Self-assembled monolayers of perfluoroalkyl amideethanethiols, fluoroalkylthiols, and alkylthiols for the prevention of silver tarnish

Self-assembled monolayers (SAM) of perfluoroalkyl amideethanethiols, F(CF2)(n)CONH(CH2)(2)SH (n = 6, 7, or 8), inhibit the corrosion of silver by hydrogen sulfide (H2S) in air. Unlike conventional hydrocarbon thiols used to protect silver from corrosion, these fluorinated amidethiols have a very low mercaptan odor, impart fluorocarbon wettability properties to the silver surfaces, and exhibit intermolecular associations via hydrogen bonding of the amide functionality. These fluorinated thiols were synthesized by reacting fluoroalkyl acid chloride with 2-aminoethanethiol, or by reacting methyl fluoroalkanoate with 2-aminoethanethiol. SAM were formed by immersing silver coupons in 0.01, 0.1, and 1 wt% solutions of the fluorinated amidethiol in propanol (CH3CH2CH2OH), or by applying a thin film of the thiol solution that rapidly, evaporated. Electrochemical impedance spectroscopy (EIS) was used to evaluate the thickness and integrity of the monolayers. The thin films of evaporating thiol solution yielded rapid monolayer formation as a result of the increasing concentration of the thiol in the solution on the silver during the evaporation of the propanol. Accelerated tarnish tests were performed in a chamber that exposed the silver to air, water vapor, and similar to1 ppm H2S for 7 h to 24 h at 313 K. The tarnish resistances associated with a fluoroalkyl thiol (1 = 1H,2H,2H-perfluorodecyl-1-thiol) and hexadecanethiol were also determined. The best tarnish resistance was attained with the hexadecanethiol, and the perfluoroalkylamide ethanethiol yielded better corrosion resistance results than the fluoroalkylthiol.