Influence of specific intermolecular interactions on the self-assembly andphase behavior of oligo(ethylene glycol)-terminated alkanethiolates on gold

A comparative study of the self-assembly and phase behavior of seven differ ent oligo(ethylene glycol) (OEG)-terminated alkanethiols on polycrystalline gold surfaces is presented. The general structure of the compounds is HS(C H2)(m)-X-EG(n), where m = 11, 15; n = 2, 4, 6, and the linkages X are amide (-CONH-), ester (-COO-), or ether (-O-) groups. The amide and eater groups give rise to the intermolecular hydrogen bonding and dipole-dipole interac tions, respectively, whereas the ether lacks specific interactions. The res ults from contact angle goniometry, null ellipsometry, and infrared reflect ion-absorption spectroscopy (IRAS) indicate that the intermolecular interac tions can be partly used to control the conformation and order of the OEG p ortion of the self-assembled monolayers (SAMs). It is shown that the latera l hydrogen bonding stabilizes the all-trans conformation of the EG(4) tails in the SAMs. Further on, the mechanism behind the thermal phase behavior o f the OEG SAMs is investigated using temperature-programmed IRAS in ultrahi gh vacuum. In the present study we show that the earlier reported helix-to- all-trans conformational transition at 60 OC in the SAM of HS(CH2)(15)CONH- EG(6) (Valiokas, R.; Ostblom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg , B. J. Phys. Chern. 2000, 104, 7565-7569.) is a result of the particular m olecular design of the SAMs through the specifically built-in lateral hydro gen bonds. A shortening of the alkyl chain to ii methylenes has no effect o n the amide-EG(6) phase behavior. Contrary, the ester- and ether- containin g SAMs undergo a melting type of transitions at 52 and 68 degreesC, respect ively, similar to that observed for poly(ethylene glycol).