Coadsorption of triphenylphosphine – triphenylphosphine oxide, triphenylphosphine – dodecyldiphenylphosphine oxide, and dodecyldiphenylphosphine oxide – tri(n)octylphosphine oxide from methanolic solutions on a polarized mercury electrode

Abstract

The coadsorption of triphenylphosphine – triphenylphosphine oxide, triphenylphosphine – dodecyldiphenylphosphine oxide, and dodecyldiphenylphosphine oxide – tri(n)octylphosphine oxide on a polarized mercury electrode from methanolic solutions of LiCl is investigated by means of electrocapillary measurements. Results show that reorientation or other molecular modifications do not take place during the coadsorption process. The adsorbed molecules maintain their orientation with the phenyl and alkyl groups in contact with the electrode surface. Thus, the adsorption maxima of the substances in the mixed adsorption are located at the same polarization region as in the single adsorption, although their separation distance increases as the bulk concentration of adsorbates rises due to changes in the electrical characteristics of the interface. This shift is not observed when the two substances are adsorbed at the same, or nearly the same, potentials (charges). In all cases the adsorption of a substance leads to a decrease in the adsorption of the coadsorbate to an extent depending on the electric field. The influence of the field on the adsorption isotherms cannot be determined by means of the surface pressure method but by determining the type of adsorption isotherm. It was found that all the systems studied follow an isotherm based on the strictly regular solutions theory. The interaction parameters are independent of the field, while the standard free energy of adsorption of an adsorbate, within experimental error, is independent of the presence of the coadsorbate. This is due to the insignificant change in the bulk adsorbate solvent interactions caused by the presence of the coadsorbed substance in the solution