ELECTROCAPILLARY STUDIES OF ADSORPTION OF SOME ORGANIC MOLECULES AT MERCURY-ELECTROLYTIC SOLUTION INTERFACE

The adsorption of 3-pentanol, chloroform, cyclohexanol, and cyclohexanone at the mercury-0.1 N perchloric acid solution interface has been studied. Interfacial tension measurements were made with a capillary electrometer of greatly improved precision. The use of a fused-quartz precision pressure gauge instead of the usual manometer enabled measurements of interfacial tension to be taken within a few seconds after renewal of the mercury surface. This procedure eliminated any error caused by diffusion of impurities to the interface. In the analysis of the electrocapillary data obtained for each adsorbate it was found that the surface pressure - In activity curves for different potentials could be superimposed by a translation along the ln a axis. Implications of this superimposability are (1) that the interactions between molecules of the adsorbate are independent of the electrical state of the interface and (2) that the surface charge density varies linearly with surface excess (at fixed potential). The Frumkin equation was used to fit the data for each adsorbate, and a standard free energy of adsorption referred to standard states based on infinite dilution both in solution and on the surface was calculated from its parameters. A correlation between these standard free energies of adsorption and polarizabilities of the molecules was established, indicating that van der Waals forces are primarily responsible for the adsorbate-adsorbent interactions. © 1969.