorption energy at the interface between the studied solution and air G ad,air Δ = -16.7 ± 0.3 kJ mol -1 . 5 Obviously, for this interface, the surfactant adsorption is predominantly due to the hydrophobic effect. The similarity of the ΔG ad values for the solution-platinum and solution-air interfaces also indirectly confirms the conclusion that hexylamine adsorption has a general nature and is mainly due to the hydrophobic effect, regardless of the interface type.…”

“…Recently, interest has renewed towards identifying the mechanism of adsorption on metals. In particular, the criteria for physical adsorption and chemisorption are considered in theoretical 3 and experimental [4][5][6] aspects.…”

“…Preliminary experiments have shown that the currents due to the transformations of surface oxides are almost two orders of magnitude smaller than the currents characterizing surfactant adsorption. 5 Therefore, the effect of surface oxides on surfactant adsorption can be neglected under our conditions.…”

“…For hexylamine in aqueous solutions x ≈ 3. 5,9 Then Eq. 7 for the system under study can be rewritten as:…”