Wf. Peng et al., ADSORPTION-DESORPTION DYNAMICS OF AMPHIPHILIC FERROCENES ON ELECTRODES STUDIED BY FLOW VOLTAMMETRY, Journal of physical chemistry, 99(18), 1995, pp. 6986-6993
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.