DISSERTATION

_____________________________________________________________ Results and Discussion
where ε has the value 2 for compact chains and δ is the thickness increment (0.11 nm per carbon
atom for a 30° tilted alkyl chain 92 ). This supports the results from Figure 3.37, in which it is
shown that the capacitance of the MCH SAM is the highest and the one of a MCHD SAM the
lowest. Furthermore, it was demonstrated previously 95 that for a low number of carbon atoms
(less than 8) the relation between thiols of different length may not be linear, which is seen also
in Figure 3.37. Considering the relationship derived from the Helmholtz model, capacitance
values of 2.7, 1.5 and 1.0 µF/cm 2 are expected for SAMs of MCH, MCU and MCHD,
respectively. The experimentally determined capacitance values are smaller, however, they are
in the same order of magnitude.
Figure 3.37. Capacitance values of fully covered thiol monolayers in dependence on the
inverse number of carbon atoms of the investigated thiol derivatives. Data obtained from
Figure 3.36. Figure adapted from ref. 89 .
From the capacitance curves it is possible to calculate the coverage kinetics of the investigated
alkylthiols using the following equation:
θ = (C − C 0)
(C f − C 0 )
(3.9)
where C0 is the initial capacitance of the bare electrode, Cf is the capacitance of a fully covered
monolayer and C is the capacitance at any time. The initial capacitance was calculated to be
6.77 µF/cm 2 as determined in background electrolyte in absence of any thiol. Calculated
coverage curves are presented in Figure 3.38, where it is shown that using the optimal potential-
3.3 Importance of controlling the surface 76