Stability of hexa(ethylene glycol) SAMs towards the exposure to natural light and repeated reimmersion

We investigate the stability of HSsingle bond(CH2)11single bond(OCH2CH2)6single bondCH3 self assembling monolayers (hexa(ethylene glycol) SAMs) on gold regarding reimmersion and exposure to natural light over long periods of time up to several months. With polarisation modulation infrared spectroscopy we were able to monitor significant changes in the fingerprint region (900–1800 cm−1) of the absorption modes of the SAMs, starting after a few days of exposure to natural light. We observed an exponential intensity decrease of modes indicating helical conformation of the SAM, as well as an exponential increase of modes indicating esters and formates suggesting a degradation of the SAM. X-ray photoelectron spectra of carbon C1s and sulphur S2p confirm the chemical nature of those changes. SAMs stored without light exposure show a drastically decreased change in the infrared spectra. In addition, we could find substantial conformational changes upon repeated drying and reimmersion in EtOH, manifesting in an intensity decrease of the absorption modes indicating hexa(ethylene glycol) molecules in helical conformation. Since the XPS data do not show changes in the chemical structure, we assume disordering effects and dissolution of molecules in solution. Our results suggest that SAMs can be stored over long periods of time in air without major changes if light exposure is avoided.