(Digital Presentation) On the Nature of Surfactant Adsorption on Metals: Adsorption of Hexylamine on Platinum

Abstract

The kinetics of the Fe2+ and Fe3+ oxidation-reduction reaction on a platinum electrode in 1 M HClO4 was investigated by the EIS method in the presence of hexylamine as a surfactant. With surfactant adsorption, the effective area of the solution-electrode interface decreases. A proportional increase in charge transfer resistance takes place. This resistance values obtained at various hexylamine concentrations. The surface coverage θ(R) for the solution-platinum interface have been calculated.The hexylamine adsorption at the solution-air interface was investigated by the maximum bubble pressure method. The surface tension γ at various hexylamine concentrations are obtained. The surface coverage θ(γ) for the solution-air interface has been calculated. The area occupied by the hexylamine molecule at the interface and the adsorption layer thickness are estimated.It is shown that adsorption process is described by the Dhar-Flory-Huggins isotherm at both interfaces. The slope tg α of the θ(R) and θ(γ) dependence on the hexylamine concentration is close to the theoretical unit in the Dhar-Flory-Huggins coordinates. The main parameters of the hexylamine adsorption (adsorption constant Kad, free adsorption energy ΔGad) were calculated for both interfaces (see Table). Solution-platinum interface Solution-air interface tg α 0.94±0.09 0.98±0.18 K ad / L mol-1 10.0±4.0 15.8±0.3 ΔG ad / kJ mol-1 –(14.8 ± 5.0) –(16.7±0.3) In the general case, the surfactant adsorption on a metal electrode can be due to the interaction of surfactant molecules with the metal or the hydrophobic effect of the surfactant displacement molecules onto the solution surface by polar water molecules. For the solution-air interface, the hydrophobic effect is the main reason for surfactant adsorption at this interface. The values of the main adsorption characteristics on both interfaces are close. Probably, the hydrophobic effect is also the predominant reason for the hexylamine adsorption on platinum. This assumption requires additional studies of the surfactant adsorption on other metals.