Temperature-Driven Phase Transitions in Oligo(ethylene glycol)-terminated Self-Assembled Monolayers

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 °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 °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 °C after stepwise heating to 70 °C.