Molecular Simulations of the Structure and Dynamics of Water Confined between Alkanethiol Self-Assembled Monolayer Plates

We have studied structural and dynamic properties of water confined between hydrophobic alkanethiol self-assembled monolayers (SAMs) using molecular-dynamics simulations. After quantifying the hydrophobic nature of the SAM surfaces via contact-angle calculations involving water droplets, we analyze the effect that the hydrophobic surfaces have on structural properties of the cofined water such as density, tetrahedral ordering, orientational structure at the SAM-water interface, and on dynamical properties via calculation of diffusion coefficients. Both the SPC/E and TIP5P water models have been utilized in the calculations. All of the analyses of the structures and dynamics of water are performed as a function of separation from the surface with a focus on determining the range of the effect of hydrophobic surfaces on the water film. We show that the effects of the surface are not noticeable at water-film depths of approximately 1 nm for the structural properties examined. However, calculated diffusion coefficients in the plane of the surface indicate the SAMs induce enhancement of water motion clearly beyond 1 nm. while the enhanced lateral diffusion coefficients persist into deeper regions of the water film than any other measure of the hydrophobic effect examined in this work, the range of influence of the surface on the dynamics of water falls dramatically short of the range for hydrophobic interactions measured some experiments.