Hydrogen Bonds Mediating the Structural and Dynamic Behaviors in Choline-Phenolic Acid Ionic Liquids: A Computational Investigation

The impact of hydrogen bonds (HBs) on the structural and dynamic behaviors of choline-phenolic acid ionic liquids ([Cho][PA] ILs) was explored by using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The most stable conformation of individual [Cho][PA] ion pairs was determined, and the interactions between charged entities were characterized, indicating the existence of robust HBs between [Cho]⁺ and [PA]^-. The bulk phase MD simulations of [Cho][PA] ILs showed that ion aggregation becomes more pronounced as more phenolic hydroxyl groups are present on [PA]^-. The results further indicated that the carboxylate oxygen of [PA]^- forms strong HBs with both the hydroxyl hydrogen of [Cho]⁺ and the phenolic hydroxyl hydrogen of [PA]^-. The former type of HBs remains robust even with more phenolic hydroxyl groups attached to [PA]^-, whereas the latter type increases in quantity with more phenolic hydroxyl groups, potentially driving ion aggregation and reducing the self-diffusion coefficients of both [Cho]⁺ and [PA]^-. Our findings are expected to contribute to a fundamental understanding of how HBs affect the structural and dynamic properties of [Cho][PA] ILs, thereby facilitating their broader applications.