ADSORPTION-DESORPTION DYNAMICS OF AMPHIPHILIC FERROCENES ON ELECTRODES STUDIED BY FLOW VOLTAMMETRY

Adsorption-desorption kinetics and equilibria were measured by using flow voltammetry. The principle involves flowing an adsorbate-free blank electrolyte solution past a working electrode and switching the flow to a solution of adsorbate, or beginning with the adsorbate solution and switching to the blank. Changes in surface concentration of adsorbate with time are monitored by fast cyclic voltammetry. Mathematical models assuming the Langmuir isotherm were developed for adsorption-desorption kinetics under hydrodynamic conditions and employed for nonlinear regression of peak current versus time data to obtain adsorption parameters. Alkylammonium surfactants having ferrocene (Fc) attached to cationic head groups and alkyl chain lengths from C8 to C16 were investigated. Results suggest that these adsorbates achieve monolayer coverage from submicromolar solutions under flow conditions. Adsorption rates are controlled by both intrinsic adsorption kinetics and diffusion rates, Residence times on the electrode were 14s for Fc-C8 and Fc-C12 and 66s for Fc-C16. Free energies of adsorption (Delta G(ads)(o) ) increased with the length of the hydrocarbon chain by about 0.75 kJ mol(-1) per CH2 group, in accord with a hydrophobic component to the driving force. Delta G(ads)(o) depended weakly on electrode potential.