Influence of Specific Intermolecular Interactions on the Self-Assembly and Phase Behavior of Oligo(Ethylene Glycol)-Terminated Alkanethiolates on Gold

Abstract

A comparative study of the self-assembly and phase behavior of seven different oligo(ethylene glycol) (OEG)-terminated alkanethiols on polycrystalline gold surfaces is presented. The general structure of the compounds is HS(CH2)m-X-EGn, where m = 11, 15; n = 2, 4, 6, and the linkages X are amide (−CONH−), ester (−COO−), or ether (−O−) groups. The amide and ester groups give rise to the intermolecular hydrogen bonding and dipole−dipole interactions, respectively, whereas the ether lacks specific interactions. The results from contact angle goniometry, null ellipsometry, and infrared reflection−absorption spectroscopy (IRAS) indicate that the intermolecular interactions can be partly used to control the conformation and order of the OEG portion of the self-assembled monolayers (SAMs). It is shown that the lateral hydrogen bonding stabilizes the all-trans conformation of the EG4 tails in the SAMs. Further on, the mechanism behind the thermal phase behavior of the OEG SAMs is investigated using temperature-programmed IRAS in ultrahigh vacuum. In the present study we show that the earlier reported helix-to-all-trans conformational transition at 60 °C in the SAM of HS(CH2)15CONH-EG6 (Valiokas, R.; Östblom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. 2000, 104, 7565−7569.) is a result of the particular molecular design of the SAMs through the specifically built-in lateral hydrogen bonds. A shortening of the alkyl chain to 11 methylenes has no effect on the amide-EG6 phase behavior. Contrary, the ester- and ether- containing SAMs undergo a melting type of transitions at 52 and 68 °C, respectively, similar to that observed for poly(ethylene glycol).