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

Self-assembled monolayers of semifluorinated n-alkanethiols having the general 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 unannealed polycrystalline Au films vapor deposited on glass supports. Determination of the adlayer capacitances through cyclic voltammetry has been validated from its independence of the potential scan rate and its constancy when using two electrolytes, NaF and KCI, whose anions have different hydrated radii. The thickness of the monolayers has been estimated on the basis of capacitance values, and an indicative average tilt angle deduced. The monolayer's organization appears to largely depend on the n and m values. Short fluorinated 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 (average 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 intermediate situation.