SELF-ASSEMBLED MONOLAYERS OF ALKANETHIOLS ON GOLD - COMPARISONS OF MONOLAYERS CONTAINING MIXTURES OF SHORT-CHAIN AND LONG-CHAIN CONSTITUENTS WITH CH3 AND CH2OH TERMINAL GROUPS

This paper describes the preparation and properties of self-assembled monolayers (SAMs) obtained by the competitive adsorption of short-chain and long-chain alkanethiols onto evaporated gold. Four systems were studied involving combinations of one short and one long chain: HS(CH2)10CH3(Sh = CH3),HS(CH2)10-CH2OH (Sh = CH2OH), HS(CH2)21CH3 (Lg = CH3), and HS(CH2)21CH2OH (Lg = CH2OH). The combinations examined were Sh = CH3/Lg = CH3, Sh = CH3/Lg = CH2OH, Sh = CH2OH/Lg = CH3, and Sh = CH2OH/Lg = CH2OH. The resulting monolayers were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and wettability by water and hexadecane. Because the two components in each SAM have different chain lengths, the thickness of a two-component SAM (as determined by ellipsometry) could be used to determine the relative amounts of each in the SAM. Intensities of XPS signals could also be used to measure the composition of the SAMs. Comparisons of compositions inferred from ellipsometry and XPS indicate that, although the two methods give results in excellent agreement, the XPS method has the higher precision and is generally superior for these types of SAMs. Comparison of the composition of the SAMs with the composition of the solutions from which they were formed suggests the order of stabilities of pairs of alkyl groups in the SAMs is long-long > short-short > long-short. This result suggests that these monolayers phase separate to some extent into microscopic islands; studies of wetting suggest that the phase separation is not complete.