DISSERTATION

_____________________________________________________________ Results and Discussion
measured by the potentiostat is sent to the lock-in amplifier input after current-to-voltage
conversion and the magnitude and the phase of the AC current response at the frequency of the
AC excitation signal are obtained. Using these values, the imaginary impedance component and
subsequently the interfacial capacitance can be calculated using the following expression,
derived from an RC series equivalent electrical circuit (for detailed calculation see Section 5.5):
C = −1
ωZ ′′ (3.7)
where ω is angular frequency and -Z" is the imaginary component of impedance.
Figure 3.32. Mercaptoundecanol (MCU) SAM formation kinetics performed at constant
potential equal to the previously measured OCP of the measuring solution (0 V vs.
Ag/AgCl/3 M KCl). Measurement was performed in 10 mM PB with 20 mM K2SO4
containing 1 mM MCU (30 % ethanol).
As mentioned earlier, the kinetics of SAM formation at OCP is quite slow after the initial fast
phase (Figure 3.32). During the first hour a relatively fast decrease in capacitance is manifested
after which a significant deceleration of the immobilization kinetics is observed. Even after
more than 12 h a compact thiol monolayer did not form, which is evident in the figure from the
absence of a capacitance plateau. The measurement was conducted by applying a constant
potential equal to the previously measured OCP. After determining the OCP, the measurement
was performed applying a constant potential equal to the OCP superimposing an AC signal of
3.3 Importance of controlling the surface 70