| Abstract
| - A Grand canonical Monte Carlo technique is used to simulate the behavior of the TIP4P model of waternear 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 theirresistance 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, higherdensity modification (hereafter t-SAM), which is experimentally observed on the Ag substrate, has theirOEG tails in the all-trans conformation. Simulations show that water molecules can penetrate into thenear surface region of the h-SAM in appreciable amounts to bring about substantial conformationaldisordering of the SAM, in agreement with recent sum frequency generation experiments. About 75% ofthe topmost oxygen atoms and even 2% of the next to the topmost oxygen atoms of the OEG tails proveto be involved in hydrogen bonds with water molecules. By contrast, the t-SAM is much more resistantto the penetration of water molecules. On interaction with water, it retains a good conformational orderand shows a noticeably lower surface density of hydrogen bonds with water molecules. Both the h- andt-SAM surfaces produce water density oscillations that extend at least 3 to 4 molecular diameters into thewater bulk. The density oscillations are less pronounced near the h-SAM, whose surface is ill-definedbecause of the conformational disordering of the OEG tails. The effect of the SAM on the structuralcharacteristics of water is short range and practically vanishes at separations larger than 1 to 2 moleculardiameters away from the SAM surface.
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