Ionic interactions and solvent-structure effects in adsorption at the mercury electrode/water interface.
Creators: name: Rocheleau, François.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1978
Subjects: subject: Chemistry, Analytical.

description: Studies of the structure and properties of the double-layer at the Hg electrode interface by measurements of the adsorption of urea and of a series of n-propylammonium salts in water, have been carried out by means of electrocapillary measurements using a drop-time tensiometer. Since the adsorption of surface-active substances at charged interfaces involves solvent displacement, an appropriate adsorption isotherm is chosen in which the relative size ratio x, of adsorbate and solvent is taken into account. The following isotherm was applied to the results obtained in the present work: Kc = theta/exp(x-1)(1-theta)x or a corresponding one with a lateral interaction term exp(-2atheta) included. Electrocapillary curves were obtained for urea adsorption but did not exhibit the striking adsorption behaviour found experimentally with thiourea in previous work, where a strong Hg-S interaction predominates. Urea was found to adsorb in a more or less perpendicular orientation to the surface with its C=O group oriented towards the solvent. The Esin and Markov coefficients and the surface pressure behavior were evaluated for urea and tend to confirm the proposed orientation. It is determined by the H-bonding between the C=O function in urea and surrounding urea molecules as well as with other surrounding water molecules in the interphase. The experimental results for the series of n-propyl-ammonium perchlorates, n-PrN+ nH4-nC10-4 , show that all the organic cations tend to adsorb more strongly when qM is negative as expected on electrostatic grounds. The increase of adsorbability with increasing number of propyl chains corresponds to increasing hydrophobicity and decreasing accessibility of the N+ charge-center to the H2O dipoles of the solvent. Evaluation of the standard free energies of adsorption for this series of cations shows that, in a general way, the DG°ads becomes more negative with increasing surface excess, Gamma, suggesting that apparent attractive interactions take place among the adsorbed cations in the interphase. At first sight, this is unexpected since repulsion between ions of the same sign in an ad-layer would be the more expected behavior. These results suggest, however, that the interaction effects amongst hydrated ions must be considered not only in terms of the electrostatic interactions involved but also in terms of interactions between overlapping hydration co-spheres of the ions in the sense of Gurney. The co-sphere overlap of R4N+ ions, as in regular solutions of R4N+ salts, gives an attractive interaction which accounts for the unusual behavior of DG°ads with increasing surface coverage. Attempts to study the structure of the mercury-electrolyte solution interphases in acetonitrile was unsuccessful. This unfortunate situation was apparently caused by the presence of traces of water and oxygen in the solvent resulting in the hydrolysis of acetonitrile to acetamide which in turn becomes the predominantly adsorbed species at the mercury surface. This difficulty, noted in other work, prevented any reliable results being obtained in this solvent.