Computer simulation of water near the surface of oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers

A Grand canonical Monte Carlo technique is used to simulate the behavior of the TIP4P model of water near the surface of an oligo(ethylene glycol) (OEG)-terminated alkanethiol self-assembled monolayer (SAM). Two structural modifications of the SAM are studied in an effort to shed light on the difference in their resistance to adsorb proteins. One modification (hereafter h-SAM), which is stable on the Au substrate, is characterized by a helical conformation of the OEG tails and a lower areal density. The other, higher density modification (hereafter t-SAM), which is experimentally observed on the Ag substrate, has their OEG tails in the all-trans conformation. Simulations show that water molecules can penetrate into the near surface region of the h-SAM in appreciable amounts to bring about substantial conformational disordering of the SAM, in agreement with recent sum frequency generation experiments. About 75% of the topmost oxygen atoms and even 2% of the next to the topmost oxygen atoms of the OEG tails prove to be involved in hydrogen bonds with water molecules. By contrast, the t-SAM is much more resistant to the penetration of water molecules. On interaction with water, it retains a good conformational order and shows a noticeably lower surface density of hydrogen bonds with water molecules. Both the h- and t-SAM surfaces produce water density oscillations that extend at least 3 to 4 molecular diameters into the water bulk. The density oscillations are less pronounced near the h-SAM, whose surface is ill-defined because of the conformational disordering of the OEG tails. The effect of the SAM on the structural characteristics of water is short range and practically vanishes at separations larger than 1 to 2 molecular diameters away from the SAM surface.