Critical influence of the fluorinated chain length in the self-assembly ofterminally perfluorinated alkanethiol monolayers on gold surfaces. An electrochemical study

Self-assembled monolayers of semifluorinated n-alkanethiols having the gene ral formula F(CF2)(n)(CH2)(m)-SH (n = 4, 6, 8, 10 and m = 2, 11) have been formed from ethanolic solutions, with an incubation time of 96 h, on unanne aled polycrystalline Au films vapor deposited on glass supports. Determinat ion of the adlayer capacitances through cyclic voltammetry has been validat ed from its independence of the potential scan rate and its constancy when using two electrolytes, NaF and KCI, whose anions have different hydrated r adii. The thickness of the monolayers has been estimated on the basis of ca pacitance values, and an indicative average tilt angle deduced. The monolay er's organization appears to largely depend on the n and m values. Short fl uorinated chains (n = 4) are unable to self-organize in a position near the surface normal. This fact is independent of the length of the hydrogenated spacer (m = 2, 11), so it appears as an inherent characteristic for these short fluorinated chains. Fluorinated chains containing eight or ten carbon atoms are able to self-organize in a packing near the surface normal (aver age tilt angle 26 degrees), independently of the length of the hydrogenated spacer length (m = 2, 11). Compounds with n = 6 and m = 2, 11 are in an in termediate situation.