Structural and Dynamical Properties of Carboxylate Ions in Bulk and Interfacial Water: An AIMD–MD Hybrid Study

Structural and dynamical properties of sodium formate, hexanoate, and benzoate salts in bulk and interfacial water have been investigated using ab initio molecular dynamics (AIMD) simulations. The interfacial behavior of these carboxylate ions plays an important role in environmental chemistry, in aerosol formation, and also in biological systems. Different structural and dynamical properties of these systems were determined in both bulk and interfacial regions. AIMD accurately captures the orientation of all three carboxylate ions near the interface, where the classical MD fails to predict the correct orientations, particularly for formate ions. Our results also show that formate ions remain predominantly in the bulk region, while hexanoate and benzoate prefer the interfacial region due to their hydrophobic substituents as confirmed from number density profiles and interfacial residence times. The preferential localization of hexanoate and benzoate ions in the interfacial region considerably affects the mobility and tetrahedral arrangement between water molecules through carboxylate–water interactions. The hydrogen-bond network is also noticeably altered at the interface, particularly for hexanoate and benzoate, leading to distinct structural rearrangements. Na[Formula: see text]binds strongly to formate ions while its interactions with hexanoate and benzoate are found to be weaker. Additionally, benzoate ions exhibit pair formation with a preferential antiparallel stacking orientation, contributing to enhanced interfacial stability.