Investigating the properties of fatty acid-based ionic liquids: advancement in AMOEBA force field

Developing the multipolar-polarizable AMOEBA force field for large molecules presents its own set of complexities. However, by segmenting the molecules into smaller fragments and ensuring that each fragment is transferable to other systems, the process of parameterizing large molecules such as fatty acids can be simplified without compromising accuracy. In this study, we present a fragment-based AMOEBA FF development for long-chain fatty acid ionic liquids (LCFA-ILs). AMOEBA enables us to incorporate polarization to measurably enhance the precision in modeling these large highly charged systems. This is of significant importance since the computational investigation of ILs needs accurate modeling. Additionally, to leverage the tunability of ILs, it is essential to test numerous anion and cation combinations to identify the most suitable formulation for each application. However, conducting such experiments can be resource-intensive and time-consuming, but accurate molecular modeling can expedite the exploration process. Here, the newly developed parameters were evaluated by comparing the decomposed intermolecular interaction energies for ion pairs with energies determined by quantum mechanics calculations as a reference. By employing this FF in molecular dynamics simulations, we predicted bulk and structural properties including density, enthalpy of vaporization, diffusion coefficient, structure factor and radial distribution function of diverse LCFA-ILs. Notably, the good agreement between the experimental data and those calculated using our parameters validates the accuracy of our methodology. Therefore, this new procedure provides an accurate approach to parameterizing large systems, paving the way for studying more complicated systems such as lipids, polymers, micelles and membrane proteins.