Local Depolarization in Hydrophobic and Hydrophilic Ionic Liquids/Water Mixtures: Car-Parrinello and Classical Molecular Dynamics Simulation

Car-Parrinello molecular dynamics (MD) was carried out to simulate pure [C(4)mim]PF6 and [C(4)mim]BF4 ionic liquids and their mixtures with polar water solvent to approach the admixing mechanism as well as hydrophobic and hydrophilic interactions from an electronic point of view. Initially, the results of density functional theory (DFT) on isolated ion pairs with partial charges assigned to atomic centers by various methods were analyzed. Next the trajectory of a 40 ps long Car-Parrinello MD were analyzed under bulk conditions. Water molecule influences substantially the hydrophilic ([C(4)mim]BF4) ionic liquid and the hydrophobic ([C(4)mim]PF6) to a different extent, which is evident by probing atomic charges of partnering constituents. The reduction in simulated dipole moment of water upon admixing with hydrophilic ionic liquid is larger than that with the hydrophobic one, which roots from stronger electrostatic screening. Water molecules tend to segregate when mixed with [C(4)mim]PF6 but mix with [C(4)mim]BF4 efficiently by interacting with BF4- anion, which interacts and resides on its cation [C(4)mim](+). When ionic liquids mixed with water, the average charge on each F atom in BF4- (PF6-) anion was -0.3261e (-0.1820e). The simulated charge on each H atom of pure water (0.3290e) can be evidently responsible for the effective H center dot center dot center dot F interaction in [C(4)mim]BF4 but ineffective in [C(4)mim]PF6. These results provide insight into the hydrophilic and hydrophobic character from an electronic point of view.