Surface Technology, 17 (1982) 229 - 237 229 ADSORPTION BEHAVIOUR AND ORIENTATION OF ADENOSINE MOLECULES AT THE MERCURY DROP ELECTRODE: CONGRUENCE OF ISOTHERMS WITH RESPECT TO THE ELECTRODE POTENTIAL AND SURFACE CHARGE DENSITY IN NEUTRAL ELECTROLYTES A. M. BEKHEET, F. EL-TAIB HEAKAL and A. A. MAZHAR Chemistry Department, Faculty of Science, Cairo University, Giza (Egypt) (Received May 31, 1982) Summary The parameters of the electrocapillary maximum of mercury in the pres- ence of adenosine, together with the results of measurements of the differ- ential capacitance of mercury in the same solutions, were used in calculating the total surface charge density q and the interfacial tension o. In 1 N solu- tions of Na2SO4, NaC104, KNO 3, KC1 and KBr the isotherms exhibited simultaneous congruence with respect to the electrode potential and surface charge density, within certain limits, for all electrolytes. The coefficient a of the adsorption isotherm was shown to have a high value, confirming that two-dimensional condensation of the adenosine molecules took place at high concentrations. 1. Introduction For a quantitative interpretation of the results of differential capac- itance measurements in the various electrolytes, the effect of the two electrical variables electrode potential and surface charge density should be considered. Results of electrocapillary and differential capacitance data have commonly been employed to study the adsorption of organic molecules at the mercury surface. The surface excess quantity F of adsorbate may be cal- culated by using the Gibbs adsorption equation: 1 F- RT E where c is the concentration of adsorbate in bulk solution, ~r is the surface pressure and E is the potential. Two methods are employed to obtain 7r: electrocapillary measurements, which give 7r directly, or integration of capacitance results, which gives ~ indirectly, can be used. In the dilute 0376-4583/82/0000-0000/$02.75 © Elsevier Sequoia/Printed in The Netherlands