INVESTIGATION OF ELECTROSORPTION OF A QUATERNARY AMMONIUM SALT USING AN ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE

Electrochemical Quartz Crystal Microbalance (EQCM) was applied to a st udy of electrosorption of Benzyldimethyldodecylammonium bromide (BDDB) on a gold coated quartz sensor from deaerated 0.1M Na2SO4. The discre te voltammetry and mass curves obtained simultaneously made it possibl e to compute the effective equivalent molar mass (EMM) of the reaction species (the ratio of effective molecular weight of the species to th e number of electrons exchanged) as a function of the potential applie d. It is found that a cathodic mass increase of gold electrode in blan k 0.1M Na2SO4 solutions can be accounted for hydrogen evolution and th e apparent quartz sensor response due to the change of the electrode-s olution mechanical coupling. This effect completely disappears in the presence of a cationic surfactant as BDDB which increases the hydrogen evolution overvoltage. The low concentrations of BDDB as 10(-5) M are found to inhibit anodic oxide formation on gold. At the concentration s higher than 10(-4) M oxidation and reduction processes appear at 1.0 4 V and 0.69 M nhe respectively. The oxidation is accompanied by inten sive electrosorption of organic species which are to a high extent des orbed upon the reduction. Although at higher concentrations, ca. 10% o f the adsorbate remains on the surface upon the reduction. The EMM val ues obtained on the basis of the voltammetric cycles and for BDDB elec trosorption at a constant potential give a clue to the analysis of the chemical transformations in the system. The process of electrosorptio n of dimethylbenzalkonium ion appears to be connected with the adsorpt ion of the Pr anions from the solution and the formation of the anodic ally adsorbed oxygen layer. The EMM values obtained for the electrosor ption in stationary conditions was 260 g/mole. The ir and XPS spectros copy data indicate that oxidation of BDDB results in a shrinkage of th e alkyl chain and formation of amide and quinonic C=O bonds, which con forms with the obtained EMM value, assuming one electron exchange.