Effect of Chain Length on the Protection of Copper by n-Alkanethiols

Abstract

Self-assembled monolayers (SAMs) formed by the adsorption of n-alkanethiols [CH3(CH2) n - 1SH; 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 O2 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 ≥ 16) exhibit a linear increase with chain length and are orders of magnitude greater than those provided by SAMs with n ≤ 12 due to the more crystalline nature of the thicker films. Upon exposure to 1 atm of O2 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.

Extraordinary IR Transmission with Metallic Arrays of Subwavelength Holes

“…Absorptions in the CH stretch region are >100-fold enhanced over those reported in RAIRS studies (28)(29)(30). The region of Snyder's CH 2 wagging progression (31) in the absorption spectrum can be seen in the expansion.…”

Multi‐channel electrochemiluminescence of luminol at a copper electrode

“…These results revealed that these three n-alkanethiols retarded the anodic oxidation of copper electrodes and longer-chained n-alkanethiols exhibited superior ability to impede corrosion and the anodic oxidation of copper electrodes than shorterchained n-alkanethiols. Laibinis and co-workers (21) have reported that the greater protection provided by the longer-chained n-alkanethiols is probably a result of Figure 8 shows the ECL spectra of various ECL peaks. The maximum emission wavelength of ECL-1, ECL-2 and ECL-3 is at about 425 nm.…”

“…SAMs of n-alkanethiols were reported to inhibit the oxidation of copper and provide great protection for the copper electrode (21)(22)(23). In this work, ethanethiol, n-hexanethiol and n-dodecanethiol were self-assembled on the surface of the copper electrodes, respectively, to study their effects on the cyclic voltammetry and ECL behaviours of luminol.…”

The Thermal Stability of Alkanethiol Self-Assembled Monolayers on Copper for Fluxless Soldering Applications

“…It has also been shown that barrier properties improve with increasing alkanethiol chain length due to an increase in the van der Waals interactions and greater coating thickness [12]. While these observations all suggest that SAM coatings make ideal preservative coatings for fluxless soldering, changes in their structure occur at elevated temperatures, which may reduce their effectiveness.…”