Solution-Phase Desorption of Self-Assembled Monolayers on Gold Derived From Terminally Perfluorinated Alkanethiols

Abstract

The solution-phase thermal desorption of three series of self-assembled monolayers (SAMs) on gold generated from terminally perfluorinated alkanethiols was examined. Series 1 SAMs, F(CF2)x(CH2)11SH, where x = 1–10, consisted of a constant hydrocarbon segment length with an increasing fluorocarbon segment length. Series 2 SAMs, F(CF2)10(CH2)ySH, where y = 2–6, 11, consisted of a constant fluorocarbon segment length with an increasing hydrocarbon segment length. Series 3 SAMs, F(CF2)x(CH2)ySH, where x = 1–10 and y = 16 – x, consisted of both hydrocarbon and fluorocarbon segments in which the segment lengths were varied while holding the total chain length constant at 16 carbon atoms. SAMs from these three series were prepared and characterized using both ellipsometry and contact-angle measurements. The resultant SAMs were shown to be highly hydrophobic and oleophobic. The SAMs were heated in decalin (DC) and perfluorodecalin (PFD) at 80 °C for various periods of time to monitor their thermal stability when exposed to hydrocarbon versus fluorocarbon solvents. In general, SAMs derived from n-alkanethiols and terminally perfluorinated alkanethiols exhibited diminished thermal stabilities upon heating in a hydrocarbon solvent (DC) versus heating in a perfluorocarbon solvent (PFD). The thermal stability of the SAMs increased with increasing lengths of the CF2 or CH2 segments. We also examined the kinetics of thermal desorption of these SAMs. From these studies, SAMs composed of higher degrees of terminal perfluorination exhibited smaller rate constants for the initial stage (fast regime) of desorption. When compared with analogous alkanethiol SAMs, the terminally perfluorinated SAMs exhibited greater thermal stabilities in both DC and PFD. In addition, values of the rate constants for desorption of the alkanethiol SAMs were approximately double those of the terminally perfluorinated SAMs having similar chain lengths.