Quantifying Adsorbate Inhibition of Metal Deposition

217-226. Both qualitative and quantitative insight can be garnered from transient X -i, i-t and r -t measurements in quiescent or stirred solutions, while measurements of steady-state behavior are best performed under well-defined hydrodynamic conditions. Typically, a rotating disc electrode (RDE), or a related method, is used to specify and/or modulate the hydrodynamic boundary layer thickness, 8. With an RDE the boundary layer is specified by 

The coverage of the reaction intermediate 0M, the inhibitor coumarin, 0A, and the number of free sites, 0O sum to unity  

In the model [57], the coumarin coverage is defined by a Langmuir isotherm (Equation 2.2) with electrochemical reduction of the adsorbed coumarin (Equation 2.9) being balanced by diffusion of coumarin to the interface (Equation 2.3). Incorporation of the coumarin in the solid was considered to be negligible. The results of the model have been summarized by three characteristic dimensionless groups, X, y and p  

The dimensionless group X reflects the relative rate of the two serial elementary reactions that describe metal deposition. Where X — 1 the second reaction is fast, resulting in minimal surface concentration of the intermediate metal species, 0M, whereas for X 0 the surface coverage of the intermediate, 0M, can become substantial if there are sufficient sites that are not blocked by the inhibitor. 

The dimensionless group y is given by the ratio of bulk inhibitor concentration to the Langmuir adsorption constant. In the absence of consumption or competition from the adsorbed metal intermediate, y = 1 corresponds to an inhibitor coverage of 0.5. Thus significant inhibition is expected when Cj, K(y l). 

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