This letter explores the phase behavior of oligo(ethylene glycol) self-assembled monolayers using temperature-programmed infrared reflection absorption spectroscopy. The monolayers are formed by self-assembly of hexa(ethylene glycol) (EG(6)) and tetra(ethylene glycol) (EG(4))-terminated and amide group containing alkanethiols on polycrystalline gold. The ethylene glycol portions of the two monolayers are known to exist in two different conformations at room temperature: EG(6) in helical and EG(4) in all-trans (zigzag). The helical phase of the EG(6) gradually diminishes upon increasing the temperature and a pronounced conformational transition occurs around 60 degrees C, leading to a rapidly increasing population of all-trans conformers along the EG(6) chain. The EG(4) SAM exhibits a much simpler phase behavior. The oligomer conformation is marginally affected upon increasing the temperature to 75 degrees C, displaying the dominating all-trans phase, which possibly coexists with a small fraction of gauche-rich (disordered) regions. The reported conformational changes are reversible upon returning to 20 degrees C after stepwise heating to 70 degrees C.

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).

Temperature-programmed desorption (TPD) of self-assembled monolayers (SAMs) on gold is investigated by using in parallel mass spectrometry (MS) and infrared reflection-absorption spectroscopy (IRAS). Monolayers formed by HS(CH2)n-OH (n = 18, 22) and HS(CH2)15-CONH-(CH2CH2O)-H (EG1) are compared to reveal the influence of specifically introduced hydrogen-bonding groups on their thermal stability. The overall desorption process of the above molecules is found to occur in two main steps, a disordering of the alkyl chains followed by a complex series of decomposition/desorption reactions. The final step of the process involves desorption of sulfur from different chemisorption states. The amide-group-containing SAM, which is stabilized by lateral hydrogen bonds, displays a substantial delay of the alkyl chain disordering by about 50 K, as compared to the linear chain alcohols HS(CH2)n-OH. Moreover, the decomposition of the alkyls and the onset of sulfur desorption occur at a temperature that is higher by approximately 25 K as compared to the HS(CH2)18-OH SAM. The desorption process is also studied for two oligo(ethylene glycol)-terminated SAMs, HS(CH2)15-X-(CH2CH2O)4-H (EG4-SAMs), where X is -CONH- and -COO- linking groups. In addition to the molecular chain disordering, the decomposition/desorption process of the EG4-SAMs occurs in two steps. The first is associated with the loss of the oligomer portion and the second with the desorption of the alkylthiolate part of the molecule. Our study points out that lateral hydrogen bonding, introduced via amide groups, is a convenient way to improve the thermal stability of alkanthiolate SAMs.