Adsorption Sites for the Head Groups

Due to the steric hindrance of the spacers and of the tail groups, the head groups do not saturate all the possible adsorption sites on the surface. For this reason, in many cases the extent of the interaction between two adjacent head groups is scarce. As an example, in the case of thiol molecules, the distance between two adjacent S atoms is approximately 5 A [48]. The formation of S-S IxMid on the surface is hard, and very few papers report formation of S-S bonds on an Au surface [49]. Organosulphur compounds, and particularly thiol molecules, possess an additional peculiarity the adsorption of the head group may on its own induce 

Specific molecules may adsorb on the surface via two head groups. In many cases, the two moieties are located at the two ends of the molecule, separated from each other by a spacer. A competition occurs between these two head groups for access to the biding sites on the substrate. The molecule can finally assume a stand-up configuration similar to the conventional monolayer (Fig. 5.7) raie moiety is 

---