Effect of chain length on the protection of copper by n-alkanethiols

Self-assembled monolayers (SAMs) formed by the adsorption of n-alkanethiols [CH3(CH2)(n-1)SH; n = 8, 12, 16, 18, 20, 22, and 29] onto copper provide a flexible method for producing coatings that can protect the underlying metal against corrosion. The ability to tailor the thickness of the coatings at the angstrom-level by choice of adsorbate allows examination of the effect of angstrom-level variations in film thickness on the performance of the SAM as a barrier layer. A combination of infrared (IR) spectroscopy and electrochemical impedance spectroscopy (EIS) was used to correlate the structure of the SAM with its barrier properties during extended exposures to 1 atm of O-2 at 100% relative humidity (RH). EIS results reveal that the coating resistances provided by SAMs with chain lengths of 16 carbons or more (i.e, n greater than or equal to 16) exhibit a linear increase with chain length and are orders of magnitude greater than those provided by SAMs with n less than or equal to 12 due to the more crystalline nature of the thicker films. Upon exposure to 1 atm of Oz at 100% RH, the barrier properties of the SAMs deteriorate as observed by impedance measurements. SAMs formed from longer-chained adsorbates are superior to shorter-chained analogues in maintaining their structural and protective properties due to their greater van der Waals interactions. The ability of a film to maintain its barrier properties scales exponentially with the chain length of the n-alkanethiol, whereby an additional five methylenes in the adsorbate yields films that are twice as effective in maintaining their barrier properties. Complementary experiments using IR spectroscopy to characterize the phase state of the films suggest that the eventual breakdown in protection for these coatings is due to a structural transformation of the SAM from a crystalline state to a less densely packed film that is much less effective as a barrier layer. The results suggest that these structural changes may be induced by roughening of the underlying copper substrate that occurs during the corrosion process.