A nanoscale view on oligo(ethylene glycol) self-assembled monolayer hydration

Self-assembled monolayers (SAMs) of oligoethylene glycol (OEG)-terminated thiols are often used in the design of protein-repellent interfaces. It is well known that their repellency depends on the crystallinity and conformation of the EG chains, and it is thought to be a consequence of interface water interacting with the OEG chains. Here, we provide novel insight into the structure and molecular conformation of OEG SAMs at a nanoscale level, focusing on the impact of immersion time (t_im) during preparation and relative humidity. Macroscopically, an increase in t_im leads to SAM densification and an increase in thickness, as well as a transition from amorphous OEG chains to predominantly helical crystalline chains. At the nanoscale, the morphology showed nano-domains of size dependent on t_im, and nano IR spectroscopy revealed direct spectroscopic evidence for different conformations prevalent in these domains. Hydration experiments conducted in an environment with increased humidity resulted in the nano-domains being hydrated to different extents. Water uptake led to an irreversible conformational transition of the helical domains. Additionally, nano IR spectroscopy demonstrated the absorbance of two differently hydrogen-bound water populations. These results shed new light on the nanophase behaviour of OEG SAMs and help to explain how nano-structure influences their protein repellency.