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