Surface chain tilt

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

These results suggested alkyl chain tilts of 15° for OTS and 26° for 1. This "tilt" is the angle between the chain axis, which bisects the C-C bonds, and the surface normal. 

A consistent description of the structure of alkanethiol monolayers on gold has emerged from an array of spectroscopic and diffraction studies. X-ray photoelectron spectroscopic (XPS) studies support the presence of anisotropically chemisorbed alkanethiolates on gold [24-29]. Ellipsometric measurements [24-27, 30], capacitance studies [30] and XPS measurements [31] confirm monolayer film thickness. Fourier transform infrared external reflective spectroscopy (FTIR-ERS) shows that the chains tilt at about 30° off the surface normal, and the plane containing the carbon backbone is twisted out of the plane of tilt by about 50° [25-27, 30, 32, 33]. 

Two-dimensional cross-sections of instantaneous equilibrium MD configurations for each of the areas are shown in Fig. 19. In all three configurations, the systems appear to exist in two phases, one with disordered chains and one with more ordered chains. In no case are the chains perpendicular to the surface. Calculations of the density distribution of methylene groups along the chain and the distribution of average chain tilt... 

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