Hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers.

Molecular dynamics computer simulations are used to study hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers (CAFSAMs). Water-water, water-CAFSAM, and internal CAFSAM hydrogen bonds are examined. Roughly half of all adjacent carboxylic acid-terminated hydrocarbon chains are hydrogen-bonded to one another. This is consistent with experimental results reflecting two pKa values for CAFSAMs. Hydrogen-bond dynamics are expressed in terms of hydrogen-bond population autocorrelation functions and are found to be nonexponential. The water-water hydrogen-bond dynamics are slower at the interface than in the bulk, which is similar to what was found at the interface between water and weakly polar liquids such as nitrobenzene. The water-CAFSAM hydrogen bonds are found to be long-lived, on the order of tens of picoseconds. Internal CAFSAM chain-chain hydrogen bonds show almost no relaxation on the simulation time scale.