Structure, wettability, and barrier properties of self-assembled monolayers on metallic surfaces

Abstract

Self-assembled monolayers (SAMs) offer a convenient approach for fabricating molecularly tailored interfaces with well-defined compositions, structures, and thicknesses. SAMs have been suggested for use as corrosion barriers, antifouling coatings, and as components of molecular electronics and lithography. Still, researchers lack the molecular description of the interfacial properties, structural features, and barrier properties of SAMs that would be useful for optimizing and tailoring the behavior of SAMs. This dissertation makes connections between the molecular level structural features of SAMs and macroscopic properties such as wettability and barrier properties using molecular dynamics (MD) simulations and experimental techniques. MD simulations were performed to explain the unexpected experimental observation that the wetting properties of some liquids on SAMs prepared using alkanethiols (CnSH) depend on whether the chain length (n) is odd or even. The difference in near-surface structure of the liquid (and not that of reorganization events by the monolayer) appears responsible for the high sensitivity of hexadecane and the general insensitivity of water to the structural differences expressed by odd- and even-chained monolayer surfaces. MD simulations were also performed to investigate the influences of molecular structure on the ability of n-alkanethiolate SAMs on gold and copper to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. The barrier resistances offered by these films towards oxygen transport, as calculated by the MD simulations, were a function of the crystallinity of the center region of the SAMs. Upon the introduction of an ether linkage within the SAM, the results from MD simulations show that when the ether linkage is too close to the metal surface or to the chain ends, the free energy barrier of SAMs towards oxygen diffusion was almost 10 kJ/mole less than that for an n-alkanethiolate SAM having the same chain length. Phytanylthiol (3, 7, 11, 15-tetramethyl-hexadecanethiol) SAM on gold was characterized by ellipsometry, wetting measurements, X-ray photoelectron spectroscopy, and MD simulations to gain insights into the effect of a branched chain on the thickness, chain packing and orientation, wettability, and barrier properties of a SAM. These experimental and computational studies indicate that phytanylthiolate SAM on gold contains a fully extended 16-carbon backbone which is more disordered as compared to n-hexadecanethiolate monolayer on gold, with the sulfur head group possibly occupying four-fold hollow sites on gold.