Structure of carboxyl-acid-terminated self-assembled monolayers from molecular dynamics simulations and hybrid quantum mechanics-molecular mechanics vibrational normal-mode analysis

Self-assembled monolayers (SAMs) are excellent models for studying interfacial reactions and various properties of thin films. Carboxylic acid-terminated SAMs (CATSAMs) are of special interest because their surface properties are highly pH-dependent. To elucidate the complicated pH-dependent structural properties of CATSAMs, we combine force-field molecular dynamics simulations with hybrid quantum mechanics-molecular mechanics (QM/MM) calculations of vibrational frequencies. These studies show that at low pH the carboxylic acids behave similarly to bulk carboxylic acids, hydrogen bonding mainly to the aqueous phase. With increasing pH, intralayer hydrogen bonds and monolayer disorder increase. The computed QM/MM vibrational data show that the observed range of carbonyl frequencies is due to inherent condensed phase properties of the CATSAMs such as fluctuating hydrogen bond environment and steric interactions. The results reported herein underscore the role H-bonding and steric congestion play in pH-dependent structural properties of CATSAMs.