Standard analyses, e.g., calculation of radial distribution functions or analysis of contacts, were all performed using Gromacs tools. Contacts between TMA and acetate were defined based on the distance between the nitrogen atom of the TMA cation (N TMA ) and the carboxyl carbon of acetate (C Ac ). Based on the radial distribution function of the N TMA and C Ac atoms, we determined the typical N TMA -C Ac distances used to distinguish direct contact and solvent-shared ion pairs. Accordingly, the number of contacts and then its distribution for each force field simulation were calculated. Moreover, to quantify the orientations of acetate relative to TMA ion, we computed the distribution of angle θ, which is the angle between TMA nitrogen, acetate carboxyl carbon, and acetate methyl carbon atoms.