STRUCTURE AND INTERFACIAL PROPERTIES OF SPONTANEOUSLY ADSORBED NORMAL-ALKANETHIOLATE MONOLAYERS ON EVAPORATED SILVER SURFACES

Monolayer films of n-alkanethiols (CH3(CH2)n(SH), n = 3-15, 17) have been spontaneously adsorbed from solution at evaporated Ag films. These monolayers have been characterized with optical ellipsometry, contact angle measurements, and external reflection infrared spectroscopy (IRS). The IRS data indicate that the monolayers with long chains (n greater-than-or-equal-to 10) are predominantly densely packed, crystalline-like structures with all-trans conformational sequences exhibiting average tilts of approximately 13-degrees from the surface normal. Although less clearly defined, the collective results from all three characterization methods point to a comparably tilted structure for the shorter chain monolayers. The chain tilts at Ag are markedly different from the approximately 30-degrees tilts for long-chain n-alkanethiolate monolayers at Au,4,10 indicating a difference in the bonding of the sulfur head group at the two metals. For short-chain monolayers, the offset of an odd-even effect by one methylene group at Ag with respect to Au for both the contact angle and IRS data further supports this conclusion. Models that are based on the crystallinity and the electronic properties of the two substrates are presented as possible explanations of the observed structures. These results demonstrate the potential of long-chain thiolate monolayers at Ag as a model molecular system for unraveling the complexities of a variety of interfacial processes.