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.

Viscoelastic properties of ultrathin films are important in such applications as polymer-supported lipid bilayers, and the quartz crystal microbalance (QCM) offers the ability to measure these properties in situ. In this study, polymer films are created by adsorbing polyacrylamide to gold and silver surfaces from aqueous solutions. The molecular weight of the polymers ranges from 10 000 to 1 000 000 g/mol, and concentrations range from dilute to the overlap concentration. Using a continuum mechanics viscoelastic film model, we extract film property parameters from frequency and dissipation change measurements for multiple harmonics. The precision of the QCM and the modeling technique are limited, however, leading to large property value ranges for any given film. The adsorbed films range from 10 to 150 Å thick, with moduli ranging from 15 to 228 kPa and viscosities ranging from 1.21 × 10-3 to 2.85 × 10-3 Pa s. Meanwhile, the bulk solution properties of the different molecular weight materials lead to drastically different adsorption profiles. While the viscosity differences between water and 10 000 M w polymer solutions lead to large frequency and dissipation responses, the large relaxation times of 1 000 000 M w polymers make their bulk solution viscosity differences from water largely undetectable by the QCM. In addition, the viscosity of the 10 000 M w solutions may be frequency dependent over the range of operating frequencies, contrary to the standard QCM assumption of frequency independence.

The adsorption of long-chain omega-alkoxy-n-alkanethiols [CH(3)(CH(2))(p-1)O(CH(2))(m)SH; m = 11, 19, 22; p = 18, 22] onto copper produces self-assembled monolayers (SAMs) that can provide protection against corrosion of the underlying metal substrate. The resulting films are 40-60 A in thickness and are isostructural with SAMs formed on copper from unsubstituted n-alkanethiols. As evidenced by electrochemical impedance spectroscopy (EIS), the barrier properties of these ether-containing SAMs depend on the chain length of the adsorbate and the position of the ethereal unit along the hydrocarbon chain. For SAMs where the ether substitution is farther from the copper surface, the initial coating resistances are similar to those projected for unsubstituted n-alkanethiolate SAMs of similar thickness. For SAMs where the ether substitution is nearer to the copper surface (m = 11), the resistances are significantly less than those for unsubstituted n-alkanethiolate SAMs of similar thickness, reflecting the effect of the molecular structure on the barrier properties of the film. Upon exposure to 1 atm of O(2) at 100% RH, the SAMs become less densely packed as observed by infrared (IR) spectroscopy, and their barrier properties deteriorate as observed by EIS. The rate that the SAMs lose their barrier properties upon exposure to oxidizing conditions is correlated to the strength of intermolecular interactions within the bulk state of the adsorbate.

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, mobile, tethered lipid bilayers were formed on a poly(ethylene glycol) (PEG) support via a two-step adsorption process. The PEG films were prepared by coadsorbing a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) and a nonlipid functionalized PEG-PDP from an ethanol/water mixture, as described in a previous paper (Munro, J. C.; Frank, C. W. Langmuir 2004, 20, 3339-3349). Then a two-step lipid adsorption strategy was used. First, lipids were adsorbed onto the PEG support from a hexane solution. Second, vesicles were adsorbed and fused on the surface to create a bilayer in an aqueous environment. Fluorescence recovery after photobleaching experiments show that this process results in mobile bilayers with diffusion coefficients on the order of 2 microm2/s. The mobility of the bilayers is decreased slightly by increasing the density of tethered lipids. The formation of bilayers, and not multilayer structures, is also confirmed by surface plasmon resonance, which was used to determine in situ film thickness, and by fluorimetry, which was used to determine quantitatively the fluorescence intensity for each 18 by 18 mm sample. Unfortunately, fluorescence microscopy also shows that there are large defects on the samples, which limits the utility of this system.

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, the properties of a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) adsorbed from ethanol and water solutions onto gold surfaces were studied using a variety of surface-sensitive techniques. X-ray photoelectron spectroscopy showed that the PEG molecules are tethered to the gold surface via thiolate bonds. When adsorbed from water, ethanol, or their mixtures, reflection−absorption infrared spectroscopy showed that amorphous PEG layers with disordered DSPE alkyl chains were formed, independent of adsorption time or solution concentration. On the basis of advancing and receding water and hexadecane contact angles on the lipopolymer films, the DSPE lipid groups appear to segregate from the PEG layer and become exposed at the surface of the polymer films. Swelling observed in surface plasmon resonance experiments and the large contact angle hysteresis observed indicate that highly swellable, mobile films capable of molecular rearrangements are formed. The self-assembling and amorphous properties of these PEG layers make them ideal candidates as polymer cushions for polymer-supported lipid bilayers. The DSPE surface concentration can be controlled, to a limited degree, by varying the adsorption time of DSPE-PEG-PDP from ethanol. A more effective strategy is to coadsorb DSPE-PEG-PDP with a non-lipid-functionalized PEG-PDP from an ethanol/water mixture, which allows the PEG thickness and density to remain constant while decreasing the density of DSPE groups.