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

Adsorption-desorption kinetics and equilibria were measured by using f low voltammetry. The principle involves flowing an adsorbate-free blan k 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 adsorb ate with time are monitored by fast cyclic voltammetry. Mathematical m odels assuming the Langmuir isotherm were developed for adsorption-des orption kinetics under hydrodynamic conditions and employed for nonlin ear regression of peak current versus time data to obtain adsorption p arameters. Alkylammonium surfactants having ferrocene (Fc) attached to cationic head groups and alkyl chain lengths from C8 to C16 were inve stigated. Results suggest that these adsorbates achieve monolayer cove rage from submicromolar solutions under flow conditions. Adsorption ra tes 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) ) in creased 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 drivi ng force. Delta G(ads)(o) depended weakly on electrode potential.