Temperature-driven phase transitions in oligo(ethylene glycol)-terminated self-assembled monolayers

This letter explores the phase behavior of oligo(ethylene glycol) self-asse mbled monolayers using temperature-programmed infrared reflection absorptio n spectroscopy. The monolayers are formed by self-assembly of hexa(ethylene glycol) (EG(6)) and tetra(ethylene glycol) (EG(4))-terminated and amide gr oup containing alkanethiols on polycrystalline gold. The ethylene glycol po rtions of the two monolayers are known to exist in two different conformati ons 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 tem perature and a pronounced conformational transition occurs around 60 degree s C, leading to a rapidly increasing population of all-trans conformers alo ng the EG(6) chain. The EG(4) SAM exhibits a much simpler phase behavior. T he oligomer conformation is marginally affected upon increasing the tempera ture to 75 degrees C, displaying the dominating all-trans phase, which poss ibly coexists with a small fraction of gauche-rich (disordered) regions. Th e reported conformational changes are reversible upon returning to 20 degre es C after stepwise heating to 70 degrees C.