On the Coadsorption of Mephenaminate and Phenylundecanoate Anions at Passive Iron
Yu. I. Kuznetsov and N. P. Andreeva
Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia
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Received August 10, 2004
Abstract — Adsorption of sodium mephenaminate (SMEP), sodium phenylundecanoate (SPU), and their mixtures by oxidized iron from a borate buffer (pH 7.4) at the constant potential E = 0.2 V was studied by ellipsometry. Like the adsorption of the previously studied SMEP, that of SPU obeys the Frumkin equation with the higher Gibbs energy of adsorption (
= 33.1 kJ/mol). The Frumkin equation fails for the adsorption of an equimolar SPU + SMEP mixture (inhibitor IFKhAN-31), which is described by the Bockris equation. The calculations showed that 
= 41.7
0.2 kJ/mol and the number of water molecules displaced by an adsorbed inhibitor species from the metal–solution interface is n = 4. It was found that SMEP anions preadsorbed at oxidized iron do not affect the shape of the SPU adsorption isotherm, yet altering its constants. In the case of preadsorbed SPU, the SMEP adsorption isotherm changes its shape and cannot be described by any known adsorption equation. In addition, SMEP begins to adsorb at the electrode in lower concentrations; i.e., PU anions stimulate the adsorption of SMEP.
= 33.1 kJ/mol). The Frumkin equation fails for the adsorption of an equimolar SPU + SMEP mixture (inhibitor IFKhAN-31), which is described by the Bockris equation. The calculations showed that = 41.7 0.2 kJ/mol and the number of water molecules displaced by an adsorbed inhibitor species from the metal–solution interface is n = 4. It was found that SMEP anions preadsorbed at oxidized iron do not affect the shape of the SPU adsorption isotherm, yet altering its constants. In the case of preadsorbed SPU, the SMEP adsorption isotherm changes its shape and cannot be described by any known adsorption equation. In addition, SMEP begins to adsorb at the electrode in lower concentrations; i.e., PU anions stimulate the adsorption of SMEP.