Sd. Campbell et Ac. Hillier, Nanometer-scale probing of potential-dependent electrostatic forces, adhesion, and interfacial friction at the electrode/electrolyte interface, LANGMUIR, 15(3), 1999, pp. 891-899
The atomic force microscope (AFM) was used to examine the influence of an a
pplied electrochemical potential on the interfacial properties of the elect
rode/electrolyte interface. Measurements of electrostatic force, adhesion,
and friction coefficient were performed at two different electrode surfaces
: glassy carbon and a thin film of sulfonate-derivatized poly(aniline) (SPA
Ni). At the carbon electrode, changes in electrostatic force between probe
and substrate exhibited a potential-dependent transition from repulsive to
attractive values at potentials negative and positive of the potential of z
ero charge (E-pzc). Simultaneous measurements of tip-substrate adhesion and
friction coefficient showed a change from low to high values over the same
potential range, suggesting a common mechanism dominated by the electrosta
tic force. Measurement of these same properties at a SPANi-coated electrode
also displayed a potential-dependent response. The electrostatic force and
the adhesion tracked with the oxidation state of the initially neutral fil
m. However, the friction coefficient appeared insensitive to the charge sta
te of the polymer. A calculation of the forces between probe and substrate
using DLVO theory accurately reflected the measured force curves as well as
the change in adhesive force as a function of surface charge. Consideratio
n of the forces that determine the friction coefficient suggested that the
influence of electrostatic interactions was strongly dependent upon the geo
metry of the tip-sample contact and the presence of microgaps between the t
ip and the substrate over which electrostatic forces could operate. The abs
ence of potential-dependent friction at the SPANi/electrolyte interface ref
lected a compliant substrate, which gave rise to a predominantly adhesive t
ip/sample contact.